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Carnegie 
Institution 

OF WASHINGTON 



Year Book 75 



1975-1976 



Library of Congress Catalog Card Number 3-16716 

Champion Press, Inc., Baltimore, Maryland 

Issued December 1976 



Contents 



Officers and Staff 

Report of the President 

Reports of Departments and Special Studies 

Department of Embryology 

Department of Terrestrial Magnetism 

Hale Observatories 

Department of Plant Biology 

Geophysical Laboratory 

Developmental Biology Research Group 
Bibliography 
Administrative Reports 

Report of the Executive Committee 

Abstract of Minutes of the Seventy-Eighth Meeting of the 
Board of Trustees 

Financial Statement 

Ten-Year Financial Summary 

Report of Independent Public Accountants 

Articles of Incorporation 

By-Laws of the Institution 

Index of Names 



v 

1 

1 

3 

105 

273 

347 

485 

859 

961 

963 

965 

967 
971 
973 
974 
989 
993 
999 



President and Trustees 



PRESIDENT 
Philip H. Abelson 

BOARD OF TRUSTEES 

William McChesney Martin, Jr. 
Chairman 

Frank Stanton 
Vice-Chairman 

William T. Golden 
Secretary 

Robert 0. Anderson 1 
J. Paul Austin 1 
Lewis M. Branscomb 
John T. Connor 
John Diebold 
Michael Ference, Jr. 
Carl J. Gilbert 
Hanna H. Gray 
Crawford H. Greenewalt 
William C. Greenough 
Caryl P. Haskins 
William R. Hewlett 
Henry S. Morgan 
William I. Myers 2 
Walter H. Page 
Robert M. Pennoyer 
Richard S. Perkins 
William M. Roth 
Robert C. Seamans, Jr. 
Charles H. Townes 
Juan T. Trippe 
James N. White 



Garrison Norton 
Charles P. Taft 
Trustees Emeriti 



1 Elected April 1976. 

2 Died January 30, 1976. 



Former Presidents and Trustees 



PRESIDENTS 

1902-1904 



Daniel Coit Gilman 

Robert Simpson Woodward 1904-1920 

Caryl P. Haskins 

T 

Alexander Agassiz 1904-05 

Lord Ashby of Brandon 1967—74 

George J. Baldwin 1925-27 

Thomas Barbour 1934-46 

James F. Bell 1935-61 

John S. Billings 1902-13 

Robert Woods Bliss 1936-62 

Amory H. Bradford 1959-72 

Lindsay Bradford 1940-58 

Omar N. Bradley 1948-69 

Robert S. Brookings 1910-29 

Vannevar Bush 1958-71 

John L. Cadwalader 1903-14 

William W. Campbell 1929-38 

John J. Carty 1916-32 

Whitefoord R. Cole 1925-34 

Frederic A. Delano 1927-49 

Cleveland H. Dodge 1903-23 

William E. Dodge 1902-03 

Charles P. Fenner 1914-24 

Homer L. Ferguson 1927-52 

Simon Flexner 1910-14 

W. Cameron Forbes 1920-55 

James Forrestal 1948-49 

William N. Frew 1902-15 

Lyman J. Gage 1902-12 

Walter S. Gifford 1931-66 

Cass Gilbert 1924-34 

Frederick H. Gillett 1924-35 

Daniel C. Gilman 1902-08 

Patrick E. Haggerty 1974-75 

John Hay 1902-05 

Barklie McKee Henry 1949-66 

Myron T. Herrick 1915-29 

Abram S. Hewitt 1902-03 

Henry L. Higginson 1902-19 

Ethan A. Hitchcock 1902-09 

Henry Hitchcock 1902 

Herbert Hoover 1920-49 

William Wirt Howe 1903-09 

Charles L. Hutchinson 1902-04 

Walter A. Jessup 1938-44 

Frank B. Jewett 1933-49 

Samuel P. Langley 1904-06 

Ernest O. Lawrence 1944-58 

Charles A. Lindbergh 1934-39 

William Lindsay 1902-09 

Henry Cabot Lodge 1914-24 

Alfred L. Loomis 1934-73 

Under the original charter, from the date of organization until April 28, 1904, the following 
were ex officio members of the Board of Trustees : the President of the United States, the Presi- 
dent of the Senate, the Speaker of the House of Representatives, the Secretary of the 
Smithsonian Institution, and the President of the National Academv of Sciences. 



John Campbell Merriam 


1921-1938 


Vannevar Bush 


1939-1955 


1956-1971 




Robert A. Lovett 


1948-71 


Seth Low 


1902-16 


Wavne MacVeagh 


1902-07 


Keith S. McHugh 


1950-74 


Andrew W. Mellon 


1924-37 


John Campbell Merriam 


1921-38 


Margaret Carnegie Miller 


1955-67 


Roswell Miller 


1933-55 


Darius 0. Mills 


1902-09 


S. Weir Mitchell 


1902-14 


Andrew J. Montague 


1907-35 


William W. Morrow 


1902-29 


Seeley G. Mudd 


1940-68 


William I. Myers 


1948-76 


William Church Osborn 


1927-34 


James Parmelee 


1917-31 


Wm. Barclay Parsons 


1907-32 


Stewart Paton 


1916-42 


George W. Pepper 


1914-19 


John J. Pershing 


1930-43 


Henning W. Prentis, Jr. 


1942-59 


Henry S. Pritchett 


1906-36 


Gordon S. Rentschler 


1946-48 


David Rockefeller 


1952-56 


Elihu Root 


1902-37 


Elihu Root, Jr. 


1937-67 


Julius Rosenwald 


1929-31 


William W. Rubey 


1962-74 


Martin A. Ryerson 


1908-28 


Henry R. Shepley 


1937-62 


Theobald Smith 


1914-34 


John C. Spooner 


1902-07 


William Benson Storey 


1924-39 


Richard P. Strong 


1934-48 


William H. Taft 


1906-15 


William S. Thayer 


1929-32 


James W. Wadsworth 


1932-52 


Charles D. Walcott 


1902-27 


Frederic C. Walcott 


1931-48 


Henry P. Walcott 


1910-24 


Lewis H. Weed 


1935-52 


William H. Welch 


1906-34 


Andrew D. White 


1902-16 


Edward D. White 


1902-03 


Henry White 


1913-27 


George W. Wickersham 


1909-36 


Robert E. Wilson 


1953-64 


Robert S. Woodward 


1905-24 


Carroll D. Wright 


1902-08 



OFFICE OF ADMINISTRATION 

1530 P Street, N .W '., Washington, D.C. 20005 

Philip H. Abelson President 

James W. Boise Bursar ; Secretary-Treasurer, Retirement Trust; 
Executive Secretary to the Finance Committee 

Marjorie H. Walburn Assistant to the President 

Sheila A. McGough Publications Officer; Editor 

Kenneth R. Henard Assistant Bursar; Assistant Treasurer, 
Retirement Trust 

Joseph M. S. Haraburda Assistant to the Bursar 

Richard E. Hewitt Administrative Officer for Services 



Marshall Hornblower Counsel 

STAFF MEMBERS IN SPECIAL SUBJECT AREAS 

Ellis T. Bolton 
Roy J. Britten 
Tatiana Proskouriakoff 

DISTINGUISHED SERVICE MEMBER IN SPECIAL SUBJECT AREA 

Barbara McClintock 

RESEARCH ASSOCIATE AT LARGE 

Harry E. D. Pollock 



Carnegie Institution of Washington adheres in all phases of its operations, 
including employment and educational programs, to a policy barring discrimina- 
tion on the basis of race, religion, color, national or ethnic origin, or sex. In its 
educational programs it admits qualified students as fellows without regard to 
race, religion, color, national or ethnic origin, or sex to « all the rights, privileges, 
programs, and activities generally accorded or made available to fellows at the 
Institution. It does not discriminate on the basis of race, religion, color, national 
or ethnic origin, or sex in administration of its educational policies, admissions 
policies, fellowship programs, and other Institution-administered programs. 



Report of 
the President 



ELECTRONICS REVOLUTION 

Earlier in the century, this country experienced a long era of sustained growth 
in many aspects. There was a steady increase in level of education, life expectancy, 
and standard of living. Growth of all kinds was welcomed, including industrial 
expansion and population increase. A feeling of progress, of achievement, of well- 
being was everywhere. As a corollary, a striving for excellence and the search 
for understanding were widely admired. 

Today the mood of this country has turned pessimistic and negative. Those 
who are so inclined can find much evidence to support these views. Growth in 
the use of energy in the form of oil was suddenly curtailed in 1974. Growth in 
consumption of the kind seen in the 1950s and 1960s will not occur again. That 
part of the standard of living that is based on large-scale consumption of energy 
is not likely to improve during this century. 

However, those who prefer optimism have reason for hope. Human ingenuity 
in solving problems is great. And native intelligence has been amplified enor- 
mously by the use of knowledge accumulated through research. An important 
product of research and a basis for hoping for a bright new future is the vitality 
of the continuing electronics revolution. This revolution has been in progress for 
about 60 years. Lately its tempo has increased greatly. Until recently its im- 
portance was overshadowed by changes brought about by the large-scale expan- 
sion in the use of energy. But the electronics revolution promises to be more 
important, of more enduring consequence, than the earlier Industrial Revolution. 

Some of the great changes brought about by this revolution have gone on 
comparatively unnoticed ; at the start they were evolutionary rather than sudden 
and drastic. The telephone, which we take for granted, was invented 100 years 
ago. Nearly every decade since then, the quality and scope of service have steadily 
improved while the cost (measured in constant dollars) is now a tiny fraction 
of what it was 50 years ago. 

Numerous applications of electronics gradually affected individuals and almost 
every component and activity of society. Americans listen to commercial radio 
an average of nearly four hours daily. Television has come to most homes. 
During the last few years the accelerated tempo of change has led to an increased 
impact on society. For example, there is the rapidly burgeoning acceptance of 
the citizen's band radio, the worldwide use of the telephone, and the current 
astonishingly low prices at which hand-held computers and electronic watches 
are being sold. Less evident to the individual but in total more important to 
society are new and increasing applications of electronics to process control for 
industry, to research equipment, to merchandising, to banking, to management, to 
national defense, and to a host of other activities. 

This revolution, which is destined to have great long-term consequences, is 
quite different in nature from the earlier Industrial Revolution. The Industrial 
Revolution was based on the profligate use of energy (mainly fossil fuels) . Much 
of its technology was crude, with only a modest scientific or theoretical base. In 
large measure what the Industrial Revolution did was to make available and to 
employ large amounts of mechanical energy. History may well describe the past 
century as the Era of Brute Force. 

In contrast, the electronics revolution represents one of the most noble intel- 
lectual achievements of mankind. Its development has been the product of the 
most advanced science, technology, and management. In many applications 
electronics requires little energy. Indeed, one factor that guarantees enduring 
impact for the electronic revolution is that it is sparing of energy and materials. 

3 



4 CARNEGIE INSTITUTION 

With electronics one can control the disposition of large amounts of energy 
and force, but the relationship is much like the use of the brain in directing the 
effort of muscles. In some aspects, electronics can be more subtle, more nimble, 
more dependable than the brain. In other applications, electronics serves as a 
great extender of human capabilities by rapidly carrying out routine but complex 
calculations, thus freeing the mind to make intuitive judgments and short cuts 
to new insights. 

In laboratories devoted to extending the electronics revolution, the use of 
powerful investigative tools based on electronics is speeding new developments. 
Ultimately, there must be limits to what can be done. However, the barriers are 
not yet obvious, and it is clear that the revolution has some distance to go. What 
has been accomplished and something of what is possible may be found in the 
history and potential of certain solid-state devices. Writing about integrated 
circuits formed on silicon chips, Robert N. Noyce has commented: 

The cost reductions resulting from higher levels of integration have 
been remarkable. The individual diffused transistor sold for a price of 
approximately one dollar in 1961, a few years after its introduction. 
Today, a 1024 bit memory including over six thousand transistors sells 
for about one dollar a few years after its introduction, representing a 
cost reduction of about 6000:1 in 15 years. The resulting effect on 
equipment costs can be seen in the electronic calculator, which has been 
reduced in cost by a factor of 500:1 in the last 8 years, creating a 
substantial new market unforeseen only 10 years ago. 

Noyce and others are confident that the performance of silicon chips will be 
improved by at least a factor of 10. Already a tiny device has the computing 
power that earlier required a roomful of tubes and other electronic components. 

One of the factors assuring a bright future for electronics is its high degree of 
social acceptance. There have been sporadic attacks on various electronic devices 
such as computers, but the intensity of criticism has diminished. In comparison 
to the level of criticism of chemical products or the environmental concerns 
associated with nuclear and fossil fuel energy, objections to electronics have been 
few. Broadly available items such as the hand-held computer, electronic watch, 
and citizen's band radio enhance the public's feeling of participating in the 
benefits of electronics while not bringing with them discernible side effects. 
In future, electronics will provide many new tools for humanity. 

There are, of course, limitations to what electronics can do. It cannot supply 
the basic human needs of food, clothing, and shelter. But it can increase the 
efficiency of the processes by which these requirements are met. It can contribute 
to economies in the use of energy, for example, by improving the engine efficiency 
of automobiles through ignition control. Better electronic communications will 
lessen the need for business travel. Solar panels of improved cost and efficiency 
may be developed, making them an important energy source. 

Just as electronics is increasingly changing the ways in which industry and 
commerce are conducted, it has also altered and continues to change the tools 
for scientific research, sometimes drastically, and the new tools are creating great 
research opportunities. 

The use of electronic instrumentation has very much affected the conduct of 
research at the Carnegie Institution. In astronomy, it has increased the volume 
of the measurable universe by as much as a factor of 50. In the physical and 
biological sciences, electronics has expanded the time horizon and vastly increased 
the sensitivity and accuracy of many measurements. Human response time is of 



REPORT OF THE PRESIDENT . O 

the order of 1/10 to 1/5 second, but electronics enables humans to conduct 
experiments with time spans as short as a picosecond (10 -12 second). 

Two examples may be cited to illustrate the increased sensitivity and ease 
of analytical determinations. At one time the analysis of a silicate rock required 
a sample of about a gram and the processing occupied about a week's time. A 
skilled chemist might complete a dozen determinations in a week. Today, using 
the electron microprobe, one can analyze a sample of perhaps 10 -10 gram in about 
10 minutes. 

Another example involves the analysis of a mixture of organic chemicals such 
as those found in petroleum. The original work to determine the composition 
of a crude oil, the Ponca City crude, required the fractionation of many barrels 
of oil — about a million grams — and efforts of many workers extending over about 
two decades. Today such an analysis can be performed in a few hours em- 
ploying a sample containing a tenth of a gram. 

Of comparable importance is the use of small but powerful on-line computers. 
These greatly facilitate the accumulation and interpretation of data and free 
investigators from dull bookkeeping tasks. 

Many of these tools are production models of major instrument makers such 
as Hewlett-Packard. But leaders in research do not necessarily wait for some one 
else to produce a new device. Indeed, they often become leaders by being first 
to develop and exploit a new tool. Developing new tools and using them has 
been a tradition and practice at Carnegie installations. 

In the past, most new research instruments used at the departments of the 
Institution were totally designed by them. The current practice is to combine 
commercially available electronic packages such as amplifiers or minicomputers 
with home-designed components to make a new device with unique capabilities. 

Any account of the electronics revolution, however brief, would be incomplete 
without mention of the key role that three Carnegie trustees have had and are 
continuing to have in it. William Hewlett's company has made enormous con- 
tributions. Scientists and engineers worldwide are impressed with the quality 
and usefulness of the many instruments that Hewlett-Packard has innovated. 
Lewis Branscomb has had a long association with electronics, and as Vice- 
President and Chief Scientist for IBM he is helping guide the development of 
new products for that company. Charles Townes, who received the Nobel Prize 
for his research in quantum electronics that led to the maser-laser principle, 
continues creative work. His current radio telescope studies of molecules in space 
and his infrared work are advancing the frontiers of astronomy. 

EVOLUTION OF THE INSTITUTION 

On the surface the Institution appears much as it did six years ago. The five 
major departments retain their names and most of the staff members. However, 
there have been substantial changes. Four new directors have taken over, research 
programs have evolved, new equipment has been obtained, and major facilities 
have been constructed. 

Department of Plant Biology 

The changes at Plant Biology are particularly satisfying. Under Winslow R. 
Briggs the department has become what some consider the world's best center 
for plant research. He has built on solid foundations laid by his predecessor. 
For many years, C. Stacy French had conducted a small though well-regarded 
department. During his tenure, French attracted fellows and guest investigators, 



6 CARNEGIE INSTITUTION 

many of whom were or became leaders in the field. He also provided leadership 
in creation and use of new instruments for plant research. 

As the time for French's retirement approached, special thought was given to 
the opportunities inherent in plant biology. It was obvious that advances in plant 
biology might provide great benefits to a world seeking solutions to its problems 
of food and energy. The great agricultural research stations could be expected 
to conduct excellent studies on existing crops. For Carnegie, the optimal oppor- 
tunities for research lay in long-term fundamental studies of plant biology that 
carry with them the potential for breakthroughs. Such new knowledge could be 
expected ultimately to lead the way to better plants and to better management 
of existing species. 

For some years the biology of plants did not enjoy so intense a study as that 
devoted to other related sciences. The substantial funds available for medically 
oriented research had led to great advances in molecular biology and biochemistry. 
The emphasis was on studies of microorganisms or animals. But these large-scale 
efforts developed concepts, techniques, and powerful equipment applicable to 
plant research. 

Briggs was fully aware of the possibilities. He was attracted also to the 
abundant opportunities for interaction with biology professors and students at 
Stanford University. It was clear, though, that to create an establishment com- 
mensurate with the opportunities would require extensive renovation of the 
laboratories and new construction. The necessary planning, design, and construc- 
tion were completed with minimal disruption of. research efforts. The resultant 
modern facilities are functional, versatile, and attractive to staff, fellows, and 
guest investigators. In addition to standard laboratory equipment for biochemical 
studies, the department has controlled plant growth chambers capable of main- 
taining a broad range of environments. The department shares a mobile labora- 
tory with Stanford (complete with minicomputer) that facilitates in situ studies 
in diverse climatic areas. Special equipment is available at the laboratory for 
observations on the photosynthetic processes at time spans as short as 10 -6 
second. Other equipment is designed to measure the absorption spectra or action 
spectra of intact plant materials or of membrane preparations. The output of 
such devices is electronic, and they arc programmed and their output is processed 
by an in-house computer that has time-sharing capabilities. 

The research program at the department includes major activities that con- 
tinue from the past — studies of the mechanisms of photosynthesis and physio- 
logical ecology. Briggs has brought a new program involving himself and others 
in the study of membranes. Membranes have a role in photosynthesis, and the 
temperature behavior of lipids of membranes is relevant to physiological ecology. 
Thus the three major thrusts of the department interact well together. 

One measure of the overall quality of the department is its attractiveness to 
Stanford biologists and to guest investigators from elsewhere. In his current 
annual report Briggs lists the many people who are now finding the department 
a satisfying place for scholarship. 

Hale Observatories 

Astronomers of the Hale Observatories continue to be pathfinders in the 
study of the universe and its contents. They are concerned with providing 
answers to basic questions: the age of the universe, the formation and evolution 
of galaxies and stars, and the nature of bizarre physical processes in x-ray 
sources, neutron stars, and black holes. These profound problems have justified 
the Institution's largest construction project in several decades — the new observa- 



REPORT OF THE PRESIDENT / 

tory at Las Campanas with its 2.5-meter Irenee du Pont telescope. Staff astron- 
omers are at the forefront in developing new instruments for measurement of 
the faintest and most remote objects that can be detected. Because optical 
astronomy can provide enormous amounts of detailed information about chemical 
composition, temperature, pressure, density, strength of gravity, electric fields, 
and magnetic fields in particular regions of stars, earth-based telescopes will 
continue to have a central role. Hale staff have interacted with observers studying 
other portions of the electromagnetic spectrum. They have also been leaders in 
developments that have extended the reach of telescopes. 

New electronic devices detect and count individual photons. The ability of 
telescopes to detect dim and very distant objects is limited by a faint but ever- 
present airglow in the earth's atmosphere: That is, even when the telescope is 
pointed to areas of the sky where there are no stars, some light is received. With 
electronic detectors, it is possible to subtract the airglow effect when spectra 
are being recorded. The use of such electronic detectors extends the distance to 
which humans can measure the universe. By such means it will be possible to 
detect light from an object so distant that the light that reaches us began its 
journey 15 billion years ago. 

For the Hale Observatories, one of the big developments of the past six years 
has been the creation of a major observatory in the southern hemisphere. 

The Mount Wilson Observatory of the Carnegie Institution was the world's 
leader in astronomy for nearly four decades with its 1.5-meter and 2.5-meter 
telescopes. With the inauguration in 1948 of the 5-meter telescope on Palomar 
Mountain by the California Institute of Technology, a new powerful tool was 
made available to Carnegie staff through an arrangement in which Mount Wilson 
and Palomar were operated as a unit in what came to be called the Hale Observa- 
tories. With the passage of time, the telescopes at Mount Wilson lost some of 
their value because of an increase in scattered light from the metropolitan region. 

If the Institution were to provide its share of facilities to the Hale Observa- 
tories, it would be necessary to add something new. The southern skies held 
relatively unexploited opportunities, and surveys showed that viewing conditions 
in north-central Chile were particularly favorable. Initial efforts to obtain outside 
support for a 5-meter telescope proved disappointing. However, a gift by 
Henrietta Swope and a later gift from the Crawford Greenewalts provided 
impetus to the building of an observatory on Las Campanas in the Norte Chico 
district- of Chile. Facilities there include a 0.55-meter telescope owned and 
operated by the David Dunlap Observatory of the University of Toronto, the 
1-meter Swope telescope, and the 2.5-meter Irenee du Pont telescope. 

Into the du Pont telescope have gone the products of the genius of two of 
the world's most experienced designers and engineers — the optical skill of the 
late Ira S. Bowen and the mechanical expertise of Bruce Rule. The 2.5-meter 
du Pont telescope is smaller than, for example, the 5-meter Hale at Palomar, so 
it takes longer to collect the same amount of light. But its capabilities are unique 
among telescopes of its size. The du Pont telescope optics employ principles of 
design that make it possible to expose photographic plates 51 centimeters square, 
or 12 times the usable area of those employed with the 5-meter at Palomar. The 
focal length is the same as the Hale 5-meter, so the du Pont can reach objects 
as faint and distant as any other instrument on earth, although requiring more 
time. But because of the enormous size of the photographic plates in the du Pont 
telescope, it will be uniquely able to perform crucial astronomical surveys. It can 
perform surveys three times faster than the huge 5-meter Hale telescope by scan- 
ning much greater areas of the cosmos. 



8 CARNEGIE INSTITUTION 

Spectrograph^ observations are crucial to studies of stellar chemical composi- 
tion and evolution. To provide maximum efficiency for spectrographic analysis of 
light from the south polar region of the sky, a radically new coude system has 
been designed that requires only three reflections of light instead of the usual five. 
Since at each reflection a considerable amount of light is lost, the new design will 
substantially increase the effectiveness of the instrument. 

Another feature of the du Pont telescope is its electronic control system that 
employs programmable microprocessors. These devices represent some of the 
latest advances in electronics. The microprocessors of the telescope's electronic 
control system are used to set the telescope accurately and quickly, move the 
dome and the windscreen automatically, provide correct rates for driving the 
telescope, and assist the astronomer with data acquisition. Automated controls 
and sensing devices will permit much of the observing to be done remotely, with 
the astronomer in the control room. An integrated closed-circuit television system 
permits guiding on very faint objects. 

With its exceptionally wide field for direct photography, the du Pont telescope 
is well adapted for searching for distant quasars. It is also well adapted for 
studying objects that are closer. Observers in the southern hemisphere have the 
advantages of studying regions of the sky that have not been so thoroughly 
examined as those of the northern. In particular, the Greater and Lesser Magel- 
lanic Clouds present special opportunities. They are our nearest galactic neigh- 
bors, and they contain very old stars, but they also contain very young ones. 
The Magellanic Clouds are very active regions of star formation. 

The new telescope is becoming operational at a time of great challenges and 
opportunities for astronomy. The subjects are truly cosmic — events at the far 
reaches of the universe, events at the beginning of time, the creation and evolution 
of galaxies, the birth and death of stars. 

Geophysical Laboratory 

During the early years of this century, the easily available oil and mineral 
resources were heavily exploited. Most of the oil that was formed during hundreds 
of millions of years was consumed. At the beginning of this century, the copper 
ore being mined contained about 3% of the metal. Currently the average grade 
is about 0.7%. The degree of exhaustion of the high grades of various elements 
differs, but the overall picture is similar to that of copper. 

Man will continue to turn to the earth as an essential source of raw materials. 
However, it will be necessary to employ much more advanced knowledge and 
technology to the processes. 

The Geophysical Laboratory has been a leading source of new information 
about the earth. The late W. W. Rubey, one of the founders of modern geology 
and a trustee of the Institution, evaluated the work of the Laboratory in an 
article published in 1974 as part of a survey of major developments in earth 
sciences occurring in the last 50 years. In discussing advances in knowledge about 
the petrology of igneous and metamorphic rocks, Rubey cited the Geophysical 
Laboratory as having made perhaps the most far-reaching contributions through 
applications of principles of physical chemistry. He summarized his discussion 
of the Institution's work by saying 

It is probably fair to state that the science of petrology has been made 
over by the results of experimental work at the Geophysical Laboratory 
and by [N. L.] Bowen's theory. Perhaps an even greater contribution 
of the Laboratory is one that extends beyond petrology into many 



REPORT OF THE PRESIDENT i> 

other fields of science. This is the demonstration that problems of 
seemingly hopeless complexity may be divided into simpler, more 
tractable parts and brought into the laboratory for analysis and solution. 

In general terms the objectives of the Geophysical Laboratory are much the 
same as they were nearly 70 years ago. However, the specific research programs 
are entirely different, and they are carried out using different equipment. Indeed, 
a history of the laboratory would necessarily emphasize a continuing change of 
equipment, techniques, and interpretations,, with many of the new tools invented 
and developed there. Especially during the past decade, there has been heavy 
and increasing use of electronics to better understand the processes involved in 
the formation and evolution of the earth. 

Various kinds of analytical tools, such as the electron microprobe, Mossbauer 
equipment, chromatography coupled with electronic detectors, spectrophotometry, 
and micro x-ray crystallography, have shortened the time for determinations by 
factors of a hundred or more while diminishing the size of sample required by 
many orders of magnitude — in some instances by factors of more than a million. 

Much of the analytical equipment utilizes dedicated minicomputers to program 
the analytical procedure and to record and analyze the results. Another important 
use of electronics is in the storage and retrieval and processing of the vast 
amounts of information produced by numerous workers. The rocks of the earth 
are many and they have been generated by a variety of processes from differing 
starting materials. Over the years many tens of thousands of chemical analyses 
have been performed on rocks in laboratories around the world. Information of 
this sort is now so voluminous and so widely dispersed as to make its retrieval 
and reduction by conventional methods grossly inefficient. Chayes has designed a 
computerized information system that efficiently scans a model data base con- 
taining 16,000 multicomponent analyses, and he is planning a much larger and 
more sophisticated world data base for igneous petrology. 

Over the years, the Geophysical Laboratory has been a leader in the invention 
and application of high-pressure techniques for the study of minerals. Using a 
device that permitted accurate control of pressure and temperature, Yoder made 
many pioneering studies at pressures of 10 to 15 kilobars during the 1950s and 
1960s. This pressure corresponds to a depth of 30 to 45 kilometers and to the 
region of many rock-forming processes. In 1960 Boyd and England developed 
a solid media press routinely capable of pressures of 50 kilobars. This device has 
been widely copied and used. During the last several years Mao and Bell have 
redesigned and extended the range of the diamond press that was originally 
developed by Van Valkenberg and others at the National Bureau of Standards. 
This year they were able to achieve a pressure of 1200 kilobars and to make 
routine measurements at 900 kilobars. They have accordingly been able to study 
behavior of minerals at pressures of the core-mantle boundary (2000-2500 km 
deep). 

Department op Terrestrial Magnetism 

Over the years a central thread of the work of the department has been the 
physics of the earth and its environs. In the early days, a major effort was de- 
voted to study of the earth's magnetic field first by expeditions, including those 
of nonmagnetic ships, and then at magnetic observatories. The program was of 
considerable practical importance, but after the accumulation of several decades 
of data, scientific interest in the work diminished. These magnetic studies were 
halted when Merle Tuve returned to the department as director following his 



10 CARNEGIE INSTITUTION 

very successful war work with the proximity fuze. Geophysical investigations of 
seismicity, radioactive dating, and cosmic rays were initiated, and some ionospheric 
studies were continued. A long-enduring program in low energy nuclear physics 
was carried out. In addition, a dynamic effort in biophysics was successfully 
launched and maintained. Interest shifted from the ionosphere to the new science 
of radio astronomy and to development of the Carnegie image tube for astro- 
nomical observations. 

These broad programs of the department continued to progress following 
Tiive's retirement in 1966. The biophysics group made many fundamental con- 
tributions to molecular biology [see Roberts in Year Book 741- Eventually, 
however, as the study of molecular biology matured, the DTM's biophysics 
program lost some of its distinctiveness. With reluctance the biophysics program 
was phased out. 

In 1975 George W. Wetherill succeeded Ellis T. Bolton, who had followed Tuve 
as director. Wetherill was trained in physics, and his research had included 
geophysics. His current work on physics of the origin and evolution of the solar 
system fits well with other research activities in the department. 

As the success of DTM's program has shown, a training in physics is particu- 
larly useful as a preparation for problem solving. Since the department is for the 
most part staffed by physicists, it is not surprising that they have invented, 
developed, or adapted many items of electronic equipment. 

For example, two developments conceived by the radio astronomy group are 
still in routine use at radio observatories. The development of the multichannel 
line spectrometer only two years after the discovery of emission from interstellar 
neutral hydrogen enabled radio astronomers to study the entire frequency spec- 
trum of a region without retuning the receiver for each frequency. The develop- 
ment of a traveling feed for large transit radio telescopes greatly increased the 
sensitivity of these instruments. Also, image intensifiers developed under the 
Carnegie image tube program were distributed to 34 observatories, and their 
worldwide use opened a new era in observational astronomy. 

The DTM geophysics group has long been recognized for its development of 
instrumentation. In the last ten years two significant new instruments have 
allowed acquisition of much valuable new seismic data. 

In the mid-1960s, Sacks designed a broad frequency band, high dynamic range 
seismograph system that for the first time allowed near-source recording of long- 
period seismic waves. These seismometers are now produced commercially, and 
special features of the magnetic tape recorder which enable stable low-speed 
recording have been copied by other seismologists as well as having been in- 
corporated into commercial recorders. 

Toward the end of the 1960s, Sacks, working with Dale Evertson from the 
University of Texas, developed a bore hole strainmeter, and it has now been 
demonstrated that this device reliably records earth strain at a sensitivity several 
orders of magnitude better than other strainmeters. Major use of this instrument 
is being made in the Japanese earthquake prediction program. 

In the field of isotope geochemistry, the DTM group was one of the first to 
make use of minicomputers for automatic control and data acquisition in the 
operation of mass spectrometers. This development increased the efficiency and 
reproducibility of mass spectrometric operation enormously, and permitted the 
reduction of quantities of data which would otherwise have been prohibitively 
large. More than anything else, this made it possible to achieve the precise isotope 
ratio measurements needed to carry out the investigations of the sources of 
oceanic basalts by Hart and his collaborators. 



report of the president 11 

Department of Embryology 

Few departments have changed as completely during the tenure of a director 
as did Embryology under James D. Ebert. When he came to the Institution in 
1956, the department was located in crowded quarters in the Johns Hopkins 
Medical School. From his predecessors Ebert inherited a world famous collection 
of human embryos, a monkey colony, and a program largely geared to primate 
embryology. Though the department enjoyed a high standing in its field, the 
challenges inherent in the study of morphology of embryos had diminished. 
Ebert gradually changed the thrust of the research and moved the department 
to a new building on the Homewood Campus of Johns Hopkins. Today there 
are practically no vestiges of the earlier program. While the study of development 
remains a general goal, the approach to it is largely in terms of molecular biology. 
A typical major effort is one devoted to study of the structure, arrangement, and 
evolution of genes. 

In his last report as director of the department, Ebert has described some of 
the changes that occurred during his tenure. A new director, Donald D. Brown, 
took over on October 1, 1976. It is too early to guess the direction in which the 
department will move. But evolve it will, for the new director has drive and 
enthusiasm, and his tastes are somewhat different from those of his predecessor. 
Moreover, the exterior environment is changing, and with it new opportunities 
will emerge. 



The Year in Review 



GEOPHYSICAL LABORATORY 

Considerable information is available from geophysical, geochemical, peno- 
logical, and geological observations at or near the surface of the earth. But one 
of the most challenging and tantalizing mysteries is the nature of the deep 
interior of the earth. 

The Geophysical Laboratory has been particularly active in furnishing the 
foundations for intelligent speculation by determining the behavior of minerals 
at temperatures and pressures prevalent in the interior of the earth. During recent 
years the Laboratory has been a leader in two such major programs. Boyd has 
led what now amounts to a sizable international effort to exploit the potential 
of nodule suites propelled upward in kimberlite pipes. These mineral assemblages, 
which may contain diamonds, originated at depths of about 200 km. In this 
year's report are presented the results of examination of nodules from South 
Africa, Siberia, and Colorado. These and earlier such studies were facilitated by 
the pioneering research at the Laboratory that produced geothermometers and 
geobarometers applicable to conditions at the site of origin of diamonds. 

Another major program is being conducted by Mao and Bell in their experi- 
ments with minerals under great pressures (about 1000 kilobars) in their new 
diamond cell apparatus. One of the first steps in developing a physical model of 
the earth's deep mantle and core is to determine the physical properties of 
minerals and mineral assemblages likely to be there. Among the principal candi- 
dates are close-packed oxides and metals including Si0 2 , MgO, iron oxides, and 
metallic iron. For work employing the new cell, MgO and eFe were chosen. 
Both phases had been studied earlier by shock wave techniques, and it was useful 
to correlate those results with findings from static pressure measurements. Com- 
pressibility of MgO and eFe was measured in the new study at 900 kilobars. It 
was found that no phase changes occurred above 130 kilobars. There was agree- 
ment between the shock wave and static results. The data serve to set constraints 
on potential mineral properties to an equivalent depth in the earth of 2000 to 
2500 km. 

Exploration for ore minerals in 1900 was a simple matter. Prospectors roamed 
the hills and inspected outcrops of rocks. Later, economic geologists established 
empirical procedures based on observations that better guided deeper exploration. 

13 



14 CARNEGIE INSTITUTION 

They observed, for example, that many ores are found in relation to geologic 
structures. They also noted that such ores seemed to have been emplaced as a 
result of motion of fluids through permeable regions associated with breakages 
in the rock. A record of processes that have occurred is left in the rocks, because 
the fluids react chemically during passage. The nature of the reaction is, of course, 
governed by the chemical content of the fluids and the physical conditions at the 
time of the reaction. Economic geologists have long used the products of such 
reactions as a guide for seeking ore, but there has been no solid theoretical base 
for systematizing field observations. 

This year Frantz and Mao made considerable progress toward such a theoreti- 
cal framework. They developed a general mathematical model to predict quanti- 
tatively the mineral sequences that occur by mechanisms known as intergranular 
diffusion and infiltration, and this year they solved the equations describing the 
thickness of zones and the relative directions of movement of the zone boundaries. 
Of particular importance, their theory aids in deducing the direction of transport 
of the various species in solution. The model accommodates variable permeabili- 
ties, diffusion constants, infiltration rates, and any number of components. 

The model has yet to be applied to natural ore deposits; but they did show 
its application to the system CaO-MgO-Si0 2 -H 2 0-C0 2 . The mineral zone se- 
quences were found to be entirely dependent on the mobilities of the various 
diffusing species. The lack of data on diffusion constants will, for the moment, 
retard application of the theory. Eventually, however, it will be necessary to 
observe only the minerals in the end zones and the thickness of the zones. By 
combining these data with the diffusion constants determined in the laboratory, 
it will be possible to predict the mineral content and thickness of the intermediate 
zones. 

The behavior of minerals containing the abundant oxides (Si0 2 , A1 2 3 , MgO, 
Na 2 0, CaO, and FeO) is extremely important to the understanding of earth 
processes. These common minerals have valuable but limited usefulness as indi- 
cators of earlier events in the evolution of igneous rocks. A basaltic lava can 
arise from a broad range of original ingredients. In contrast, trace elements can, 
in principle, provide much sharper diagnostic tools and they are also of interest 
because of potential economic value. The behavior of a given trace element is 
partly governed by the major components of the rocks or magmas with which 
it is associated. If one or more of the elements present have ionic radius and 
valence similar to that of the trace element, the trace element can readily enter 
a crystal structure as proxy for the abundant element. When the radii and charge 
differ, substitution becomes more difficult and complex. A particularly interesting 
set of trace elements are the rare earths. For the most part these have the same 
valence (3) but their ionic radii differ systematically. For more than a decade 
it has been known that the rare earths are fractionated differently when magmas 
are formed or are partially crystallized. But few attempts have been made at 
calibrating trace element partitioning within the range of natural abundances. 
Mysen has now developed a general method for measuring their fractionation. 
In one application he studied the partitioning of samarium between olivine, 
orthopyroxene, and a hydrous silicate liquid, using the number of beta-tracks 
generated by samarium- 151 as a measure of the amount of samarium present 
in the various phases. By this means he was able to determine accurately the 
concentration of samarium at levels as low as parts-per-billion. 

Mysen found that only approximately 1 ppm Sm can enter olivine and ortho- 
pyroxene before the activity coefficients of samarium become dependent on the 
samarium concentration. That is, the trace element concentration range of be- 



REPORT OF THE PRESIDENT 



15 



havior according to Henry's law is limited; the range is 25% larger in ortho- 
pyroxene than in olivine. This concentration range in both minerals increases 
about 15% per 100°C. He also found the crystal-liquid partition coefficients to 
increase with pressure ; yet increasing temperature results in increased partitioning 
of samarium into the olivine relative to liquid, whereas the partitioning decreased 
in orthopyroxene relative to liquid. These results are already leading to a more 
definitive understanding of what constitutes trace element behavior. The deter- 
mination of partition coefficients between crystals and between crystals and 
liquid under known conditions will provide a novel method, presumably inde- 
pendent of major components, for ascertaining the conditions of formation of 
igneous rocks. 



DEPARTMENT OF TERRESTRIAL MAGNETISM 

During the past decade we have learned a great deal about the origin and 
evolution of the earth. We are now reasonably certain that the solar system was 
formed 4.6 billion years ago. The oldest rocks that have been dated on earth are 
about 3.7 billion years old. From that time to the present, rocks of different ages 
were formed, attesting to a restless, changing earth. What of the gap between 
4.6 b.y. and 3.7 b.y.? If the earth were our only source of specimens, we could 
not hope to learn much about the hiatus. But space exploration has brought 
information from other bodies such as meteorites, Mercury, Mars, and especially 
the moon. As a result of a detailed examination of lunar samples, events on the 
moon from 4.6 b.y. to 3.7 b.y. ago are now fairly well known. The moon, though 
a comparatively small body, was at least partially molten very early. It is 
generally agreed that the necessary heat was derived from gravitational impacts. 
A large body such as the neighboring earth must have had a similar history, 
with early melting. The lunar samples also provide information about large-scale 
impacts that occurred about 4.0 b.y. ago. Thus there is now a reservoir of infor- 
mation about the solar system that merits study and interpretation. Wetherill 
has devoted considerable attention to this matter and has arrived at a model 
for the accretional history of the solar system that is somewhat different from 
the currently accepted view. 

It is commonly believed that a very rapid, 10 3 to 10 4 year time-scale is a rather 
firm constraint on the formation of the moon, and by inference, of the terrestrial 
planets as well. The principal observational basis for this is the evidence for very 
early chemical differentiation of the moon, coupled with thermal history calcula- 
tions that purport to show that this requires rapid accretion to prevent the 
limited gravitational energy of accretion from being wasted by radiation into 
space. If true, such a constraint would be very useful in limiting the range of 
permissible speculation. If false, it would be an obstacle to understanding what 
really happened. 

The problem is that all serious quantitative theories of planetary accretion 
lead to time-scales of approximately 10 8 years, a discrepancy of 4 or 5 orders of 
magnitude (the principal contributors in this area are V. S. Safronov and his 
colleagues at the Institute for the Physics of the Earth in Moscow). Safronov 
and Ruskol (his wife) have pointed out that the presence of large (about 100 
km) accreting bodies could overcome the thermal problem if the heat were buried 
below the large craters resulting from their impact. Unfortunately, a quantitative 
application of Safronov's theory leads to an initially cool moon. 

During the past year Wetherill has pursued this problem, using a stochastic 



16 CARNEGIE INSTITUTION 

j 

simulation of the solar system based on earlier work by Opik and Arnold. This 
leads to much larger bodies and higher velocities than those found by Safronov, 
and consequently much greater accretional energy — now primarily the kinetic 
energy of heliocentric motion of the bodies rather than the gravitational energy 
resulting from falling into the gravitational "potential well" of the moon. These 
differences arise from the "lumpiness" of the planetesimal distribution and 
particularly the presence of the biggest lump, the embryo of Venus accreting 
nearby. From this point of view, the initial temperature of the moon and planets 
would be high on any accretional time scale, and the range of permissible thermal 
and petrological models greatly increased. 

Since the beginning, much of the research of the Department of Terrestrial 
Magnetism has been strongly oriented toward global problems of the earth and 
its history. Today a major program at the department is the study of the earth's 
mantle, using the most advanced techniques of geochemistry and geophysics. 

In the first few years of the "plate tectonic revolution" it seemed likely that 
continents were a superficial feature of the earth, riding along on lithospheric 
plates similar to those in oceanic regions. This view has been changing, as a 
consequence of several developments: (1) Surface wave dispersion down to the 
wavelength-limited depth of 200 km showed the absence of a low velocity zone 
under South America. (2) Sacks and Okada have shown that beneath continents 
Q and seismicity are high down to 350 km but decrease below that depth. (3) . 
Isotopic and geochemical data on mantle-derived continental rocks show that 
heterogeneities are preserved on a time-scale long compared to that of rates of 
continental drift. 

Because of these developments it is now believed that continental lithospheres 
are fundamentally different from those below oceans, and some workers doubt 
that there is any asthenosphere at all beneath continents. 

This year Sacks and Snoke report direct evidence for a lithosphere-asthenosphere 
boundary under South America at a depth of 400 ± 30 km associated with a 
transition zone only a few kilometers thick. This evidence is in the form of a 
seismic arrival that comes in between the p and s arrivals, which is interpreted 
as resulting from an incident s phase converted to compressional waves at a 
boundary. Unlike other models, this one fits all aspects of the data, such as 
amplitude ratios of the different components. First-motion studies show the 
polarity of the boundary — low velocity below, high velocity above. Thus the 
bottom of a continent has been found, replacing the Moho, which has been known 
to be superficial since acceptance of sea floor spreading and continental drift a 
decade ago. 

In some astrophysical work, Ford, Peterson, and Rubin report very high 
resolution spectra of an e'liptical galaxy, spectra of such quality that at first 
glance one would think they were those of a star rather than a galaxy. Previously, 
high resolution spectra had been obtained for spirals in which the high abundance 
of excited gas results in prominent emission lines. In contrast, these emission lines 
are absent in the gas-poor elliptical galaxies; these new spectra are absorption 
spectra, resulting from the integrated absorption of the stars in the galaxy. From 
these spectra, velocity fields, a preliminary mass model, and a mass/light ratio 
as a function of radius have been calculated. This development opens up for 
observation the chemical and dynamical history of elliptical galaxies, a pre- 
requisite for progress in understanding the reasons galaxies occur in two such 
different forms. This development resulted from taking advantage of the high 
spatial and spectral resolution of the 4-meter Carnegie image tube spectrograph 
at Kitt Peak Observatory. The spectrograph is also being used to study absorp- 



REPORT OF THE PRESIDENT 17 

tion spectra of other galactic features, such as the stars in the central bar of 
barred spiral galaxies. 



HALE OBSERVATORIES 

Earlier in this report I discussed the approaching completion of the Las 
Campanas Irenee du Pont telescope and described some of its novel features. 
Obviously, design and construction of such an instrument places heavy burdens 
on staff, on some more than others. But the new construction in Chile was only 
one of the tasks to be addressed. The other Hale telescopes must be continually 
mantained and upgraded. Electronic control systems have evolved. In addition, 
the use of electronics at the observing end of the telescope continues to increase, 
and Hale staff are necessarily involved in the design, development, testing, and 
use of such equipment. Added to all this are time-consuming tasks such as allo- 
cation of observing time and dealing with 60 or more guest investigators. 

In spite of all these burdens, which are borne by a relatively small staff, the 
Hale Observatories are remarkably productive of advances in astronomy. 

The new electronic devices create opportunities, and these are being exploited. 
A major new frontier has been created by instruments that can count individual 
photons. Sky background can be subtracted so that very distant, faint objects 
that had been inaccessible can now be studied. 

There are now available digital or linear photoelectric devices capable of 
precise photometry on large numbers of picture elements within an optical image. 
Such devices are equally important in the focal plane of a telescope and at the 
output end of a spectrograph that is attached to the telescope. But the photo- 
graphic plate, especially with the most modern emulsions of high information 
content and optimum color sensitivity, continues to be improved and is in- 
dispensable, particularly for wide-angle recording. 

At the Hale Observatories, Sandage and Kristian, using the photographic 
method, and Gunn and Oke, using an image tube, are reporting the discovery of 
impressive numbers of new faint clusters of galaxies. Such clusters are important 
because the brightest galaxy in a cluster has a standard luminosity very nearly 
the same for any cluster. From this information the relative distances to such 
clusters can be derived, and if the redshift of several galaxies in each cluster can 
be measured with the spectrograph, new points in the Hubble diagram of redshift 
versus distance can be plotted. 

Not only are dozens of distant clusters of galaxies being found, but the pro- 
duction of spectra and the measurement of redshifts of galaxies in them is 
proceeding at a record rate. For this work Sandage, Kristian, and Westphal are 
using an SIT (silicon intensifier target) Vidicon on a prism spectrograph at the 
5-meter telescope, while Gunn and Oke are using the SIT- Vidicon with a grating 
spectrograph. 

Most of the new redshifts being measured are greater than z = 0.2 and a 
substantial fraction have z > 0.4. Each of these galaxies is of small angular 
extent, and most are much fainter than the uniform light of the sky (airglow 
and zodiacal light) that overlies them. Spectroscopy involves subtracting elec- 
tronically the spectrum of the sky from that of the object plus sky. The principal 
limitation now is not measuring the spectra but rinding faint candidates and 
positioning them on the slit. Observing depends upon low-level closed-circuit TV 
systems for these operations, which formerly involved the much more difficult 
technique of making blind offsets from nearby stars. 



18 CARNEGIE INSTITUTION 

Sandage has made an important discovery of substantial quantities of dust in 
regions lying above the galactic plane. Though the finding itself was serendipitous, 
it illustrates the benefits of intensive, long-term observational programs of the 
kind the Hale Observatories can sponsor. During the last five years while looking 
for remote clusters of galaxies, Sandage surveyed extensively the sky in the 
region of the north and south polar gaps. During that survey he noted occasional 
high latitude fields that have bright nebulous regions often in patterns correlated 
over distances of degrees. He was able to show that these structures are reflection 
nebulae whose illumination comes from our galactic plane. 

Schmidt has analyzed spectroscopic and photometric observations of a com- 
plete sample of quasi-stellar sources in the 6-cm National Radio Astronomy 
Observatory survey. The value of V/V max , which measures the position of an 
individual source within the available volume, appears to be correlated with the 
radio spectral index. Sources with steep radio spectra have a mean V/V max 
around 0.7, similar to values found previously for quasars in the 3C catalogue of 
radio sources and the revised 4C catalogue, most of which have steep radio 
spectra. This implies that the space density of these quasars rises steeply with 
redshift (declines with cosmic time) by a factor of around 1000 to a redshift of 
2.5. In contrast, quasars with flat or inverted radio spectra have a lower mean 
value of V/Vmax around 0.52 ± 0.05 (m.e.), consistent with a uniform distri- 
bution in space. 

Searle is making a comparative study of the chemical history of the globular 
cluster systems of the Galaxy, of M31, and the Magellanic Clouds. A more 
sensitive method of determining the metal abundance in individual red giants 
belonging to a cluster has been developed. In this report year Searle and Zinn 
calibrated the new system and began a program to determine the metal abundance 
of all globular clusters for which color-magnitude arrays are available. They 
are especially interested in remote clusters and other stellar systems on the fringes 
of the Galaxy. 

In a related investigation Searle has obtained low resolution spectrophoto- 
metric scans of 70 of the globular clusters in M31. So far all the observations 
appear to be consistent with the idea that the M31 globular clusters differ from 
one another in one parameter only — presumably the metal abundance. However, 
there are differences between the globular clusters of M31 and the Galaxy, and 
those of the Magellanic Clouds. There are also important differences between the 
clusters of the Small and Large Clouds. 

DEPARTMENT OF PLANT BIOLOGY 

In the past ten years very powerful techniques have been developed to study 
the structure and action of DNA. Substantial contributions to this important 
effort have been made by Carnegie staff. The work of Britten and others at DTM 
and of Brown and others at Embryology has been outstanding. Their work has 
largely been devoted to animal DNA. 

These techniques are now being applied to plants by Thompson and colleagues 
at Plant Biology. Thompson reports this year on an analysis of the sequence 
organization of pea DNA. The pea genome has a pattern much like that of the 
toad Xenopus, with single-copy DNA sequences less than 3000 nucleotides long 
interspersed with repetitive sequences. It is already clear that the study of plant 
DNA will be a powerful tool in the elucidation of evolutionary relationships in 
plants. A typical question is the origin of C 4 photosynthesis in Atriplex (saltbush) 
and the DNA sequence comparison between C 3 and C 4 species of that genus. 



REPORT OF THE PRESIDENT 19 

Using techniques available for examining DNA, Rogler, a fellow, and Thomp- 
son have made a useful contribution to knowledge of crown gall tumors and to 
the controversial topic of recombinant DNA. 

Wounded plant tissue infected with Agrobacterium tumefaciens will, under 
appropriate conditions, form tumors that have several physiological properties 
in common with mammalian cancer. These properties include non-self-limiting 
growth on synthetic medium devoid of hormones and the inability to differentiate 
into organized, functional tissue in response to treatments causing such differenti- 
ation in normal cultured cell lines. Crown gall tumor cells retain these properties 
when cultured in the absence of the inciting organism and, when grafted into 
healthy plants, produce tumorous overgrowths. 

Recently it was discovered that all pathogenic strains of Agrobacterium 
tumefaciens contain one or more large plasmids 1 and that loss of the plasmid 
results in loss of virulence. 2 On the other hand, transformation with plasmid 
DNA results in acquisition of virulence by the strain acquiring the plasmid. 
These two lines of evidence strongly support the hypothesis that the plasmid is a 
necessary genetic element for virulence in Agrobacterium. 

Rogler and Thompson have described changes in plasmid DNA sequences in 
Agrobacterium during either the loss or the acquisition of virulence. In their 
report they discuss an instance in which a large deletion in the plasmid genome 
is associated with loss of virulence. They also present evidence that acquisition 
of virulence may involve replacement of a preexisting avirulent plasmid with a 
virulent plasmid derived from another strain of Agrobacterium. 

Rogler and Thompson do not attempt to relate their findings to the current 
controversy over DNA work — perhaps wisely so. One can choose to view the 
results with alarm or with equanimity. Today much recombinant DNA work 
uses plasmids of Escherichia coli. The fact that a change in a plasmid of another 
bacterium gives rise to crown gall tumors argues for caution when producing 
new kinds of plasmids in Escherichia coli. A contrary view is that the patho- 
genicity of Agrobacterium tumefaciens arose naturally and doubtless is the 
result of innumerable experiments performed by nature. It may well be true, as 
some argue, that among the countless experiments on DNA performed by nature 
everything and every possible combination has already been tested. 

The physiological ecology group continues to make good progress in under- 
standing the factors that enable plants to cope with stress and thus to exist under 
extreme conditions of temperature, moisture, light intensity, and other variables. 
The general approach is to combine field observations using mobile equipment 
with laboratory studies. When plants having unusual characteristics are noted 
in nature, their photosynthetic machinery can be studied in the laboratory. 

In the current report, Mooney, Bjorkman, Ehleringer, and Berry describe 
some of their efforts in studying the native flora of Death Valley, California. 
Although Death Valley is often identified as one of the hottest and driest en- 
vironments on earth, these extremely high air temperatures (exceeding 50°) occur 
only in the summer. In winter, relatively cool temperatures (20°C) predominate, 
and in spring and fall air temperatures are often around 30° C. Consequently, 



1 Zaenen, I., N. van Larebeke, H. Teuchy, M. van Montagu, and J. Schell, J. Mol. Biol, 85, 
109-127, 1974. 

2 van Larebeke, N., O. Engler, M. Holsters, S. van den Elsaker, I. Zaenen, R. A. Schilperoort, 
and J. Schell, Nature, 225, 169-170, 1974. 

Watson, B., T. C. Currier, M. P. Gordon, M. D. Chilton, and E. W. Nester, J. Bacteriol., 
123, 255-264, 1975. 



20 CARNEGIE INSTITUTION 

Death Valley provides a broad variety of thermal environments and a unique 
opportunity to study the ways plants adapt to these seasonal thermal regimes. 

Utilizing the mobile laboratory, the photosynthetic capacity of plants was 
measured during the winter (January), spring (March), summer (late May), 
and fall (October) seasons. Plants studied included a winter annual (Camissonia 
claviformis) , a summer active herbaceous perennial (Tidestromia oblongifolia) , 
and two evergreen shrubs (Larrea divaricata and Atriplex hymenelytra) . 

Under natural conditions the two evergreen shrub species were photosyntheti- 
cally active throughout the year, although their maximum photosynthetic rates 
were considerably below those of the two that hold their leaves for a short period. 
The two herbaceous species, Tidestromia (C 4 ) and Camissonia (C 3 ), have ex- 
tremely high maximum photosynthetic rates; yet their physiological activities 
are restricted for the most part to a single season. 

The Cs winter-active annual Camissonia germinates following heavy winter 
rains and can, under certain conditions, complete its entire life cycle in six 
weeks. This short-lived species has a remarkable capacity to capture sunlight 
and fix carbon. Camissonia converts incident photosynthetically active radiation 
(400-700 nra) to chemical energy with an efficiency of 8.5%. This is possible 
because of the lack of light saturation at midday irradiances, a feature that is 
also characteristic of Tidestromia. The in situ midday photosynthetic rate of 
Camissonia was higher than the rate measured for such productive crop species 
as corn, sorghum, and sugar cane. 

Earlier studies had identified Tidestromia oblongifolia as a plant with a high 
rate of photosynthesis at elevated temperatures. During the year Bjorkman, 
Boynton, and Berry participated in studies to determine the thermal stability of 
key components of the photosynthetic machinery responsible for the marked 
differences in temperature tolerances between thermophilic higher plant species 
such as T. oblongifolia and cool-temperate plants such as Atriplex sabulosa. As 
reported last year, the two C 4 species differ in their high temperature stability 
in several respects, including the maintenance of semipermeability of the cell 
membranes, respiratory activity, and photosynthetic activity. While onset of high- 
temperature damage differed by about 10°C in the two plants, photosynthesis 
was affected at considerably lower temperatures than either respiration or mem- 
brane semipermeability. 

This year they studied the effect of temperature on the quantum yield for 
photosynthesis by intact leaves with its effect on photosystem I and II activities; 
ribulose 1,5-diphosphate carboxylase activity (the C 3 enzyme for C0 2 fixation) ; 
phosphoenolpyruvate carboxylase activity (the C 4 enzyme for C0 2 fixation) ; and 
heat coagulability of soluble proteins as a measure of irreversible denaturation. 
It is clear that the best fit with thermal inhibition of photosynthesis in both 
cases is with system II activity. In addition, however, there were interesting 
differences in heat stability of the C 3 enzyme from the two plants, suggesting that 
not just lipids but proteins may differ between plants adapted to high tempera- 
tures and those adapted to low temperatures. 



DEPARTMENT OF EMBRYOLOGY 

James D. Ebert, the retiring director, leaves this department after having 
brought it successfully through a major era of evolution from classical embryology 
to a new form of developmental biology heavily based on molecular biology. 

The two major classes of studies at this department are (1) broad-ranging 



REPORT OF THE PRESIDENT 21 

fundamental research on DNA — its structure in chromosomes and its behavior 
as genetic material and (2) researches aimed at better understanding of the 
structure, behavior, and interactions of membranes. The investigations of DNA 
are progressing very well and the department has compiled an impressive record 
of achievements in this field. The studies of membranes are a newer, evolving 
activity in the department which have already been fruitful and which have 
great potential. The staff investigators (Fambrough, Muller, and Pagano) are 
young and have been brought together fairly recently. Of the three, Fambrough 
has been at the department longest and has accumulated an impressive record 
of accomplishment in studies of nerve-muscle interactions with emphasis on the 
acetylcholine (ACh) receptors in skeletal muscle fiber membranes. In a series 
of articles Fambrough and his students and colleagues report on the appearance 
of ACh receptors during muscle development, the production and incorporation 
of ACh receptors into the surface membranes of myotubes and their subsequent 
degradation, the clustering of ACh receptors at newly formed neuromuscular 
junctions both in vitro and in vivo, the number and distribution of ACh re- 
ceptors in adult rat skeletal muscle fibers, and the time course of change in these 
parameters following denervation. They are continuing to explore physiological 
aspects of receptor function, examining further the biosynthesis and degradation 
of ACh receptors and obtaining more accurate estimates of ACh receptor density 
on denervated muscle fiber membranes. 

In this year's report, Fambrough and his student Peter Devreotes present a 
model for the major events in receptor metabolism. The protein subunits that 
are to form the ACh receptor are synthesized inside the muscle cells and are 
rapidly assembled into receptor units in the cytoplasm of the muscle cells where 
they constitute a "precursor pool." The average residence of a new receptor in 
the precursor pool is 2 to 3 hours. Receptors are transferred from this pool to 
the surface by an energy-requiring process and are then able to function. The 
average half-life of an ACh receptor in the plasma membrane is 22 hours. Re- 
ceptors are degraded by an energy-requiring process that involves the internali- 
zation of the receptor and transport to secondary lysosomes and then proteolytic 
destruction. 

In his continuing professional development, Pagano illustrates one of the 
principles on which the Institution is based: Andrew Carnegie's advice that the 
Institution seek out talented individuals and support them. 

Eber-t followed this course in choosing and fostering Pagano. Four years ago, 
when this young scholar came to the department, his background was that of a 
physical chemist who had worked with some nonliving model membrane systems. 
He has since acquired substantial experience in cellular biology, and he and his 
colleagues have been using artificially generated lipid vesicles to perturb mem- 
brane components in living cells. In the report of the director they present findings 
on the interactions of phosphatidyl choline vesicles with murine lymphocytes, 
and the physiological consequences of these interactions. Pagano and postdoctoral 
fellows Huang and Ozato have shown that the pathways of vesicle-cell inter- 
actions enumerated in previous reports (vesicle-cell fusion, adsorption, and lipid 
exchange) are exquisitely sensitive to the molecular composition and physical 
properties of the artificial vesicles. In the lymphocyte system, the resulting 
membrane perturbations lead to altered surface properties and greater prolifera- 
tive activity of these cells when stimulated by various mitogens and antigens. 

Muller, the most recently added staff member, is tackling the major question 
of how single nerve cells are organized into a nervous system. He is especially 
looking at the pathways neurons take to reach their targets. For this purpose he 



22 CARNEGIE INSTITUTION 

has been studying activities in the nervous system of the leech. This creature 
has a very simple nervous system, yet its neurons behave in many ways like 
the neurons of higher animals. 

Neurons in this segmented worm are grouped 350 together to form essentially 
identical ganglia that are linked in a chain by axon bundles; each ganglion 
mediates the reflexes and controls the behavior of its segment. It has been 
possible to monitor electrical activity from within single sensory and motor 
neurons with microelectrodes and to correlate the physiological activity of func- 
tionally identified neurons with their characteristic shape and position within 
each ganglion. By recording from pairs of neurons, one can trace synaptic con- 
nections, which have been found to be surprisingly uniform and in that sense 
predictable between specific neurons. 

In his current report, Muller describes the structural details of chemical 
synapses, delineates their arrangement on sensory neurons, and explores the 
sprouting of sensory neuron terminals. 

Staff members active in DNA research include Brown, Dawid, Reeder, and 
Suzuki. In a series of investigations inspired by Brown and Dawid, this group 
together with fellows and guest investigators have made a significant impact in 
developmental, molecular, and evolutionary biology. Studies of "developmental 
genetics by gene isolation" have led to new and brilliantly successful approaches 
to fundamental questions in development. The department has originated and 
exploited techniques for gene purification. Studies of "gene anatomy" — the struc- 
ture and arrangement of genes — have resulted in new insights into the mecha- 
nism by which multiple tandem genes evolve together. 

Brown has a long-term goal of reassembling the macromolecules that enable 
DNA to be transcribed with fidelity in the living cell. In this way he hopes to 
elucidate the control mechanisms of animal genes. For this purpose he and his 
colleagues are now concentrating on the family of genes from Xenopus which 
code for the small RNA molecule of ribosomes termed 5S RNA. 

This year Brown's group reports progress along several lines. The "transcrip- 
tion unit," that is, the region of the Xenopus DNA that is transcribed into 5S 
RNA in the living cell, is relatively simple — only 120 base pairs long. Moreover, 
the form of RNA polymerase that transcribes this gene in vivo has been identi- 
fied. The fact that the mature RNA transcript does not appear to be derived from 
a higher molecular weight precursor is especially important: It means that the 
true initiation site of transcription from 5S DNA is known. Therefore, one can 
detect faithful transcription when it occurs in vitro. 

Three different kinds of 5S DNA have been cloned in E. coli by means of the 
new recombinant DNA methology, thereby providing an order of magnitude 
more material to work with and permitting the careful examination of a single 
repeating unit of a tandem gene family. The "restriction maps" of these cloned 
DNAs — the sites where they are cleaved by certain restriction enzymes — are 
described by fellows Jeffrey Doering and Scott Emmons. Emmons is attempting 
to develop an assay for proper transcription. Fellow Nina Federoff is exploring 
DNA sequencing methods to determine the exact nucleotide sequence in the 
spacer regions of 5S DNA, particularly those regions adjacent to the initiation 
site of the gene. 

DNA structure continues to hold the attention of several staff members and 
fellows. Reeder and Dawid, along with fellow Peter Botchan, report on the 
structure of nontranscribed spacer regions in Xenopus ribosomal DNA, based on 
studies using cloned fragments of rDNA generated with the restriction endo- 
nuclease EcoRI. Fellow Peter Wellauer and Dawid, a staff member, have under- 



REPORT OF THE PRESIDENT 



taken the first stages of a study of the ribosomal DNA of Drosophila; Dawid 
and his colleagues are cloning Xenopus mitochondrial DNA; and Carnegie fellow 
Ohshima and staff member Suzuki are attempting to clone the silk fibroin gene. 
In all these studies, the department has adhered to the safety standards pro- 
posed by the Asilomar Conference on Recombinant DNA. 



EDUCATION 

To an unprecedented extent this country has become a nation of the here-and- 
now with a great part of its efforts and resources devoted to instant gratification. 
One of the few but important investments we make for the future is in education. 
For most people, formal education stops at graduation from college or before; 
comparatively few of these go on for advanced degrees in the sciences to form 
the cadre from which our future leaders in science and technology will emerge. 
But of the cadre only a fraction achieve creativity. After twelve years of primary 
and secondary schools and four years of university, most young people have 
mastered the memorizing of facts, have developed some judgment and some 
synthetic abilities, and have arrived at a personal identity. But the effort needed 
to become a creative scientist and to establish a professional identity is far more 
demanding than the struggles of adolescence. Among other things, what is in- 
volved is a metamorphism from a fact-accumulating to an idea-producing 
organism — a change from depending on the direction of others to confidently 
relying on one's own judgments and initiatives. The last two predoctoral years 
and the first three postdoctoral years are the critical period. During that time 
only those few who achieve the metamorphism are launched on a productive 
career. There is evidence to show that a crucial factor in the outcome is the 
intellectual environment in which the young scholar is immersed. At the Carnegie 
Institution we have diligently sought, and I believe we have succeeded in pro- 
viding, optimum circumstances for the metamorphism of our fellows. In turn we 
feel that our fellowship program is one of the best kinds of investment that the 
Institution and the nation makes for the future. 

Each year the departments of the Institution makes contributions to knowledge 
that win for them continuing recognition as leaders in their fields. At the same 
time staff members enjoy a high reputation for innovation. This aura of success 
attracts young scholars, many of whom seek to associate with experts in their 
chosen fields and to savor the excitement and intellectual growth that a creative 
group fosters. Since the departments can choose from among many excellent 
candidates, the quality of our predoctoral and postdoctoral fellows is excellent. 
They merit careful tuition and fostering, and they get them. 

An important factor is the Institution's policies with respect to staff and 
fellows. Staff members are expected to participate actively and personally in 
research and teaching. They do not sit at command posts in offices while others 
work for them. They devote almost all their time to scholarly activities with 
few interruptions for administrative activities. Thus they are highly available 
for discussions. Since the ratio of staff to fellows is about one to one, the young 
scholars can have all the consultation and instruction they want. The situation 
is a modern version of the ideal enunciated nearly a century ago of professor and 
student sitting on a log discoursing together. 

Optimal intellectual growth of fellows is promoted by another policy of the 
Institution. They are encouraged to pursue projects of their own choosing, and 



24- CARNEGIE INSTITUTION 

they are given the kind of back-up in equipment and supplies that is enjoyed by 
staff members. 

The precise patterns of training differ from department to department, reflect- 
ing the needs and opportunities of the respective fields of inquiry. However, some 
general patterns prevail throughout the Institution, and these are outlined and 
illustrated in an essay on education in the Department of Embryology that was 
prepared for this report by the retiring director, James D. Ebert. His comments 
follow: 

For the past two decades training has been an integral part of the 
department's program. It was clear in 1956, as the department prepared 
to embark in new directions in molecular developmental biology and 
genetics, that even if ample funds and space were available immedi- 
ately to undertake all the tasks that had been set, the scientific talent 
was not. The world's pool of able developmental biologists then was 
small. President Haskins and the director agreed that the catalysis 
provided by young minds — graduate students and postdoctoral fellows — 
would be a vital ingredient in the shaping of these new fields. 

The wisdom of that decision has been demonstrated repeatedly. A 
glance at today's staff tells a part — but only a small part — of the story. 
Director-designate Donald Brown and staff members Igor Dawid, 
Douglas Fambrough, Ronald Reeder, and Yoshiaki Suzuki all joined the 
department initially for training. Thus, students become teachers and 
they, in turn, set standards of excellence for new generations of students 
to follow. 

Douglas Caston, Professor and co-Director of the Developmental 
Biology Center at Case Western Reserve University, emphasized this 
point recently when he wrote "The experience I gained as a fellow [at 
Carnegie] has been the major force in shaping not only my personal 
career but in my helping junior colleagues shape their careers." 

The department's purpose in offering its training program is to 
encourage the development of research scientists. First consideration 
is given to students who appear likely, by their intellectual gifts and 
intentions, to contribute to the advancement of the field whether in 
biology or in medicine. The previous training of applicants, whether in 
embryology, molecular biology or genetics, pediatrics or physics, may be 
of secondary importance, preference being given to the most promising 
students. Thus the department's program has had a profound impact 
not only in molecular developmental biology but in the clinical and 
mission-oriented fields that draw from the same spring. Caston is a 
molecular development biologist, yet his experience is matched by that 
of Glenn Rosenquist, one of the nation's leading students of cardiology 
and birth defects, now Professor of Pediatrics at the University of 
Nebraska Medical Center, who wrote "The two years I spent at Car- 
negie Institution were probably the most important years of my entire 
career." 

Although over the years opportunities have been provided for the 
training of undergraduate students of exceptional promise, most of the 
department's teaching has been focused on graduate students and 
postdoctoral fellows. It is at the graduate and postgraduate levels that 
the department's unique resources — both human and scientific — can 
have their greatest impact. Several generations of fellows have written 
about the department's unique intellectual climate. The department 
provides well-organized and well-equipped laboratories where, in Dorothea 
Rudnick's words, "Everything peripheral [is] taken care of." In such 
an environment the most promising young people can be afforded the 



REPORT OF THE PRESIDENT 

opportunity to exercise and extend their abilities, enabling them to take 
their critical steps toward independence. 

A listing of the department's graduate students and postdoctoral 
fellows constitutes a veritable "Who's Who" of developmental biology. 
Even a cursory glance reveals Carnegie fellows teaching and carrying 
forward their research at Harvard, Princeton, Brown, and Rockefeller 
Universities; at Virginia, Emory, Case Western Reserve, and Washing- 
ton University, St. Louis; at the Universities of California (Berkeley), 
Kyoto, Tokyo, and London. They are working actively in departments 
of biology, in schools of medicine, and in an enormous variety of both 
basic and clinical research institutions. 

Thus far we have emphasized the "products" of training, that is, the 
students themselves. Yet there is another factor to be considered: The 
greatness of the department's contribution to science can itself be 
attributed in part to its insistence that teaching and research are 
inextricably interwoven. We spoke earlier of the catalysis provided by 
young minds. The truth of that observation is reiterated year after 
year in the department's Annual Reports. The interaction between staff 
member and student is truly reciprocal, with students being an intimate 
part of some of the department's greatest achievements. The classic 
demonstration of the role of the nucleolus in the synthesis of ribosomal 
RNA was provided by a staff member, Brown, working with a fellow, 
Gurdon. Similarly, two fellows, Yoshiaki Suzuki and Patrick Gage, 
worked closely with Brown in the isolation and characterization of the 
gene for silk fibroin. Fambrough's far-reaching studies of the acetyl- 
choline receptor sites in muscle have been advanced through the con- 
tributions of graduate students Hartzell and Devreotes. Fellow Peter 
Wellauer, working with Brown, Dawid, and Reeder, has led the way 
in elucidating gene structure. 

The crucial nature of the intellectual climate needed for the training 
of young investigators is emphasized in our earlier expression "steps 
toward independence." Each student or fellow must be considered an 
individual. It is difficult, some may say impossible, to teach another 
individual how to do research except by example. There must be a 
special interplay between student and preceptor. The student or fellow 
must be afforded an ample opportunity to, in the words of former 
fellow Malcolm Steinberg, now Professor of Biology at Princeton Uni- 
versity, "follow his own nose." Steinberg went on to say "The trail I 
picked up as a fellow in the Department of Embryology has broadened 
' into a significant field of investigation now pursued by many scientists 
the world over." Turning again to the clinical side, Gilbert Greenwald, 
Professor of Obstetrics and Gynecology at the University of Kansas 
Medical Center, wrote "The freedom that I had to pursue a number 
of my own ideas gave me the confidence to think that I could embark 
on my own career as an independent and . . . imaginative investigator. 
I believe the last point is the most important objective to be gained from 
any postdoctoral experience. 



LOSSES . . . 

I am sorry to report that William I. Myers died in January. For 27 years the 
board of trustees benefited from the insight and knowledge he acquired during 
a career of distinguished service in government and education. 

As one of President Roosevelt's top agricultural aides from 1933 to 1938, 
Dr. Myers established and headed the Farm Credit Administration. He also 



26 CAENEGIE INSTITUTION 

served as president of the Federal Farm Mortgage Corporation and the American 
Farm Economics Association. In addition, he was director of the Federal Surplus 
Relief Corporation and the Commodity Credit Corporation. 

In 1938 Dr. Myers accepted the chairmanship of the Department of Agri- 
cultural Economics at Cornell University. He became dean of the department 
in 1943 and held that post until his retirement in 1959. He was director of the 
Eisenhower Exchange Fellowships from 1953 to 1969 and was a trustee of the 
Agricultural Development Council from 1954 to 1971. He was also a trustee of 
the Rockefeller Foundation (1941-1957) and later served on its Board of Con- 
sultants for Agricultural Sciences until 1965. 

At home in New York State, he was part of the Science and Technology 
Foundation and a member of the Council of Economic Advisers and a director 
of the Association for Crippled Children and Adults of New York. Dr. Myers 
was a director or trustee of six companies as well as Marine Midland Banks, Inc., 
and the New York State Gas and Electric Corporation. 

Another loss to the Institution is Berwind P. Kaufmann, the last director of 
the Department of Genetics (1960-1962), who died in September 1975. After 
distinguishing himself as a professor at Southwestern College and at the Uni- 
versity of Alabama, he joined Carnegie Institution in 1936 and remained with 
us until our research in genetics was phased out in 1962. 

At the department, Dr. Kaufmann's cytological and cytogenetic research made 
important contributions to the field, among them the pioneering discovery that 
chromosomes contain double spiral bands and the first demonstration of inter- 
change between X and Y chromosomes. His studies indicated the association of 
RNA with a histone-type protein and of DNA with a tryptophane-rich protein 
many years before confirmatory biochemical data were obtained. He also studied 
the effects of radiation, especially on chromosomes, and was a member of the 
National Academy of Sciences' Committee on Genetic Effects of Atomic Radiation. 

After retiring from the Institution, Dr. Kaufmann continued his genetics 
research at the University of Michigan, Ann Arbor, as professor of zoology and 
botany and senior research scientist. He was made professor emeritus when he 
retired from the University in 1967. 

I must also report with sorrow the death in January of Rudolph Minkowski, a 
staff member of the Hale Observatories from 1937 to 1960. A native of Germany, 
he fled to this country in 1935 and became an American citizen in 1940. 

Throughout his career he collaborated with another German-born Observa- 
tories' astronomer, Walter Baade. Together they studied supernovae, which they 
divided into two types on the basis of long sequences of supernovae spectra first 
obtained by Minkowski. With Baade, he became interested in radio astronomy, 
and in the 1950s was the first to identify and interpret correctly a radio source, 
the remnant of a long-dead supernova in the constellation of Cassiopeia. For 
years Dr. Minkowski supervised the National Geographic Society-Palomar 
Observatory Sky Survey. 

Dr. Minkowski received the Bruce Medal of the Astronomical Society of the 
Pacific in 1961 and an honorary doctorate from the University of California, 
Berkeley, in 1968. After his retirement from Hale, he worked at the University 
of California's radio astronomy laboratory. 

The decision this year by an exceptionally gifted scientist and administrator 
to leave the Institution is, inevitably, cause for regret. Yet our sense of loss is 



REPORT OF THE PRESIDENT dl 

assuaged by our pride in the accomplishments of James D. Ebert during the 
twenty years he was director of the Department of Embryology. And we look 
forward to Dr. Ebert's contributions to developmental biology at the Marine 
Biological Laboratory, Woods Hole, Massachusetts, where he is the first full-time 
president and director. 

Since his appointment as director in 1955 by the late Vannevar Bush, Dr. 
Ebert has guided the Embryology laboratory through a metamorphosis that 
took it from a place in the forefront of classical embryology to a leading role 
in molecular developmental genetics. 

Among the outstanding achievements of the Department during Dr. Ebert's 
administration are his own work on the cellular and molecular basis of heart 
development in embryos, and his studies on the graft-versus-host reaction, the 
relation of viruses to tumor formation, and the regulation of DNA synthesis. 
Moreover, his leadership and encouragement created a stimulating and supportive 
environment in which the research of his colleagues and students also flourished. 

In addition to its standing as a foremost research center, the Department of 
Embryology has become, under Dr. Ebert's guidance, internationally known for 
its advanced training program in developmental biology. 

Dr. Ebert's contributions to the scientific community go beyond his research 
and his service to his "home" institution. He is professor of biology and pro- 
fessor of embryology at The Johns Hopkins University, and he has lectured as 
a visiting professor at other institutions in the United States and abroad. He is 
the first chairman of the Assembly of Life Sciences of the National Academy of 
Sciences; past president of the American Institute of Biological Sciences, the 
American Society of Zoologists, and the Society for the Study of Development 
and Growth. He is a trustee of the Board of Scientific Overseers of the Jackson 
Laboratory, a member of the Corporation of Woods Hole Oceanographic Institu- 
tion, and he is on the board of visitors of ten universities. He has served as a 
consultant to the U.S. Department of Health, Education and Welfare. And he 
has been the editor or a member of the editorial board of half a dozen biological 
journals. 



. . . AND GAINS 

It is our privilege to welcome to the Institution this year two men of out- 
standing achievement in their fields. Robert 0. Anderson and J. Paul Austin were 
elected trustees at our annual meeting of the board on April 30. 

Robert 0. Anderson, chairman of the board and chief executive officer of 
Atlantic Richfield Company, has held those positions since 1965. His experience 
in the business world is extensive and varied. He joined the American Mineral 
Spirits Co. in his native Chicago soon after his graduation from the University 
of Chicago in 1939. Within two years he had set up his own corporation, Malco 
Refineries, Inc., which later became Hondo Oil and Gas Co. and merged with 
Atlantic Richfield in 1963. At that time Mr. Anderson became one of the directors 
of Atlantic Richfield. 

Although the focus of Mr. Anderson's business career has been in the petroleum 
industry, he has been involved in such diverse enterprises as mining, milling, and 
general manufacturing. In addition, he operates his own livestock company in 
Lincoln County, New Mexico. His experience is further broadened by being a 



28 CARNEGIE INSTITUTION 

trustee of three universities, and a director of the First Chicago Corporation and 
the First National Bank of Chicago. His participation in cultural, civic, and 
charitable organizations includes chairmanships with the Aspen Institute for 
Humanistic Studies and The Lovelace Foundation. 

J. Paul Austin, chairman of the board and chief executive officer of the Coca- 
Cola Company, has enjoyed a distinguished career with that firm since 1949. 
Joining the legal department of the Company in that year, Mr. Austin trans- 
ferred to the Coca-Cola Export Corporation the following year and became 
the Corporation's president in 1959. In 1962 he was elected the tenth president 
of the Coca-Cola Company, in 1966, its chief executive officer, and in 1970, 
chairman of the board. 

Mr. Austin also holds directorships with other well-known companies, among 
them Morgan Guaranty Trust Co. of New York; Dow Jones and Co., Inc.; and 
the General Electric Co. He is a trustee of numerous organizations, including the 
United States Council of International Chambers of Commerce, the National 
Industrial Conference Board, the Rand Corporation, the Twentieth Century 
Fund, and the California Institute of Technology. He is also a regent of the 
Smithsonian Institution. 

Mr. Austin was born in Georgia in 1915 and received his A.B. (1937) and 
LL.B. (1950) degrees from Harvard University. He served with the U.S. Navy 
in the Pacific during World War II, achieving the rank of lieutenant commander. 
He was awarded the Legion of Merit. 

When James Ebert moved to Woods Hole, we were especially fortunate to 
have Donald D. Brown to take over the direction of the Department of Embryol- 
ogy. Since 1961 when he was brought to the department as a promising fellow 
by Ebert, Brown has matured in the Carnegie environment, becoming a pioneer 
in developmental biology and molecular genetics and a gifted teacher. 

Brown attended Dartmouth College and received the M.S. (in biochemistry) 
and the M.D. degrees from the University of Chicago in 1956. After interning 
at Charity Hospital, New Orleans, he embarked upon a research career, first as 
a research associate in the National Institute of Mental Health (1957-1959) 
and then as a special fellow of the National Cancer Institute in the Service de 
Biochimie Cellulaire, Institut Pasteur, Paris. After a year as fellow at the 
Department of Embryology, he was appointed staff member. 

A leader in developing techniques for the recombination of animal genes with 
bacterial DNA molecules, Dr. Brown was one of the first to isolate animal 
genes — from the South African clawed toad Xenopus laevis. He has also studied 
the fibroin gene of the larval silkworm Bombyx mori. In recognition of these 
accomplishments he was chosen by the National Academy of Sciences to receive 
the U.S. Steel Foundation Award in Molecular Biology (1973) and was awarded 
the V. D. Mattia Lectureship of the Roche Institute of Molecular Biology (1975). 
In 1976 he received the Distinguished Alumni Award from the University of 
Chicago Medical School and an honorary D.Sc. from the University of Chicago. 

A frequent contributor to scientific journals, Dr. Brown became editor of 
Developmental Biology in 1975. He has been president and treasurer of the 
Society for Developmental Biology and is a member of the National Academy of 
Sciences and a fellow of the American Academy of Arts and Sciences. Con- 
current with his work at Carnegie, Dr. Brown is part-time professor of biology 
at The Johns Hopkins University. 



REPORT OF THE PRESIDENT «B 

The following honors have been awarded to Staff Members during the year: 

L. Thomas Aldrich of the Department of Terrestrial Magnetism was re-elected 
General Secretary of the American Geophysical Union. 

Peter M. Bell of the Geophysical Laboratory received the Exceptional Scien- 
tific Achievement Medal of the National Aeronautics and Space Administration. 

Donald D. Brown of the Department of Embryology was honored by his 
alma mater with the Distinguished Alumni Award of the University of Chicago 
Medical School and a D.Sc. degree from the University of Chicago. He also 
received the V. D. Mattia Lectureship of the Roche Institute of Molecular 
Biology and gave the John M. Prather Lectures in Biology at Harvard University. 

Jesse L. Greenstein of the Hale Observatories received several awards, includ- 
ing the Distinguished Public Service Medal of the National Aeronautics and 
Space Administration, a Medal of the University of Liege, and the Gold Medal 
of the Royal Astronomical Society. 

James E. Gunn, Hale Observatories, was the first George Ellery Hale Visiting 
Professor at the University of Chicago. 

Elburt F. Osborn was given the 29th Albert Victor Bleininger Award by the 
Pittsburgh Section of the American Ceramics Society. 

The Institution has been fortunate in the generosity of its friends and associates. 
Gifts from staff, former staff, and trustees, as well as other individuals and 
organizations, have added $1,909,000 to the funds for research over the past six 
years. Some of the recent donors are the late Dr. Vannevar Bush, Mr. William 
Golden, Mr. and Mrs. Crawford H. Greenewalt, Mr. William McChesney Martin, 
Jr., Mr. John C. Meyer, Mr. Henry S. Morgan, Mr. Walter H. Page, Dr. George 
W. Preston, and Mr. Orley H. Truman. 



30 CARNEGIE INSTITUTION 

FACULTY, FELLOWS, AND STUDENTS 
1975-1976 



DEPARTMENT OF EMBRYOLOGY 
Baltimore, Maryland 

Director 

James D. Ebert 

Director Designate 
Donald D. Brown 

Staff Members 
Igor B. Dawid 
Douglas M. Fambrough 
Kenneth J. Muller 
Richard E. Pagano 
Ronald H. Reeder 
Yoshiaki Suzuki 

Fellows 

Peter Botchan 
Diana Card 
S. T. Carbonetto 
Jeffrey Doering 
Scott Emmons 
Nina Fedoroff 
Paul Geshelin 
Elizabeth Godwin 
Leaf Huang 
Yasumi Ohshima 
Keiko Ozato 
Aileen K. Ritchie 
Alex Sandra 
Masatoshi Takeichi 
Katherine Tepperman 
Harvey Wahn 
Peter K. Wellauer 

Students 

Peter Devreotes 
John Gardner 
Carol Kaushagen 
Jose Ramirez 



GEOPHYSICAL LABORATORY 
Washington, D.C. 

Director 

Hatten S. Yoder, Jr. 

Carnegie Institution Distinguished Professor 
Elburt F. Osborn 



REPORT OF THE PRESIDENT SI 

Emeritus 
Emanuel G. Zies 

Staff Members 
Peter M. Bell 
Francis R. Boyd, Jr. 
Felix Chayes 
John R. Cronin 
Gordon L. Davis 
David H. Eggler 
Larry W. Finger 
John D. Frantz 
P. Edgar Hare 
Thomas C. Hoering 
T. Neil Irvine 
Thomas E. Krogh 
Ikuo Kushiro 
Ho-Kwang Mao 
Yoshikazu Ohashi 
Douglas Rumble III 
David Virgo 

Fellows 
Richard J. Arculus 
Nicholas T. Arndt 
Timothy M. Benjamin 
John M. Ferry 
Anthony A. Finnerty 
Toshitsugu Fujii 
John I. Hedges 
Frank E. Huggins 
Gifford H. Miller 
Bj0rn 0. Mysen 
Howard R. Naslund 
Frank S. Spear 
Juergen Trochimczyk 
Rosemary J. Vidale 
E. Bruce Watson 
Bernard J. Wood 

Students 
Julia A. Dill 
Dora Y. Lee 
Catherine A. McCammon 



HALE OBSERVATORIES 
Pasadena, California 

Director 

Horace W. Babcock 

Associate Director 
J. Beverley Oke 



CARNEGIE INSTITUTION 



Staff Members 
Halton C. Arp 
Eric E. Becklin 
Edwin W. Dennison 
Jesse L. Greenstein 
James E. Gunn 
Robert F. Howard 
Jerome Kristian 
Robert B. Leighton 
Guido Munch 
Gerry Neugebauer 
S. Eric Persson 
George W. Preston 
Bruce H. Rule 
Allan R. Sandage 
Wallace L. W. Sargent 
Maarten Schmidt 
Leonard T. Searle 
Stephen A. Schectman 
Arthur H. Vaughan, Jr. 
James A. Westphal 
Harold Zirin 

Staff Associates 
Robert J. Brucato 
Michael W. Werner 

Fellows 
A. Ger de Bruyn 
Eduardo Hardy 
Mark Hartoog 
Francois Schweizer 
Christopher P. Wilson 
Hirokazu Yoshimura 
Robert J. Zinn 

Carnegie-Chilean Fellow 
Maria Teresa Ruiz 

Student Observers 
Steven V. W. Beckwith 
Kirk Borne 
Todd Boroson 
France Cordova 
David J. Diner 
Jonathan H. Elias 
Richard F. Green 
Paul Hickson 
John G. Hoessel 
John P. Huchra 
Steven Kent 
John Kormendy 
Barry J. Labonte 
Philip Massey 
Jorge Melnick 
Daniel Nadeau 



REPORT OF THE PRESIDENT S3 

William C. Priedhorsky 
Douglas M. Rabin 
Russell 0. Redman 
Anneila I. Sargent 
William L. Sebok 
David Sholle 
Richard J. Terrile 
Edwin L. Turner 
Barry E. Turnrose 
Richard Wade 
Theodore B. Williams 
Steven P. Willner 



DEPARTMENT OF PLANT BIOLOGY 
Stanford, California 

Director 

Winslow R. Briggs 

Staff Members 
Joseph A. Berry 
Olle Bjorkman 
Jeanette S. Brown 
David C. Fork 
Malcolm A. Nobs 
William F. Thompson 

Emeritus 

C. Stacy French 
William M. Hiesey 

Fellows 

Michael Blatt 
Steven J. Britz 
James Collatz 
John Cross 
Ulrike Pohrmann 
James Ehleringer 
Algirdas J. Jesaitis 
George Johnson 
Aaron Kaplan 
John Mackenzie 
Bruce E. Mahall 
Norio Murata 
Michael Murray 
Charles E. Rogler 
Ulrich Schreiber 
Diana Stein 
Alan Stemler 
Heather Strong Belford 

Students 

Brian Anderson 
Robert D. Brain 
Daniel Cosgrove 



34 



CARNEGIE INSTITUTION 



William Curtis 
Mary Enama 
Thomas Payne 
Richard Preisler 
Karen Swift 



DEPARTMENT OF 
TERRESTRIAL MAGNETISM 
Washington, D.C. 

Director 
George W. Wetherill 

Distinguished Service Member 
Merle A. Tuve 

Emeritus 
Scott E. Forbush 

Staff Members 
L. Thomas Aldrich 
George E. Assousa 
Louis Brown 
Dean B. Cowie 
W. Kent Ford, Jr. 
Stanley R. Hart 
Albrecht W. Hofmann 
David E. James 
Alan T. Linde 
Nancy R. Rice 
Richard B. Roberts 
Vera C. Rubin 
I. Selwyn Sacks 
Norbert Thonnard 
Kenneth C. Turner 

Research Associates 
Tom I. Bonner 
Mordeckai Magaritz 
J. Arthur Snoke 
Kiyoshi Suyehiro 



Fellows 
John R. Bannister 
Charles L. Bennett 
Arturo Cuyubamba 
Gregory S. DeWitt 
John R. Evans 
Antonio Flores 
George H. Pepper 
Charles J. Peterson 
R. Sundar Rajan 
Michael Schneider 
Christine M. Seber 
Stuart J. Weidenschilling 
David J. Whitford 



Reports of Departments 
and Special Studies 



Department of Embryology 

Department of Terrestrial Magnetism 

Hale Observatories 

Department of Plant Biology 

Geophysical Laboratory 

Developmental Biology Research Group 



Department of Embryology 



Baltimore, Maryland 

James D. Ebert 
Director 

Donald D. Brown 
Director-Designate 



Carnegie Institution of Washington Year Book 75, 1975-1976 



Contents 



Introduction 7 

Developmental Genetics by Gene Isolation: The Dual 5S DNA System in Xenopus . . 12 

Preparation of 5S DNA recombined with the plasmid PMB9 12 

Restriction enzyme maps of two cloned fragments of X. mulleri oocyte-type 

5S DNA 14 

Characterization of a new 5S DNA from X. mulleri 15 

Reconstruction of 5S RNA transcription 16 

Sequencing studies on Xenopus 5S DNA 17 

The Structure of Ribosomal DNA from Xenopus 20 

Studies of Human X-chromosome DNA 22 

Chromosomal Proteins and the Control of Gene Function 23 

Isolation and characterization of amplified nucleoli 24 

The subunit organization of mouse satellite chromatin 27 

The Polarity of Ribosomal DNA in Xenopus 28 

Determination of polarity with DNA exonucleases 28 

Determination of polarity using rDNA transcription complexes 29 

Ribosomal DNA in Drosophila melanogaster 30 

Purification of Drosophila rDNA 31 

Analysis of EcoRI fragments of Drosophila rDNA 32 

Length heterogeneity in the spacer region 35 

Arrangement of length heterogeneity 35 

Distribution of rDNA repeat lengths on the X and Y chromosomes of 

D. melanogaster 36 

Structural and Functional Studies of Fibroin Genes 36 

Physical purification of the gene coding for silk fibroin 37 

Cloning of fibroin gene plasmid 37 

Accentuated expression of fibroin genes in vivo and in vitro 38 

Biogenesis of Mitochondria 45 

Cloning of Xenopus laeuis mitochondrial DNA 46 

Maps ofX. laeuis andX. mulleri mtDNA: restriction sites, the D-loop and the 

rRNA genes 46 

Mapping of 4S genes on X. laeuis mtDNA 48 

Mapping studies with mtDNA from Drosophila melanogaster 49 

Mapping and sequence comparisons of goat and sheep mtDNAs 50 

Phospholipid-Cholesterol Dynamics in Mammalian Cells 52 

Interactions of phospholipid vesicles with murine lymphocytes: vesicle-cell 

adsorption and fusion as alternative pathways of uptake 53 

Evidence for a stable vesicle-cell adsorption 53 

Vesicle-cell fusion and lipid exchange 56 

Interaction of Phospholipid vesicles with murine lymphocytes: vesicle-cell 
adsorption leads to altered surface properties and 

enhanced proliferative response 57 

Enhancement of proliferation by lipid vesicle pretreatment 58 



Alterations in lectin-induced agglutinability and lectin-binding 

in vesicle-treated cells 59 

Alteration of cell surface properties by vesicle adsorption 60 

Studies of membrane adhesion 61 

Vesicle to cell adhesion 62 

Cell to cell adhesion 63 

Distribution of lipophilic fluorescent probes in intact cells 66 

Membrane Transplantation by Fusion 67 

Mechanism of Syngeneic Response Induced by Lectins 73 

Studies on Skeletal Muscle and Neuronal Plasma Membranes 76 

ACh receptor biosynthesis 78 

Autoradiographic analysis of receptor degradation 81 

Effect of muscle activity on receptor turnover 83 

Organization of muscle plasma membrane in vivo 84 

Studies on the mobility of extrajunctional ACh receptors in plasma membrane ... 85 

Preparation of a fluorescent derivative of a-bungarotoxin 86 

Properties of fluorescent a-bungarotoxin 86 

Use of FITC-a-bungarotoxin as a probe in studying membrane structure 87 

Turnover of plasma membrane proteins 87 

Studies on neuronal plasma membranes 88 

Signaling in the Nervous System: Synapses of Specific Neurons in the Leech 90 

Chemical synapses in the leech 90 

Structural details of the synapse 90 

The arrangement of synapses on sensory neurons 91 

Sprouting of sensory neuron terminals 91 

An electrical synapse in the leech 95 

Interruption of the Rohde neuron 96 

The Collection of Human Embryos 97 

Developmental stages in human embryos 98 

Development of the nervous system 98 

The Bluntschli Collection 98 

Staff Activities 99 

Bibliography 102 

Personnel 103 



INTRODUCTION 



In Pentimento* , Lillian Hellman 
wrote, "Old paint on canvas, as it ages, 
sometimes becomes transparent. When 
that happens it is possible, in some pic- 
tures, to see the original lines: a tree 
will show through a woman's dress, a 
child makes way for a dog, a large boat 
is no longer on an open sea. That is 
called pentimento because the painter 
'repented,' changed his mind. Perhaps 
it would be well to say that the old con- 
ception, replaced by a later choice, is a 
way of seeing and then seeing again." 

There have been three conceptions of 
the Department of Embryology: the 
conception of Franklin P. Mall and 
George Streeter (1913-1940), the vis- 
ion of George W. Corner (1940-1955), 
and of the writer (1956-1976). Now 
there will be another. By the time this 
report is in the hands of its readers, the 
Department's fifth Director, Donald D. 
Brown, will be charting its course. 

The departmental canvas is a large 
one, painted and repainted over more 
than six decades, not just by four di- 
rectors, but by perhaps twenty "mas- 
ters" and ten times that number of 
their students. Here and there, espe- 
cially in the older sections, the paint is 
transparent, and one can see and see 
again the classic works for which the 
Department is justly renowned. 

The Department was, for five dec- 
ades, the world's leading center for the 
study of the human embryo. It 
pioneered in the development of pri- 
mates for research, having the earliest 
successful breeding monkey colony. 
Using these animals, large strides 
were made toward understanding 
menstruation and cyclic changes in the 
ovaries and uterus, laying much of the 
groundwork for more recent advances 
in family planning. 

Earlier studies, culminating in Ram- 
sey's classic works, set the standard for 



^Pentimento: A Book of Portraits, 1973, Little, 
Brown & Co., Boston. 



research on the primate placenta. 
Burns' studies of sex transformation 
similarly provided a standard in the 
field of sex differentiation. Although 
the techniques of cell culture were born 
in the neighboring Department of 
Anatomy of the Johns Hopkins Medical 
School, they came of age in the De- 
partment of Embryology especially in 
the contributions of M. R. and W. H. 
Lewis and, later, Irwin Konigsberg. 

During the past two decades the field 
of developmental biology has been re- 
shaped, as students of development 
began to perceive that it should be pos- 
sible to understand the factors regulat- 
ing the synthesis of specific products 
and their assembly during the shaping 
of a new organism. The Department of 
Embryology has played a part — at 
times a key part — in this reshaping 
process. However, the paint in these 
newer sections of the canvas is not yet 
transparent enough for the viewer — 
especially one as biased as the 
writer — to make out all of the land- 
marks. A few do stand out in bold re- 
lief. Donald Brown pioneered in the 
isolation and characterization of ani- 
mal genes and their immediate prod- 
ucts, the ribonucleic acids. His study 
with Carnegie Fellow John Gurdon of 
an anucleolate mutant strain of the 
South African clawed toad Xenopus 
constitutes a modern classic, offering 
decisive proof of the role of the nuc- 
leolus in the synthesis of ribosomal 
RNAs. By further exploitation of the 
anucleolar mutant, Brown was able to 
map the genes encoding these RNAs. 
He and his co-workers then proved that 
during oogenesis the genes for 
ribosomal RNAs are amplified into 
about 900 copies. His research then 
evolved to the point where it became 
possible for him to isolate the genes 
coding for the 28S and 18S ribosomal 
RNA fragments — the first genes to be 
isolated from any eukaryotic cell. A 
further extension of these techniques 



CARNEGIE INSTITUTION 



culminated in the isolation and charac- 
terization of the genes coding for 5S 
ribosomal RNA. 

Brown and his co-workers then 
turned to the question of whether the 
amplification of specific genes can be 
demonstrated in specialized cell types. 
Together with Suzuki, he isolated the 
messenger RNA coding for the silk pro- 
tein fibroin in the larval silkworm 
Bombyx mori. Using this mRNA as a 
probe, Brown was able to show that the 
fibroin gene is not amplified in the cells 
of the silk glands as compared to other 
cells, thereby providing strong evi- 
dence that the amplification of specific 
genes is not a generalized mechanism 
of cell differentiation. Brown's and 
Suzuki's studies have provided what 
may be the most complete account thus 
far available of the waxing and waning 
of gene action during development. 

The group's continuing investiga- 
tions have made a significant impact in 
developmental, molecular, and evolu- 
tionary biology. Studies of "develop- 
mental genetics by gene isolation" 
have led to new and brilliantly success- 
ful approaches to fundamental ques- 
tions in development. The Department 
has pioneered in the development and 
exploitation of techniques for gene 
purification, genes of known function 
having been characterized with the 
long-term goal of reconstructing their 
controls in vitro. Studies of "gene 
anatomy," the structure and arrange- 
ment of genes, have resulted in new in- 
sights into the mechanism by which 
multiple tandem genes evolve to- 
gether. 

It was just a decade ago, in his now- 
classic articles in Journal of Molecular 
Biology and Proceedings of the Na- 
tional Academy of Sciences, that Igor 
Dawid proved that the so-called cyto- 
plasmic DNA of amphibian oocytes, 
long an enigma to embryologists, is in 
fact mitochondrial. In cooperation with 
a series of able postdoctoral fellows and 
students, he has explored in depth the 
mitochondrial genes (mtDNA) and 
their products, the mitochondrial 
ribosomal RNAs (mt-rRNAs) and 
transfer (mt-tRNAs). 



Cell and tissue interactions are the 
hallmark of development. The dif- 
ferentiation of most cells requires, at 
one or more points in their life his- 
tories, interactions with their neigh- 
bors, and failures in close range tissue 
interactions may result in alterations 
in the normal patterns of development. 
The question has been posed in a gen- 
eral way as follows. How does the mic- 
roenvironment make its impact on cell 
function via the cell membrane? Genes 
direct the synthesis of the components 
of cell membranes, and must, in turn, 
be regulated by signals from the cell 
membrane. 

A better understanding of the role of 
the microenvironment demanded the 
capability to analyze cell differentia- 
tion in vitro. Of the Department's 
achievements in cellular develop- 
mental biology in the 1960s one stands 
out: Konigsberg's clonal analysis of 
myogenesis. His was the first clonal 
study of any differentiating cell. We 
are indebted to T. T. Puck of the Uni- 
versity of Colorado, who first developed 
the "feeder layer" technique, enabling 
him and others to clone a number of 
established cell lines. It was 
Konigsberg, however, who modified 
the technique and applied it signifi- 
cantly in developmental biology. It de- 
serves special mention, too, that two 
other members of the Department, 
Coon and Kaighn, followed Konigs- 
berg's initial success with uncommonly 
important clonal analyses of chondro- 
cytes and liver, and that a former Car- 
negie Fellow, Tokindo Okada, has used 
his experience in the Department to 
great advantage in his studies of 
"transdifferentiation" of pigmented re- 
tinal to lens cells. 

Another significant advance in cell 
biology came in DeLanney and Ebert's 
description of the graft-versus-host 
reaction. . 

But single "major discoveries" do not 
tell the whole story. Equally important 
is the steady flow of consistent, repro- 
ducible findings that, over the years, 
illuminate an entire subject. For 
nearly two decades the Department 
was the Nation's, indeed the world's, 



DEPARTMENT OF EMBRYOLOGY 



9 



leading center of research on the devel- 
opment of the heart, with the work of 
DeHaan, Rosenquist, and Ebert being 
especially noteworthy. 

Finally, mention must be made of 
the studies of a number of contributors, 
notably Kaighn, Lee, Hay, and 
Yoshikawa-Fukada, on the interplay of 
tumor viruses and differentiating cells. 
Their findings provided many of the 
"building blocks" in our current under- 
standing of viral oncogenesis. 
Yoshikawa-Fukada's first description 
of the viral oncogenic sequences, lost 
sight of at times because of the flood of 
related findings in the cancer virus 
field, nonetheless marked her as an in- 
novating spirit. 

1975-1976 

In recent years, no single research 
program has attracted more attention 
to the Department than Douglas Fam- 
brough's continuing exploration of the 
acetylcholine receptors in skeletal 
muscle fiber membranes. Acetyl- 
choline (ACh) receptors are of special 
interest to Fambrough and his col- 
leagues, both as membrane glycopro- 
teins and as elements figuring promi- 
nently in neuromuscular interactions. 
In a series of definitive articles, they 
have reported on the appearance of 
ACh receptors during muscle develop- 
ment, the production and incorporation 
of ACh receptors into the surface mem- 
branes of myotubes and their sub- 
sequent degradation, the clustering of 
ACh receptors at newly formed 
neuromuscular junctions both in vitro 
and in vivo, the number and distribu- 
tion of ACh receptors in adult rat 
skeletal muscle fibers, and the time 
course of change in these parameters 
following denervation. They are con- 
tinuing to explore physiological aspects 
of receptor function, examining further 
the biosynthesis and degradation of 
ACh receptors and obtaining more ac- 
curate estimates of ACh receptor den- 
sity on denervated muscle fiber mem- 
branes. 

In this year's report, Fambrough and 



his student Peter Devreotes discuss in 
detail their model for the major events 
in receptor metabolism. Its main fea- 
tures can be summarized briefly. The 
protein subunits of which the ACh re- 
ceptor is composed are synthesized 
with the usual protein synthesizing 
mechanisms of the cells and are rapidly 
assembled into receptor units resem- 
bling those in the plasma membrane. 
These newly synthesized receptors 
occur upon some membrane systems in 
the cytoplasm of the muscle cells and 
are collectively referred to as the "pre- 
cursor pool." The average residence of a 
new receptor in the precursor pool is 
2-3 hours. Receptors are transferred 
from this pool to the surface by an 
energy-requiring process and are then 
able to function. The average half-life 
of an ACh receptor in the plasma 
membrane is 22 hours. Receptors are 
degraded by an energy-requiring pro- 
cess which involves the internalization 
of the receptor and transport to second- 
ary lysosomes and then proteolytic de- 
struction. Interested readers will find 
the evidence supporting these conclu- 
sions in the body of the report, along 
with indications of the group's new di- 
rections, including a foray into the 
membranes of neurons. 

Richard Pagano and his associates 
are also focusing attention on the cell 
membrane, their work being concerned 
with the possible dynamic role which 
membrane phospholipids and choles- 
terol may play in the regulation of cell 
behavior. During the past several 
years they have utilized artificially 
generated lipid vesicles to perturb 
membrane components in living cells 
in a controlled and systematic way. In 
the present report they offer findings 
on the interactions of phosphatidyl 
choline vesicles with murine lympho- 
cytes, and the physiological conse- 
quences of these interactions. Pagano, 
and postdoctoral fellows Huang and 
Ozato, have shown that the pathways 
of vesicle-cell interactions, enumer- 
ated in previous reports (vesicle-cell 
fusion, adsorption and lipid exchange) 
are exquisitely sensitive to the molecu- 
lar composition and physical properties 



10 



CARNEGIE INSTITUTION 



of the artificial vesicles. In the lympho- 
cyte system, the resulting membrane 
perturbations lead to altered surface 
properties and an enhanced prolifera- 
tive activity of these cells, when stimu- 
lated by various mitogens and anti- 
gens. 

In a separate line of research, Car- 
negie Fellow Masatoshi Takeichi has 
explored some of the adhesive prop- 
erties of a fibroblastic cell line and re- 
ports on his attempts to identify and 
characterize membrane proteins which 
participate in this phenomenon. Paral- 
lel studies on the adherence of lipid 
vesicles of known composition to cells 
are also discussed. 

The importance of cellular interac- 
tions is illustrated strikingly in Ken- 
neth Muller's studies of synapses in the 
leech. The role that a neuron assumes 
in the nervous system is dependent 
upon the connections that it makes 
with other neurons or with peripheral 
targets. Therefore, an essential step 
both in determining the forces that act 
initially to organize the nervous sys- 
tem and in characterizing the mecha- 
nisms by which a neuron handles, or 
integrates, information is to ascertain 
the relationship between a neuron's 
function and the arrangement of its 
synapses. A related problem in 
neurobiology has been to determine the 
extent that a neuron's shape and other 
properties are intrinsic, or whether its 
contacts or associations with other 
neurons are essential for certain as- 
pects of its growth and maintenance. 

The nervous system of the leech 
Hirudo medicinalis has been particu- 
larly favorable for study of the physio- 
logical role and functioning of specific 
neurons, largely because its organiza- 
tion is relatively simple. Neurons in 
this segmented worm are grouped 350 
together to form essentially identical 
ganglia that are linked in a chain by 
axon bundles; each ganglion mediates 
the reflexes and controls the behavior 
of its segment. It has been possible to 
monitor electrical activity from within 



single sensory and motor neurons with 
microelectrodes and to correlate the 
physiological activity of functionally 
identified neurons with their charac- 
teristic shape and position within each 
ganglion. By recording from pairs of 
neurons one can trace synaptic connec- 
tions, which have been found to be sur- 
prisingly uniform and in that sense 
predictable between specific neurons. 

In his current report, Muller de- 
scribes the structural details of chemi- 
cal synapses, delineates their ar- 
rangement on sensory neurons, and 
explores the sprouting of sensory 
neuron terminals. He also describes 
the unexpected discovery of an electri- 
cal junction between unpaired Rohde 
neurons in neighboring ganglia. 

Brown and his colleagues are now 
concentrating on the family of genes 
which code for the small RNA molecule 
of ribosomes termed 5S RNA. The 
"transcription unit," that is the region 
of this DNA which is transcribed into 
RNA in the living cell, is relatively 
simple — only 120 base pairs in length. 
Moreover, the form of RNA polymerase 
which transcribes this gene in vivo has 
been identified. The fact that the ma- 
ture RNA transcript does not appear to 
be derived from a higher molecular 
weight precursor is especially impor- 
tant: It means that the true initiation 
site of transcription from 5S DNA is 
known. Therefore, one can detect faith- 
ful transcription when it occurs in 
vitro. Using this system Brown hopes 
to reassemble the macromolecules 
which enable this DNA to be trans- 
cribed with fidelity in the living cell. In 
this way he hopes to elucidate the con- 
trol mechanisms of an animal gene. 

This year his group reports progress 
along several lines. Three different 
kinds of 5S DNA have been cloned \r\E. 
coli by means of the new recombinant 
DNA methodology, thereby providing 
an order of magnitude more material to 
work with and permitting the careful 
examination of one single repeating 
unit of a tandem gene family. The "re- 



DEPARTMENT OF EMBRYOLOGY 



11 



striction maps" of these cloned DNAs, 
i.e., the sites where they are cleaved by 
certain restriction enzymes, are de- 
scribed by Fellows Jeffrey Doering and 
Scott Emmons. The latter is attempt- 
ing to develop an assay for proper 
transcription. Fellow Nina Fedoroff is 
exploring DNA sequencing methods to 
determine the exact nucleotide se- 
quence in the spacer regions of 5S 
DNA, particularly those regions adja- 
cent to the initiation site of the gene. 

DNA structure continues to hold the 
attention of several Staff Members and 
Fellows. Reeder and Dawid, along with 
Fellow Peter Botchan, report on the 
structure of nontranscribed spacer re- 
gions in Xenopus ribosomal DNA, 
based on studies using cloned frag- 
ments of rDNA generated with the re- 
striction endonuclease EcoRI. Fellow 
Peter Wellauer and Dawid have under- 
taken the first stages of a study of the 
ribosomal DNA of Drosophila, Dawid 
and his colleagues are cloning Xenopus 
mitochondrial DNA, and Carnegie Fel- 
low Ohshima and Staff Member Suzuki 
are attempting to clone the silk fibroin 
gene. 

In all of these studies, the Depart- 
ment has adhered to the safety stan- 
dards proposed by the Asilomar Con- 
ference on Recombinant DNA. 

Mention has already been made of 
the contributions of several fellows 
and students. Peter Wellauer, whose 
time in 1975-1976 was shared between 
California Institute of Technology and 
this Department, will return to Swit- 
zerland to a new position in the Swiss 
Cancer Research Institute in 
Lausanne. Scott Emmons will continue 
his postdoctoral training at the Uni- 
versity of Colorado. Leaf Huang has 
taken up his new responsibilities in the 
Department of Biochemistry of the 
University of Tennessee, and 
Katherine Tepperman will join the 
University of Cincinnati's faculty. 
Keiko Ozato will move to the Johns 
Hopkins School of Medicine-Good 
Samaritan Hospital. 



Continuing in the Department are 
Diana Card, a Fellow of the Muscular 
Dystrophy Associations of America, 
Inc., with Fambrough; Peter Botchan 
and Harvey Wahn, with Reeder; 
USPHS Fellows Jeffrey Doering and 
Nina Fedoroff with Brown; Paul 
Geshelin and Yasumi Ohshima, with 
Suzuki; Alex Sandra, with Pagano; 
Elizabeth Godwin, with Dawid; and S. 
T. Carbonetto, with Muller and Fam- 
brough. 

This brief listing of postdoctoral fel- 
lows beings me full circle to my start- 
ing point in 1956. In his autobiog- 
raphy, Anatomist at Large, George 
Corner wrote that the secret of his suc- 
cess was his ability to place himself 
under wise leaders. His words ring true 
for me as well. I would emphasize still 
another ingredient: the catalysis pro- 
vided by young minds — fellows and 
students. When President Haskins and 
I began to chart the Department's 
course in 1956, a vital principle 
emerged. It was clear that even if 
ample funds and space were available 
immediately to undertake all the tasks 
we had set for ourselves, the scientific 
talent was not. The pool of able devel- 
opmental biologists then was small. It 
was agreed, therefore, that training 
would henceforth be an integral part of 
the Department's program. The wis- 
dom of that decision is demonstrated by 
a glance at today's staff — Brown, 
Dawid, Fambrough, Reeder and Suzuki 
all joined the Department initially for 
training. Thus the reshaping of the 
Department has been gradual, not 
cataclysmic. As I wrote in 1956 (Year 
Book 55, p. 270), the Department may 
be likened to an organism. Although 
there has been considerable change or 
turnover within its parts, as a whole it 
enjoys stability. We achieve what 
Rudolph Schoenheimer called a condi- 
tion of stability with flux — working 
toward a better understanding of the 
mechanisms of development, using for 
that purpose whatever methods be- 
come necessary. 



12 



CARNEGIE INSTITUTION 



DEVELOPMENTAL GENETICS BY GENE ISOLATION: THE DUAL 
5S DNA SYSTEM IN Xenopus 

D. D. Brown, J. Doering, S. Emmons, N. Fedoroff, and E. Jordan 



In Year Book 74 (p. 12) we described 
our reasons for concentrating on the 
family of genes which code for the 
small RNA molecule of ribosomes 
termed 5S RNA. The "transcription 
unit," that is, the region of this DNA 
which is transcribed into RNA in the 
living cell, is relatively simple — only 
120 base pairs in length. The form of 
RNA polymerase which transcribes 
this gene in vivo has been identified by 
Roeder as form III. The mature RNA 
transcript does not appear to be derived 
from a higher molecular weight pre- 
cursor. This observation is especially 
important because it means that we 
know the true initiation site of trans- 
cription from 5S DNA and therefore 
can detect faithful transcription when 
it occurs in vitro. Using this simple 
system we hope to reassemble the mac- 
romolecules which enable this DNA to 
be transcribed with fidelity in the liv- 
ing cell. In this way we hope to eluci- 
date the control mechanisms of an 
animal gene. 

In this report we summarize progress 
made along several lines. Three dif- 
ferent kinds of 5S DNA have been 
cloned in E. coli by means of the new 
recombinant DNA methodology (see 
Year Book 74). This approach provides 
an order of magnitude more material to 
work with and permits us to examine 
carefully a single repeating unit of a 
tandem gene family. We describe the 
"restriction maps " of these cloned 
DNAs, i.e., the sites where they are 
cleaved by certain restriction enzymes 
(Doering and Emmons). Various 
schemes are being investigated to de- 
velop an assay for proper transcription 
(Emmons). DNA sequencing methods 
will determine the exact nucleotide se- 
quence in the spacer regions of 5S 
DNA, particularly those regions adja- 
cent to the initiation site of the gene 
(Fedoroff). We have two new species of 
Xenopus obtained from the collection of 



M. Fischberg of Geneva, Switzerland. 
We have purified partially the princi- 
pal 5S DNA component from each of 
these. Although we will not describe 
these experiments in this report, we 
have a specific purpose for isolating the 
genes from different species which en- 
code the same function. The spacer re- 
gions that separate the genes have di- 
verged considerably in their nucleotide 
sequences. Control regions within 
these spacers are likely to be con- 
served. By comparing the nucleotide 
sequences in comparable regions of 5S 
DNA isolated from different species we 
can identify possible control regions. 



Preparation of 5S DNA 

Recombined with the Plasmid 

PMB9 

D. D. Brown and E. Jordan 

We have used exclusively the plas- 
mid PMB9 for recombining with, and 
cloning, Xenopus 5S DNA. This circu- 
lar plasmid has a molecular weight of 
3.5 x 10 6 (5400 base pairs). It has a 
single site for several restriction en- 
zymes into which foreign DNA can be 
introduced and joined. We have used 
the plasmid site that is cleaved by the 
enzyme Hindlll for insertion because 
three different Xenopus 5S DNAs also 
have Hindlll sites. This enzyme leaves 
at each end four nucleotide single- 
stranded regions that are complemen- 
tary and can be joined by ligase. 

Xenopus laevis oocyte-type 5S DNA 
has a single Hindlll site once per re- 
peat, and Xenopus mulleri oocyte 5S 
DNA has irregular Hindlll sites which 
occur on the average about once every 
four repeats. The newly isolated 5S 
DNA from Xenopus mulleri also has a 
single Hindlll site per repeat. The 
scheme for insertion is as follows. The 
plasmid is digested to completion with 
Hindlll. The 5S DNA is digested either 



DEPARTMENT OF EMBRYOLOGY 



13 



completely or partially with Hindlll, 
depending upon whether monomers or 
multimers are to be inserted. The 
plasmid and 5S DNAs are mixed and 
joined with T4 ligase. The bacteria are 
transformed with the recombined 
DNA. Those bacteria which incorpo- 
rate a plasmid can be selected by their 
resistance to colicine or tetracycline. 
Grunstein and Hogness (Proc. Nat. 
Acad. Sci. 72, 3961, 1975) devised a 
simple screening test to detect bacteria 
that contain a specific inserted gene. 
Many individual colonies are grown on 
a filter paper disc and their DNA is 
immobilized on the paper after al- 
kaline denaturation. The filter is then 
hybridized with iodine- 125 labeled 5S 
RNA. Only the clones that contain 5S 
DNA hybridize with the 5S RNA and 
become radioactive. The radioactive 
clones are located by exposing the fil- 
ters to x-ray film. Individual clones are 
grown up for analysis. Figure 1 demon- 
strates one such screening experiment. 
This method of colony hybridization 
not only simplifies the identification of 
bacteria that contain 5S DNA sequen- 
ces, but it avoids growing large amounts 
of bacteria that might contain un- 
known DNA sequences. Such bacteria 
remain on the agar plate and are auto- 
claved. Since 5S DNA occurs as a tan- 
dem gene family, no other DNA 
molecules are likely to be inserted into 
a plasmid with the 5S DNA fragment. 
These considerations are important be- 
cause of our continuing concern with 
safety precautions, especially when 
there is a possibility of cloning un- 
known animal DNA fragments. The 
colony isolation method described 
above minimizes this possibility. In 
addition we have introduced a "chemi- 
cal" containment step during the isola- 
tion of recombinant DNA. When we 
grow E. coli cultures containing a re- 
combined plasmid, we kill the bacteria 
before isolating their plasmid DNA. 
This step is achieved simply by adding 
small amounts of chloroform to the cul- 
tures just before harvesting. No viable 
bacteria remain after five minutes of 
chloroform treatment. 




Fig. 1. Autoradiograph of bacterial colonies 
that had been hybridized with 1J3 I-labeled 5S 
RNA. Tetracycline-resistant colonies (containing 
the plasmid or plasmid plus 5S DNA insert) were 
grown on the filter paper, treated with alkali to 
denature DNA, neutralized, dried, and hy- 
bridized with the radioactive RNA. The excess 
radioactivity was removed, and the labeled col- 
onies containing 5S DNA molecules were de- 
tected by autoradiography. Top, a photograph of 
three filter papers with dried colonies. The mid- 
dle filter contains bacteria that had been trans- 
formed with PMB9 recombined with X. laevis 
oocyte-type 5S DNA. Bacteria on the other two 
filters were transformed with PMB9 recombined 
with X. mulleri oocyte-type 5S DNA. The film 
itself is shown in the middle panel, and it has 
been placed over the filters to show which ones 
contain the 5S DNA insert (bottom panel). 



14 



CARNEGIE INSTITUTION 



The Structure of X. laevis Oocyte-Type 
5S DNA 

A model for one repeating unit of X. 
laevis oocyte-type 5S DNA (XLo) was 
first published in Year Book 74. This 
model is refined and corrected in Fig. 2. 
The A+T-rich spacer region was placed 
erroneously at the 3' end of the trans- 
cription unit (the 5S RNA gene). We 
know that there is a short A + T-rich 
region adjacent to the 3' end of the 
gene from the experiments of R. D. 
Brown {Year Book 74). However, the 
recent sequencing studies by Claude 
Jacq and George Brownlee at the Med- 
ical Research Council Laboratory of 
Molecular Biology in Cambridge, Eng- 
land, have shown that the principal 
A+T-rich spacer region is at the 5' end 
of the transcription unit. Our current 
model (Fig. 2) incorporates this change 
in orientation. The A+T-rich 15-mer 
oligonucleotides that comprise most of 
this region are designated by short dot- 
ted lines. Their exact location and 
number are unknown. Jacq and 
Brownlee also partly sequenced an in- 
teresting region called the "pseudo- 
gene," which differs from the gene se- 
quence by only six bases in the region 
from residue 45 to residue 100. The 
terminal 20 nucleotides are completely 
different; sequencing of the first 44 
nucleotides is incomplete. The recogni- 
tion site for the restriction enzyme 
Haelll (Fig. 2) occurs in a part of the 
pseudogene which is analogous to a 
corresponding region in the gene. The 
other Haelll sequence found in the 
gene is missing in the pseudogene. The 
function of the pseudogene is unknown. 



Restriction Enzyme Maps of Two 

Cloned Fragments ofI. mulleri 

Oocyte-Type 5S DNA 

J. Doering and S. Emmons 

The restriction enzyme Hindlll cuts 
X. mulleri oocyte-type 5S DNA (Xm ) 
occasionally yielding fragments which 
are heterogeneous in length. Frag- 
ments from a complete Hindlll digest 
of Xm were cloned by the methods de- 
scribed above in E. coli HB101 using 
the plasmid PNB9. Two of these cloned 
fragments were selected for detailed 
study. 

Xm 3 is a 2700 base-pair fragment. 
It is cut by the restriction enzyme 
Hpall into four fragments, the smallest 
of which is present in two copies. To 
determine the order of these frag- 
ments, Xm 3 was end-labeled using 
a- 32 P-labeled ATP and T4 polynuc- 
leotide kinase. The labeled DNA was 
digested with the enzyme and the 
fragments run on a gel. Radioactive 
terminal fragments were identified by 
autoradiography. The order of the re- 
maining fragments was deduced from 
the sizes of partial digestion products 
(Fig. 3). The pattern of fragments ob- 
tained after double digestion with 
Hpall and Haelll was indistinguish- 
able from that obtained with Haelll 
alone, indicating that all Hpall sites 
are very close to Haelll sites. This im- 
plies that Hpall cleaves only within 
the gene sequence itself, since the rec- 
ognition sequence for Hpall occurs 
next to an Hae site in X. mulleri 
oocyte-type 5S RNA. The restriction 
map in Fig. 3 was constructed using 



A + T regi on 



Hind 

nr 



G + C region 

Hind 
Gene Pseudogene HE 
*-$' 



327- 507- 



Hae Hae 

->+« 120 



Hae 
-103 -H 



Fig. 2. Model of one repeat ofX. laevis oocyte-type 5S DNA. Restriction enzyme sites are included. 
The numbers refer to base pairs. 



DEPARTMENT OF EMBRYOLOGY 



15 



Hind 

m 



Hind 

m 



->!&*- 



+*fi 190*^190^7*- 



190 100 



t=Hpa H 
I =Hae m 
: EHae HI (UNCERTAIN) 

Fig. 3. Restriction map of oneX. mulleri oocyte-type cloned fragment (Xm„3). The numbers refer to 
base pairs. Some cleavage sites are tentative. They are shown as dotted lines. 



end-labeled fragments, partial digests, 
and Haelll digests of each Hpall frag- 
ment. 

Xm l is an 800 base-pair fragment 
containing one Hpall site, one Hhal 
site, and six Haelll sites. A map is 
being constructed using the methods 
described above. 

Characterization of a New 5S 
DNA fromI. mulleri 

J. Doering 

In the search for the 5S DNA coding 
for the somatic-type 5S RNA, new 5S 
DNA-containing satellites were iso- 
lated from both X. laeuis and X. mul- 
leri. The new X. laevis 5S DNA was 
found to code for a 5S RNA different 
from both the oocyte and somatic types 
(Year Book 74, p. 13). 

The kind of 5S RNA gene carried by 
the newX mulleri 5S DNA is still un- 
known. Table 1 lists some characteris- 
tics of Xm-new DNA compared with 
those of the oocyte-type 5S DNA from 
X. mulleri. The partially purified new 
5S DNA from X. mulleri (Xm-new) is 
cut by the restriction enzyme Hindlll 
into homogeneous 900-base-pair frag- 
ments (Fig. 4). Higher molecular 
weight DNA is also present after diges- 
tion, but only the 900-base-pair frag- 
ments hybridize with 5S RNA. Frag- 
ments from a complete Hindlll digest 
of Xm-new were cloned by the methods 
described above. The Xm-new DNA 
from one clone has been subjected to 
detailed study. 




Fig. 4. Electrophoresis in a 2% acrylamide, 
0.5% agarose composite gel of Hindlll digests of 
Xm-new 5S DNA (left), purified from the ani- 
mal's DNA, PMB9 containing one Xm-new 5S 
DNA insert (middle), and a mixture of these two 
samples (right). Electrophoresis is from top to 
bottom. 



16 



CARNEGIE INSTITUTION 



TABLE 1. Characteristics of the Newly Isolated 5S DNA fromX. mulleri 
Compared toX. mulleri Oocyte 5S DNA 



Oocyte-5S DNA 


New 5S DNA 


Density (g/cm :i ) 1.695 


1.712 


Average no. of repeats 9000 


700 


Repeat length (base pairs) 1400 


900 


Length heterogeneity great 


not detectable 


Strand separation in alkaline CsCl yes 


yes 


5S RNA coding strand light 


light 


Restriction Enzyme Sensitivity (cuts/repeat) 




EcoRI resistant 


resistant 


Hindlll less than one 


1 


Hpall variable 


3 


Hhal variable 


2 


Haelll variable 


> 5 



The cloned Xm-new DNA has a 
biphasic melting curve, and both re- 
gions are high in G+C content. Haelll, 
which cuts DNA at the sequence 5'- 
GGCC-3', digests about one-third of 
the 900-base-pair fragments into pieces 
of less than 40 base pairs. This 
suggests that the spacer has a short re- 
peating sequence that contains a re- 
petitive Haelll cleavage site. The 
spacer region of cloned Xm-new DNA 
has little, if any, homology with Xm 3 
DNA by hybridization criteria. While 
E. coli RNA polymerase was found to 
transcribe the noncoding strand of X. 
laeuis oocyte-type 5S DNA preferen- 
tially, the enzyme transcribes both 
strands of cloned Xm-new DNA about 
equally. 

A restriction enzyme map of the 
cloned Xm-new DNA is being con- 
structed in preparation for sequencing 
studies to determine what type of 5S 
gene the new DNA carries. 



Reconstruction of 5S RNA 
Transcription 

S. Emmons 

Our efforts to devise a system in 
which to detect and study 5S RNA 
transcription in vitro {Year Book 74, p. 
19) have continued over the past year. 
The immediate aim is to develop a sen- 
sitive and specific assay for the initia- 
tion of transcription at the beginning of 
the 5S RNA gene. 



For this purpose we have available to 
us a number of small (one hundred to a 
few hundred base pairs), well- 
characterized DNA "restriction frag- 
ments" cut from various regions of 5S 
DNA by restriction enzymes. Some of 
these fragments carry the initiation 
site for 5S RNA transcription, being 
produced by a cut before the gene re- 
gion and a cut within the gene region. 
We envisage that these fragments may 
be used in one of two ways in transcrip- 
tional studies. First, they may be used 
as hybridization probes to determine 
whether a population of RNA trans- 
criptions contains any molecules whose 
5' ends are identical to the 5' end of 5S 
RNA. In this method, RNA transcripts 
are hybridized to the restriction frag- 
ment and treated with a single- 
strand-specific ribonuclease. RNA 
molecules resulting from a correct in- 
itiation event will give rise to an RNA 
fragment equal in length to the known 
length of the gene region carried on the 
restriction fragment. This RNA frag- 
ment could be identified by gel elec- 
trophoresis and nucleotide sequencing. 
In the second method, the DNA restric- 
tion fragment is used as a template for 
RNA polymerase or for RNA- 
synthesizing extracts. Since the frag- 
ment ends within the gene, correct in- 
itiation of transcription again results 
in an RNA fragment of known length. 

Both of these approaches have been 
attempted using the (approximately) 
450 base-pair fragment produced by 



DEPARTMENT OF EMBRYOLOGY 



17 



the restriction enzyme Haelll from X. 
laeuis oocyte 5S DNA. This fragment 
carries all of the A + T-rich spacer, the 
hypothetical promotor region(s) before 
the gene, and the first eight nuc- 
leotides of the gene itself (Fig. 2). To 
determine whether this fragment could 
be used as a hybridization probe, at- 
tempts were made to hybridize it with 
125 I-labeled 5S RNA. For this purpose, 
the fragment was denatured and bound 
to nitrocellulose filters, and RNA an- 
nealed to it under conditions favoring 
the stability of the very short hybrid ( 1 
M NaCl, 0°C). Under these conditions a 
13-nucleotide-long oligomer (produced 
from 5S RNA by ribonuclease Tl) an- 
nealed to 5S DNA, but shorter 
oligonucleotides did not, and 125 I-5S 
RNA did not anneal to the restriction 
fragment. It is presumed that this is 
because the RNA was not at a high 
enough concentration. Unfortunately, 
higher concentrations are not obtaina- 
ble easily in an assay using a mixture 
of transcripts, so it is concluded that if 
restriction fragments are to be used as 
hybridization probes they must carry 
more than the first eight nucleotides of 
the gene. 

The 450-nucleotide Haelll fragment 
from X. laevis has also been used as a 
template in transcription studies. A 
protein extract of X. laevis ovaries was 
prepared following procedures of 
Roeder (J. Biol. Chem. 249, 241-248, 
1974; personal communication), and its 
RNA polymerase activity analyzed 
using various templates. This extract 
was found to be relatively inactive on 
Xenopus 5S DNA fragments, compared 
with poly dAT and withE". coli plasmid 
fragments. No activity at all (conver- 
sion of precursor to TCA insoluble ma- 
terial) could be observed even with 
high concentrations of the 450 nuc- 
leotide fragment. In one experiment, 
the reaction mixture was nevertheless 
fractionated in a system (homo- 
chromatography on DEAE paper) that 
would have revealed the presence of an 
eight nucleotide oligomer, but none 
was seen. 

The 5S gene system of Xenopus re- 



mains an optimal one for attempts to 
reconstruct eukaryotic transcription in 
vitro. Purified 5S DNAs from several 
species are available as templates, and 
the structure of these DNAs is being 
studied intensely. Two classes of genes, 
expressed in different tissues (somatic 
and oocyte) exist, allowing an analysis 
of the mechanisms operating to control 
their expression. In addition, the pri- 
mary transcription product in this sys- 
tem is small and of known sequence. 

Sequencing Studies on 
Xenopus 5S DNA 

N. Fedoroff 

We have focused on the introduction 
and development of direct DNA 
sequencing techniques. DNA sequenc- 
ing methodology has been evolving 
rapidly over the past few years. The 
approach that has proved most fruitful 
is identification of the terminal nuc- 
leotide in successively shorter frag- 
ments having one fixed end. With 
technical improvements in oligonuc- 
leotide fractionation procedures, the 
number of nucleotides that can be or- 
dered in a single analysis has increased 
steadily. Several laboratories are en- 
gaged in developing and improving dif- 
ferent methods for generating the req- 
uisite fixed-end fragments and iden- 
tifying terminal nucleotides. 

Our immediate interest is in the nuc- 
leotide sequence of the spacer region 
adjacent to the 5 ' terminal portion of 
the 5S RNA gene in Xenopus, since this 
is likely to contain a control region for 
transcription. All of our studies so far 
have been done on a single repeating 
unit of X. laevis oocyte-type 5S DNA. 
Quantities sufficient for sequencing 
studies have been obtained by integra- 
tion of the DNA into bacterial plasmids 
as described in previous sections. The 
distribution of restriction endonuclease 
Haelll sites within this DNA, shown in 
Fig. 2, makes the region of interest ac- 
cessible to direct sequencing in several 
ways. 

We have, for example, used two dif- 



18 



CARNEGIE INSTITUTION 



t, if 
T 3 5 T 



5' A + T - r ic h region 3 

5S gene 



DN A poly mera se 
a-P 32 d NTP's 



HaeUr site 




WAAAA/VWAAV**- 



Hae n digest ion 
denat lira t ion 



Hoe HI site 



V r 3 

A+T- rich region 55 gene 



X Pol y nucle 



ir DNase I 



7 



tide kinase 
P" AT P 



exonucl ease 



,Hae HI site 



denaturation 



\A/VWWWVA/<AA«. 



\aava\*w 



Fig. 5. Two schemes for direct DNA sequencing applied to the region adjacent to the initiation site 
of 5S DNA. A series of labeled DNA fragments (wavy lines) are generated which have a unique 5' 
terminus and a known 3'-end; that is, the nucleotide at the 3'-end is known to be A, T, G, or C. These 
different-sized fragments are separated by polyacrylamide gel electrophoresis, (a) The Sanger- 
Coulson method, (b) A modification developed in this laboratory. Details are given in the text. 



ferent procedures to generate frag- 
ments which have a common 5 ' ter- 
minus and which extend over the re- 
gion adjacent to the gene (Fig. 5). Fig- 
ure 5a represents a direct application 
to 5S DNA of the scheme developed by 
Sanger and Coulson (J. Mol. Biol. 94, 
441_448, 1975). The strand of 5S DNA 
containing the 5S RNA sequence (non- 
coding strand) is used as a template for 
E. coli DNA polymerase I. The Haelll 
fragment extending from residue 9 to 
residue 66 of the 5S RNA sequence 
(Fig. 2) is used as a primer for the en- 
zyme. The strand of the fragment com- 
plementary to 5S DNA (coding strand) 
is annealed to the template extended 
with DNA polymerase in the presence 
of cv- :i2 P-labeled nucleotides. This gen- 
erates a population of labeled frag- 
ments that begins at residue 8 of the 5S 
gene and extends into the adjacent 
spacer region (Fig. 5a). The identity of 



the terminal nucleotide in the labeled 
fragments is determined by the follow- 
ing procedure. The extended primer- 
template complex is purified and 
aliquots are reincubated with DNA 
polymerase in the presence of either 
one or three unlabeled nucleotides. 
Under the conditions used, the labeled 
fragments are degraded until they 
terminate at the single included nuc- 
leotide, or extended until they termi- 
nate just prior to the single omitted 
nucleotide. For example, the reaction 
mixture containing TTP will yield only 
those oligonucleotides terminating in T 
residues. Since all fragments have a 
common 5 ' end, the position of T resi- 
dues in the sequence can be deduced 
directly from the length of the 
T-terminated fragments. Separation of 
oligonucleotides that differ in length 
by a single residue is carried out by 
electrophoresis in high-percentage de- 



DEPARTMENT OF EMBRYOLOGY 



19 



naturing poly aery 1 amide gels. 

One difficulty with this approach is 
inherent in the organization of the 5S 
DNA. As described in an earlier sec- 
tion, each repeating unit of Xenopus 
laeuis oocyte 5S DNA contains both a 
gene and a very similar sequence, 
termed the "pseudogene" (see Fig. 2). 
The gene and pseudogene differ by ap- 
proximately four nucleotides within 
the segment covered by the primer 
molecule. The primer, therefore, hy- 
bridizes to both regions, and the ex- 
tended primer molecules represent se- 
quences adjacent to both gene and 
pseudogene. One way around this diffi- 
culty is to determine the sequence of 
the short fragment released by digest- 
ing the extended template-primer 
complex with Haelll as shown in Fig. 
5a. Since the pseudogene lacks this 
Haelll site, the radioactive fragments 
will be released from primer molecules 
on gene sequences but not pseudogene 
sequences. In principle, this permits 
determination of at least the first 50 
residues of the spacer sequence adja- 
cent to the gene before the longer 
fragments extended from the 
pseudogene interfere. Using this ap- 
proach, we have obtained useful se- 
quence information over a 30- 
nucleotide region beginning between 
10 and 20 nucleotides from the first in- 
tragenic Haelll site. The first 10 to 20 
nucleotides are recovered in very low 
yield or not at all because of differen- 
tial labeling and probably also because 
of differential degradation of short 
oligonucleotides. 

We have developed an alternative 
procedure for generating the same 
population of labeled fragments that 
includes the region which is adjacent to 
the gene. In this procedure, illustrated 
in Fig. 5b, the population of labeled 
oligonucleotides is derived directly 
from the Haelll fragment of 5S DNA 
which contains the A+T-rich spacer re- 
gion and extends to the 8th residue of 
the gene sequence (Fig. 2). The coding 
strand of the fragment is labeled at the 
5' end using a- 32 P-labeled-ATP and 



polynucleotide kinase. Label is intro- 
duced in only one of the two strands, 
essentially as described by Maniatis et 
al. (Proc. Nat. Acad. Sci., U.S.A. 72, 
1184, 1975). A larger Haelll fragment 
containing a Hindlll site is labeled at 
both ends and cleaved with Hindlll. 
The two resulting fragments, each 
labeled in only one strand, are recov- 
ered separately. The labeled fragment 
of interest is then nicked internally 
with DNase I at a low level. The nicks 
are enlarged into gaps by the 3 ' to 5 ' 
exonuclease activity of either the T4 
DNA polymerase or snake venom 
phosphodiesterase. The gapped com- 
plex is equivalent to the extended 
primer-template complex in Fig. 5a, 
and subsequent steps in the sequencing 
procedure are identical. The fragments 
are extended with DNA polymerase in 
the presence of either one or three of 
the cold nucleotides and the lengths of 
the specifically terminated fragments 
are determined on a gel. As with the 
previous method, we observed a dif- 
ferential loss of short oligonucleotides. 
Using a fragment of known sequence 
(the intragenic Haelll fragment), we 
have been able to derive an accurate 
sequence to within about 20 nuc- 
leotides from the 5 ' end of the 
molecule. Another difficulty we have 
encountered is that certain nicks in- 
troduced by DNase I appear to be rela- 
tively resistant to one or the other of 
the exonucleases used. 

In general, the strengths and weak- 
nesses of these enzymatic approaches 
are those discussed by Sanger and 
Coulson. The principal advantage of 
this method is that each terminal nuc- 
leotide is identified separately and 
each sequence is, in effect, determined 
twice within the same experiment. A 
shortcoming of this method is in 
sequencing runs of the same nuc- 
leotide, since a run is usually repre- 
sented only by the oligonucleotide ter- 
minating just before the run and the 
oligonucleotide containing all mem- 
bers of the run. 

In addition to the enzymatic methods 



20 



CARNEGIE INSTITUTION 



described above, we are using a chemi- 
cal sequencing method for DNA which 
has been developed in the laboratory of 
W. Gilbert. In this procedure, termi- 



nally labeled DNA fragments are 
cleaved chemically at specific bases, 
followed by the gel electrophoretic sep- 
aration described above. 



THE STRUCTURE OF RIBOSOMAL DNA FROM Xenopus 

R. H. Reeder, P. Botchan, and I. B. Dawid 



The nontranscribed spacer regions in 
Xenopus rDNA are heterogeneous in 
length; the physical basis for this 
length heterogeneity was described in 
Year Book 74 (pp. 19-36). The studies 
of spacer structure involved the use of 
cloned fragments of rDNA generated 
with the restriction endonuclease 
EcoRI. EcoRI cuts each repeating unit 
of rDNA (gene plus spacer) into just 
two fragments — one containing only 
gene sequences and a second fragment 
that contains all of the nontranscribed 
spacer region plus some gene region on 
either end. Four of these spacer con- 
taining fragments (each with a dif- 
ferent length spacer) have been cloned 
using a bacterial plasmid as vector. 
These cloned EcoRI fragments were 
used for the heteroduplex mapping ex- 
periments reported last year and have 
also been used in the restriction map- 
ping described here. The structure of 
one of the fragments, Xlrl4, is shown 
in Fig. 6. 



28S a B 



500 f 900 800 
430 



760 660 660 660 
BAM 



Fig. 6. Structure of the cloned rDNA fragment 
XIrl4. This fragment was generated by cleavage 
with EcoRI. As shown on the upper map, it con- 
tains some transcribed gene region on each end 
and all of the nontranscribed spacer region in the 
middle. The location of regions A, B, C, and D 
were defined from heteroduplex mapping. The 
dotted lines indicate regions that contain repeat- 
ing sequences. The lower map shows the location 
of BAMHI cutting sites on the same fragment. 
Molecular weights are in base pairs. 



Measurement of heteroduplexes 
formed between long and short cloned 
spacers led to the conclusion that each 
spacer contains two regions, each com- 
posed of a short repeating nucleotide 
sequence. One such region (region B in 
Fig. 6) is near the 28S sequence and the 
other (region D) is near the 18S se- 
quence. Regions B and D appear to be 
separated from each other by a non- 
repetitious sequence (region C). In 
order to gain more insight into the 
structure of the nontranscribed spacer, 
we have begun to dissect these cloned 
fragments with other restriction en- 
zymes. 

The procedure we have used for 
mapping restriction cuts is as follows. 
Cloned spacer-containing fragments 
were labeled at both ends of the 
molecule using polynucleotide kinase 
to put a 52 P atom on the 5' end of each 
DNA chain. The label at the 28S end 
can be eliminated by restriction with 
endonuclease Hindlll, which cleaves 
once, 500 bases from the 28S end. If the 
fragment now labeled only at the 18S 
end is partially cleaved with a restric- 
tion endonuclease, an overlapping set 
of fragments is created, all possessing 
the same :i2 P labeled 18S termini. If 
the partial digestion products are then 
electrophoresed on a 1% agarose gel 
and their positions determined by au- 
toradiography, the order of the termi- 
nal digestion products can be deter- 
mined by subtracting the molecular 
weight of one partial product from the 
molecular weight of the next largest 
partial product. This procedure has 
been used for ordering the fragments 
produced by endonucleases Smal, from 
Serratia marcescens, and BAMHI, from 
Bacillus amyloliquefaciens. Results of 



DEPARTMENT OF EMBRYOLOGY 

12 13 14 15 

6450- 
5150- 

4000 - 



2420- 



1430- 



"ip(w- 



860 



500- W Jf «* m - 

Fig. 7. Autoradiographs of partial Smal digests 
of four cloned spacer containing fragments. Four 
EcoRI-generated spacer fragments (Xlrl2, 13, 
14, and 15) were end-labeled with polynucleotide 
kinase, digested with Hindlll to remove the 28S 
end, and then partially digested with Smal. After 
electrophoresis on 1% agarose the gels were 
dried and autoradiographed. The bands at 5150 
and 6450 are from ColEl plasmid carrier. 
Molecular weights are in base pairs. 

Smal partial digests on four cloned 
spacer-containing fragments are 
shown in Fig. 7 (the 500-base-pair ter- 
minally labeled fragment has not been 
separated out and appears as the 
bottom-most band). The Smal diges- 



21 



tions reveal that the fragments pro- 
duced from the 18S end of all clones are 
identical up to a point 2420 base pairs 
from the 18S end. At this point the lo- 
cation of Smal sites diverges sharply 
among the different clones. The loca- 
tion of the divergence boundary agrees 
well with previous estimates of the 
gene-spacer boundary made from 
RNA-DNA heteroduplex measure- 
ment. Therefore, it appears that the 
nontranscribed spacer region shows 
some divergence close up to the gene- 
spacer interface. Within the transcrip- 
tion region, however, no sequence var- 
iation has been found among the four 
clones. 

Smal also appears to recognize a re- 
peating sequence in region B. The 
clearest example is Xlrl2 in Fig. 7 
where Smal produces a series of finely 
spaced bands starting at 4000 bases 
from the 18S end. 

Mapping of the restriction fragments 
produced by BAMHI reveals that this 
enzyme recognizes sites only within 
the spacer region of all four clones. 
With three of the four clones BAMHI 
produces a fragment of about 3300 base 
pairs containing the 18S end, and from 
the same clones a fragment of about 
2100 base pairs containing the 18S 
end. Internal to these terminal frag- 
ments is a region of expanding size 
which accounts for most of the size 
heterogeneity of the four clones. The 
internal spacer fragments vary from 
about 500 base pairs on the smallest 
clone to a 660 base-pair fragment that 
tandemly repeats itself five times on 
the largest clone. The exact order of 
BAMHI fragments on cloned Xlrl4 is 
shown in Fig. 6. 

Further restriction analysis has been 
done on the 2100 base-pair BAMHI 
fragment containing the 28S end. This 
fragment overlaps region B (Fig. 6) 
which has been shown by electron mic- 
roscopy to contain a highly repeating 
sequence 100 base pairs or less in size. 
Digestion of the BAMHI fragment with 
either of the restriction endonucleases 



22 



CARNEGIE INSTITUTION 



Hpall, RII, Smal, HHA or Haelll re- 
veals that all recognize a higher order 
repeating sequence of about 250-300 
base pairs in length. Since all these en- 
zymes recognize either the sequence 5 ' 
CCCGGG 3 ' or a simple variation of 
this, it is possible that the basic small 
repeat in this region is closely related 
to this sequence. In summary, the re- 
striction analysis when combined with 
the electron microscopy reported last 
year indicates that: (a) the length 
heterogeneity of the clones is largely 
confined to one region of the spacer, 



near the 28S end (region D; Fig. 6); (b) 
the spacer is composed of a short re- 
peating sequence that is probably 100 
base pairs or less in size; (c) the D re- 
gion and possibly the B region also ex- 
hibit a higher order repeating struc- 
ture superimposed upon the smaller 
sequence repeat. The latter two obser- 
vations are characteristic of all "satel- 
lite" DNAs so far analyzed with re- 
striction enzymes. The interest in this 
case is that the satellite is regularly 
interspersed between repeated gene 
sequences. 



STUDIES OF HUMAN X-CHROMOSOME DNA 



B. Migeon 



We have begun attempts to isolate 
the human X-chromosome or portions 
of it. The X-chromosome in mammals 
is subject to a control mechanism 
termed heterochromatinization in 
which all but one X-chromosome in a 
cell are in a condensed, inactive form in 
interphase cells. Although there has 
been much speculation, little is known 
about the molecular basis for this 



unique form of gene regulation. 

In metaphase, inactive X-chromo- 
somes (although indistinguishable 
from their active homologues by all 
chromosome staining techniques) can 
be identified because they replicate 
very late during DNA synthesis and 
are the last chromosomes to terminate 
replication. The inactive X is also iden- 
tifiable in interphase nuclei because of 




- 1.750 



-1.700 



650 



600 



10 15 

Fraction Number 

Fig. 8. BrdU-labeled DNA from isolated chromosomes centrifuged to equilibrium in CsCl. The 
purified chromosome fraction of human lymphocytes that had been stimulated to divide with a 
mitogen was labeled with :1 H-BrdU for 1 hr and chased for 3 hr. cts/min (x x) A. (1( , ( ). 



DEPARTMENT OF EMBRYOLOGY 



23 



its highly condensed state at a time 
when the majority of DNA from other 
chromosomes is dispersed. It may be 
possible to isolate this highly con- 
densed X (or Barr body) from inter- 
phase cells, but this requires a highly 
specific assay for X-chromosome DNA. 

As a preliminary step we have ini- 
tiated experiments designed to obtain 
X-specific DNA sequences that may 
provide the essential probe for the Barr 
body isolation. We are attempting to 
isolate X-specific DNA from the inac- 
tive X-chromosome because the timing 
of its replication enables us to label the 
X-DNA selectively with bromo- 
deoxyuridine (BrdU). The newly syn- 
thesized DNA has a greater buoyant 
density and can be separated on that 
basis from other chromosomal DNA. 

We have established short-term cul- 
tures of leukocytes from the peripheral 
blood of human females and exposed 
these cells to a one-hour pulse of 3 H- 
BrdU (S.A. 1 Ci/mM) during the termi- 
nal period of DNA synthesis. After sev- 
eral hours cells that had incorporated 
BrdU into DNA reach metaphase and 
are blocked at that stage with col- 
chicine. The next step is to purify 
chromosomes from interphase cells in 
the culture. Chromosomes are sepa- 
rated from contaminating interphase 
nuclei by velocity sedimentation in a 
sucrose gradient. The fractions con- 



taining chromosomes — but no nu- 
clei — are pooled, and the DNA is 
extracted. Only late replicating DNA 
incorporates the BrdU, thereby alter- 
ing the buoyant density of DNA and 
permitting the separation of the 
BrdU-substituted DNA from the un- 
substituted DNA by cesium chloride 
density gradient centrifugation. 

The BrdU-labeled DNA can be a 
template for the synthesis of com- 
plementary RNA and the cRNA ob- 
tained can be hybridized in situ to 
human metaphase chromosomes to de- 
termine its specificity. 

Examination of chromosomes from 
cultured cells obtained just prior to cell 
lysis has revealed that under the condi- 
tions used, BrdU is incorporated into 
the chromosomal DNA — and that most 
of the label is confined to the 
X-chromosome. However, under these 
conditions, only a few metaphase cells 
have labeled chromosomes. We have 
been able to obtain a small smount of 
BrdU-labeled DNA isolated as a satel- 
lite of the main band (Fig. 8). The 
quantity, however, has so far been in- 
sufficient to use as template for cRNA 
synthesis. 

We are attempting to increase the 
field of specifically labeled DNA and 
decrease the small amount of contami- 
nation released from breakdown of in- 
terphase nuclei. 



CHROMOSOMAL PROTEINS AND THE CONTROL OF GENE 

FUNCTION 

R. H. Reeder, A. Bokhon'ko, andH. L. Wahn 
with the assistance ofE. Hogan 



We have continued to improve the 
method for isolating nucleoli from 
Xenopus oocytes so that we can recover 
not only the amplified ribosomal DNA 
as chromatin but also several enzyma- 
tic activities associated with it. We 
have demonstrated both RNA poly- 
merase I activity previously known to 
be associated with rDNA as well as an 
enzymatic activity that relaxes 
superhelical turns in DNA. Some evi- 



dence suggests this latter activity may 
be involved in transcribing rDNA. 

In Year Book 74, pp. 27-28, we pre- 
sented evidence that at least some frac- 
tion of the rDNA in Xenopus cultured 
cells is in subunit configuration with 
chromosomal proteins. This year we 
expanded this observation by demon- 
strating that the A+T-rich satellite 
DNA of mouse is also organized with 
proteins in the same subunit fashion. 



24 



CARNEGIE INSTITUTION 

















1.25 
















1.24 
1.23 
1.22 r 
1.21 




1.0 


_ 


Nucleoli 








2.0 








1 










1 






h 






Whole 




i 


' 










Cells " 


1.5 l 


o 


























Nuclei 


\ 


6 


< 


0.5 




1 ' '\ 

/ ' • \ 
/ ' < \ 




\ 


0_ 








/; r 
/ 1 * 








o 

1 

I 








/ 1 ° 








fO 








/ ? . 

y / / • 

;_ / 








0.5 £ 
o 

E 
a. 




r\ m 


a_S 


•B-W" * 1 




1 


W*<o 







u 


5 10 




15 


20 








Fract 


ion 


No. 







nIOOO 

< 

- 500 or 

k. 

Q_ 
CVJ 

ro 
-10 



Fig. 9. Isolation of amplified nucleoli on a metrizamide density gradient. A homogenate of immature 
Xenopus oocytes was used to pour a metrizamide density gradient as described in the text. Fractions 

were analyzed for light scattering. A 650 (• •); ribosomal DNA hybridization (• •); and RNA 

polymerase activity (O O). 



Isolation and Characterization 
of Amplified Nucleoli 

H. L. Wahn andR. H. Reeder 

We have improved the previous 
method for isolating amplified nucleoli 
by the addition of a density gradient 
separation as the final step. Immature 
pigment-free oocytes were isolated es- 
sentially as described in Year Book 73 
(p. 28). The oocytes were then homo- 
genized in a solution of metrizamide 
and the homogenate used to pour a 
linear metrizamide density gradient. 
Metrizamide is an inert tri-iodinated 
benzamido derivative of deoxyglucose 
which can be used to form gradients 
that cover the entire density range of 
biological macromolecules. Since met- 
rizamide is non-ionic, the gradients do 
not dissociate nucleoli or other cellular 
organelles and they can be isolated in 
an apparently native state. 

Figure 9 shows the result when a 
metrizamide density gradient was used 



to fractionate a whole oocyte homoge- 
nate. Examination of each fraction by 
phase contrast microscopy revealed 
that nucleoli were banded at a rela- 
tively high density location well sepa- 
rated from contaminating nuclei. 
Measurement of light scattering at 650 
nm (metrizamide absorbs ultraviolet 
light) showed a peak of scattering that 
coincided with the location of nucleoli 
in the gradient and which coincided 
with a peak of rDNA as measured by 
hybridization with radioactive rRNA. 
If nucleoli were rebanded in a second- 
round metrizamide gradient, it was 
possible to reduce bulk chromosomal 
DNA contamination to less than 10% of 
the total DNA found in the nucleolar 
peak (as judged by analytical ultracen- 
trifugation analysis). 

Nucleoli isolated by this method 
have all the characteristics expected 
for nucleoli that are active in rRNA 
synthesis. As seen in Fig. 10 they show 
RNA polymerase activity. This RNA 
polymerase is sensitive to actinomycin 
D and resistant to high concentrations 



DEPARTMENT OF EMBRYOLOGY 



25 



of a-amanitin. Therefore, it has the 
properties of the template requiring 
polymerase I or nucleolar polymerase. 
In addition, these isolated nucleoli ex- 
hibit the normal morphology charac- 
teristic of active nucleoli when 
analyzed by electron microscopy, and 
they contain both 40S and 30S rRNA 
precursor. 

However, when nucleoli are re- 
banded on a shallow metrizamide gra- 
dient, it is apparent that not all are 
equally active in transcription. Figure 
10 shows another gradient in which 
RNA polymerase activity and rDNA 
amount were measured in each frac- 
tion. The more dense nucleoli have a 
much higher polymerase to rDNA ratio 
than do the less dense nucleoli. Al- 
though we do not yet know the basis for 
this subfraction of nucleoli, it has 



1500- 



1000- 



m 500 

CVJ 



•^ 4 
o 

< 

S 2 

x 

o 

0) 

a: 



rDNA 




/ 


\ Bulk DNA 




\ 9 \ 
\ / o 

V 


Relaxing 
Activity 


Polymerase 


- j! W*A 

i/ \ \ / 


or J 

-•-U 1 


1 1 



- 500 



.6, 
< 

rr 

o 

< 



250 *-> 

3 

m 

o 

V 
i 

0.3 ^ 

Q_ 

I- 
-|0.2 V 

X 



5 10 15 

Fraction no. 

Fig. 10. Association of RNA polymerase- 
relaxing and DNA-relaxing activities with nuc- 
leoli. Nucleoli were banded in a second-round 
metrizamide density gradient, and fractions 
were assayed in various ways. Upper panel: 

Ribosomal DNA hybridization (• •). Bulk 

chromosomal DNA hybridization (o o). 

Lower panel: DNA-relaxing activity (• •). 

RNA polymerase activity (o o). 



proved very useful in helping to deter- 
mine which proteins are associated 
with transcriptionally active rDNA. 

Figure 11 shows an SDS-acrylamide 
gel electrophoresis of total nucleolar 
proteins from each fraction across a 
second-round metrizamide gradient. 
The distribution of RNA polymerase 
activity and rDNA is also shown for the 
same gradient. Our next major task is 
to sort out these proteins, subtract 
those which are involved only in ribo- 
some assembly, and determine the 
function and location of those proteins 
which bind to the rDNA. Some pro- 
teins, such as the four major histones, 
have distributions that appear to coin- 
cide with the distribution of rDNA. 
Several other protein bands, however, 
follow the distribution of RNA 
polymerase activity and these are of 
prime interest for future study. 

One highly interesting protein that 
distributes with the polymerase activ- 
ity is an enzyme that relaxes super- 
coiled DNA. To assay for this enzyme, 
nucleolar fractions were incubated 
with supercoiled Col El plasmid DNA 
and the DNA was electrophoresed on a 
1% agarose gel. A densitometer trace of 
the stained gel is shown in Fig. 12. 
Some of the supercoiled DNA has been 
completely relaxed while some of it has 
been trapped at intermediate levels of 
supercoiling, giving rise to a series of 
discretely spaced bands in the gel. The 
relative level of DNA-relaxing enzyme 
was assessed by measuring the amount 
of completely supercoiled DNA left 
after incubation for a standard time 
interval. The distribution of relaxing 
activity across a metrizamide gradient 
is shown in Fig. 10. It is clear that the 
relaxing enzyme tracks with RNA 
polymerase activity and not with the 
rDNA distribution. This suggests that 
the relaxing enzyme may be an inte- 
gral part of the transcription ma- 
chinery. It seems unlikely that the pre- 
sence of this relaxing enzyme is due to 
the presence of DNA replication, since 
these nucleoli are in a replication dead 
end. They have previously replicated 
their DNA weeks to months before iso- 



26 



CARNEGIE INSTITUTION 



H 123456789 




-130 

- 68 

- 60 

-43 



25.7 



H2b 



H3 



* 



H2a 
H4- ■ 



- 17.2 

- 11.7 



1500- 




3 4 5 6 7 8 9 

Fraction number 

Fig. 11. SDS-acrylamide gel electrophoresis of proteins from nucleoli banded in metrizamide. Nuc- 
leoli were banded in a second-round metrizamide gradient, and protein from each fraction was elec- 
trophoresed on a 15% acrylamide-SDS gel. Aliquots of each fraction were also assayed for RNA 

polymerase activity (o o) and ribosomal DNA location (• •). Lane H shows acid-extracted 

Xenopus histones. Molecular weights are in daltons x 10~ 3 . 



lation and they will never enter the re- 
plication cycle again. Furthermore, we 
cannot detect any significant DNA 
polymerase activity associated with 
these nucleoli even after adding an 
exogenous template. 



Since amplified rDNA is most likely 
present as circular molecules, it is 
quite reasonable that a relaxing en- 
zyme might be necessary for some 
phase of transcription. Initiation by 
RNA polymerase requires localized 



DEPARTMENT OF EMBRYOLOGY 



27 




Fig. 12. Assay of nucleolar DNA relaxing en- 
zyme. Supercoiled ColEl plasmid DNA was in- 
cubated in the presence (upper trace) and in the 
absence (lower trace) of nucleoli purified through 
metrizamide. The ColEl DNA was then elec- 
trophoresed on a 1% agarose gel, and the DNA 
was located by staining with ethidium bromide. 
Direction of electrophoresis is from right to left. 
Supercoiled ColEl (major peak in lower trace) 
has a higher mobility than relaxed circular 
ColEl DNA (major peak in upper trace). The in- 
termediate bands differ from each other by inte- 
grals of one supercoil turn. 



strand separation of the DNA and 
therefore must introduce superhelical 
strain into the DNA. It is also possible 
that chain elongation causes similar 
physical changes in DNA structure, 
especially if the polymerase must dis- 
lodge histones as it transcribes the 
gene. Therefore, an enzymatic activity 
that would release physical strains 
placed on the DNA by transcriptional 
processes would be required to return 
the DNA to its normal state. 



The Subunit Organization of 
Mouse Satellite Chromatin 



A. Bokhon'ko 



Most of the DNA in a eukaryotic 
chromosome is complexed with his- 
tones to form subunits resembling 



beads on a string. This structure can be 
observed with the electron microscope 
and can be demonstrated in several in- 
direct ways. One property of this sub- 
unit organization is that DNA con- 
tained within the bead is more resis- 
tant to endonuclease attack than is 
DNA between the beads. Thus, limited 
digestion of chromosomes with nu- 
clease cleaves the DNA into small 
fragments that are multimers of about 
200 base pairs of DNA. An example of 
this phenomenon is shown in Fig. 13. 
In this experiment mouse liver nuclei 
were briefly digested with nuclease, 
and the DNA was deproteinized and 
electrophoresed on an acrylamide gel. 
An oligomeric series of bands is ob- 
served, resulting from histone protec- 
tion in the chromatin subunits. 

It is of interest to know if there are 
any specific DNA sequences that are 
exempt from this type of subunit or- 
ganization. Last year we presented 
evidence that some fraction of the 
ribosomal genes were so organized and 
suggested that histones may be present 
on actively transcribed sequences. This 
year we have used similar techniques 
to examine the A+T-rich satellite se- 
quences in mouse DNA. This DNA is 
located in highly condensed cen- 
tromeric heterochromatin and appears 
never to be transcribed in the cell. 

The experiment is shown in Fig. 13. 
After brief digestion of mouse liver 
nuclei with micrococcal nuclease the 
protected DNA fragments were elec- 
trophoresed on an acrylamide gel. The 
gel was then sliced and the DNA in 
each slice was hybridized with radioac- 
tive RNA transcribed in vitro from 
purified mouse satellite L-strand. The 
hybridization essentially follows the 
distribution of the bulk DNA, and we 
conclude that mouse satellite is also 
organized in a similar subunit fashion. 

Together, these results increase the 
likelihood that histones are associated 
with all of the chromosomal DNA re- 
gardless of its function. 



28 



CARNEGIE INSTITUTION 




SLICE NO. 

Fig. 13. Hybridization of mouse satellite cRNA to micrococcal nuclease fragments from mouse liver 
nuclei. Mouse liver nuclei were partially digested with micrococcal nuclease, and the DNA fragments 
were purified and electrophoresed on a 6% acrylamide gel. The gel was sliced and the DNA in each 
slice was hybridized with :! H cRNA transcribed in vitro from the L-strand of purified mouse satellite 
DNA. The bulk DNA was located by staining with ethidium bromide, (a) Fragments from nuclei 
digested for 5 min. (b) Fragments from nuclei digested for 20 min. The arrow indicates the position of 
the presumed monomer at approximately 170 base pairs. 

THE POLARITY OF RIBOSOMAL DNA IN Xenopus 

I. B. Dawid, R. H. Reeder, P. K. Wellauer, T. Higashinakagawa 
in collaboration with O. L. Miller, Jr.* 



Ribosomal RNA (rRNA) in Xenopus 
and other eukaryotic nuclei is trans- 
cribed as a precursor molecule contain- 
ing both 18S and 28S RNA regions and 
transcribed spacer segments. The ques- 
tion of whether the 18S or the 28S re- 
gion is closer to the 5' end of the pre- 
cursor has been answered in opposing 
ways (Year Book 69, 565; Year Book 72, 
38; Year Book 73, 45; and Year Book 
74, 20). We have undertaken a re- 
investigation of this question which 
leads to the strongly supported conclu- 
sion that the 18S RNA sequence is 
closer to the 5' end (transcription ori- 
gin) of the precursor than is the 28S 

*University of Virginia. 



RNA sequence. This order of sequences 
is almost certainly correct for rRNA 
precursor molecules of other 
eukaryotes as well. Transcription in 
the direction of small-to-large rRNA is 
a general feature of genomes, including 
prokaryotes, eukaryotic nuclei, 
chloroplasts and mitochondria. A 
model of a segment of Xenopus laeuis 
ribosomal DNA with the correct polar- 
ity of the precursor is shown in Fig. 14. 

Determination of Polarity with 
DNA Exonucleases 

The first test of polarity in rRNA 
that we describe depends on the 



DEPARTMENT OF EMBRYOLOGY 
5' 



29 



3' 



5' 



//- 



40S -*- I // 

I8S 28S NTSp I8S 



Xlr 



3 5 



H h 



// 



5" 3' Xlr 12 



X- EXONUCLEASE 



18 S 



28 S 



EXONUCLEASE IE 



28 S 



I8S 



Fig. 14. Determination of rDNA polarity with DNA exonucleases. The figure is described in the 
text. 



availability of fragments of rDNA 
whose ends have been mapped with re- 
spect to the rRNA (see Fig. 14) and of 
DNA exonucleases with known speci- 
ficities. Exonuclease III from E. coli 
(specific for 3' ends) and lambda 
exonuclease (specific for 5' ends) were 
donated by former fellows of our De- 
partment, John Chase and John Mor- 
row, respectively. The principle of the 
experiment is illustrated in Fig. 14. At 
the top we show a model of a segment of 
rDNA with the polarity that is sup- 
ported by the experiments reported 
here. EcoRI cleaves each repeat twice, 
creating two types of fragments. Such 
fragments have been cloned in E. coli 
and two particular ones are named 
Xlr 11 and Xlrl2. In Fig. 14 the coding 
strand is the "heavier" line and the po- 
larity of each strand is indicated in 
agreement with the polarity of the 
RNA. Such rDNA fragments were par- 
tially digested with each of the two 
exonucleases until about 20% of the 
DNA was hydrolyzed. The models in 
Fig. 14 illustrate the resulting 
molecules. Lambda exonuclease diges- 
tion of Xlrll removes the anti-coding 



strand in the 18S region, thereby ex- 
posing 18S sequences on the coding 
strand. At the other end of Xlrll the 
same exonuclease digests the coding 
strand only. The resulting DNA frag- 
ment with single-stranded tails is 
therefore expected to hybridize with 
18S rRNA only, as indicated in the fig- 
ure by dotted line. Similar models for 
the other fragments and both exonuc- 
leases are shown in Fig. 14. When the 
opposite polarity is assumed in the 
rRNA, the predictions for exonuclease 
III and lambda exonuclease are simply 
interchanged. We consider the con- 
clusion from these experiments to be 
firm because the specificities of the two 
exonucleases have been well estab- 
lished and because the test involves 
four separate experiments (two frag- 
ments, two exonucleases) all of which 
give highly consistent data in support 
of the same polarity. 

Determination of Polarity 

Using rDNA Transcription 

Complexes 

The second test of polarity makes use 
of the fact that ribosomal gene trans- 



30 



CARNEGIE INSTITUTION 



cription complexes can be spread for 
viewing in the electron microscope. 
This technique was first developed by 
O. L. Miller, Jr., at the University of 
Virginia, and this experiment was 
done with his collaboration. The nas- 
cent rRNA chains in such transcription 
complexes are seen to be short at one 
end of the complex (the 5' end of the 
precursor) and to increase in length 
until they are released at the other (3') 
end. To determine which end the 18S 
rRNA sequence was near, we digested 
transcription complexes with the re- 
striction enzyme EcoRI which cuts 
rDNA at two known locations within 
the transcribed region. 

The cut transcription complexes 
were then spread for electron micros- 
copy, and we measured the distances 
from cut sites to either termination (3' 
end) or initiation (5' end) sites. 



The experiment is diagrammed in 
Fig. 15. In panel (a) a full-length trans- 
cription complex is shown at top with 
the location of EcoRI sites indicated, 
assuming the 18S sequence is proximal 
to the 5' end of the precursor. Below 
are shown the four types of transcrip- 
tion complex fragments that would re- 
sult from this model. Four types of cut 
ends are generated because proteins on 
the rDNA block the EcoRI sites with a 
certain probability so that only partial 
digestion is achieved. Panel (b) shows 
the predicted fragments if the opposite 
polarity is assumed. 

Measurement of 100 EcoRI cut 
transcription complexes yielded results 
that agreed well with model (a) and 
strongly disagreed with model (b). This 
supports the conclusion that the 18S 
sequence is proximal to the 5', or initi- 
ation, end of the rRNA precursor. 



a) 5'-l8S 



* «H .„„ I — h 



I8S 28S 

t t 

EcoRI EcoRI 



b) 5'-28S 



I I ,„„ I — ' — f 



28S ' ' I8S 

t t 

EcoRI EcoRI 



.33 



.93 



.07 



.67 



.67 



.33 



.93 



Fig. 15. Determination of rDNA polarity using EcoRI on transcription complexes. The figure is 
described in the text. 



RIBOSOMAL DNA IN Drosophila melanogaster 

P. K. Wellauer and I. B. Dawid 

in collaboration with K. D. Tartof of the Institute for Cancer Research Philadelphia, 

and with the assistance ofM. Rebbert 

The genes that code for the large occur in multiple copies. The ribosomal 
ribosomal RNA (rRNA) molecules DNA (rDNA) forms clusters of redun- 



DEPARTMENT OF EMBRYOLOGY 



31 



dant genes in the nuclei of all or- 
ganisms studied. Since the rDNA of 
many species has special physical 
properties, it may be isolated and 
characterized. The rDNA of Xenopus 
laevis was the first eukaryotic gene to 
be purified (Year Book 67, 401), and 
this DNA has since been characterized 
extensively (see Year Book 74, p. 19, 
and this Year Book). In spite of the fact 
that Xenopus rDNA had been isolated 
some time ago and the isolation of 
rDNA from other organisms seemed 
technically feasible, few reports on the 
purification and detailed characteriza- 
tion of rDNAs have appeared in the lit- 
erature. It is clear that the properties 
of rDNA are of great interest, and that 
generalizations about the structure 
and evolution of this DNA can be based 
only on the comparison of results with 
several organisms. We decided to study 
rDNA in Drosophila melanogaster be- 
cause this organism offers the advan- 
tage of producing many known mu- 
tants that affect the ribosomal locus, 
and because it shows the interesting 
phenomena of compensation and mag- 
nification of rDNA. 



Purification of Drosophila rDNA 

In constrast to X. laevis, Drosophila 
rDNA is not a density satellite, i.e., the 
buoyant density of rDNA is very close 
to the density of the average DNA. We 
achieved purification of the rDNA in 
two different ways. In the first proce- 
dure Drosophila DNA, purified from 
embryos, was centrifuged in cesium 
sulfate in the presence of 0.2 moles of 
mercury ions per mole of nucleotide. 
The rDNA was located by hybridiza- 
tion with rRNA, the corresponding re- 
gion of the gradient pooled, and the 
DNA centrifuged in cesium chloride in 
the presence of actinomycin D. Again, 
rDNA was pooled and centrifuged in 
cesium sulfate with mercury at a ratio 
of 0.08. The rDNA pooled from this 
third gradient was usually "pure," 
which means that we estimate its pur- 
ity above 80% (see below). 

The second method for rDNA purifi- 



cation is based on the R-loop procedure 
developed by White and Hogness of 
Stanford University. When native 
DNA and RNA are annealed in high 
concentrations of formamide at the 
right temperature, the RNA invades 
the duplex and displaces one of the 
DNA strands. The resulting complex is 
suitable for mapping by electron mi- 
croscopy and also for preparative pur- 
poses, since the RNA/DNA hybrid is 
more dense than native DNA. Thus, 
Drosophila DNA was incubated with 
rRNA and centrifuged in cesium sul- 
fate. The rDNA region, which was 
slightly more dense than the main 
band of DNA, was pooled and cen- 
trifuged again in the same way. The 




r 



Fig. 16. Electrophoretic patterns of Drosophila 
rDNA digested with the restriction endonuclease 
Smal. Electrophoresis is from top to bottom; the 
gels were stained with ethidium bromide and 
photographed in ultraviolet light. Lane a, 
marker fragments from a Hindlll digest of 
lambda DNA; lane b, partially purified rDNA 
containing 10-20% rDNA; lanec, rDNA purified 
by the mercury-actinomyein D method. 



32 



CARNEGIE INSTITUTION 



resulting rDNA was pooled and the 
RNA removed. The product was effec- 
tively pure rDNA of high molecular 
weight. 

The purity of rDNA is most easily 
checked by digestion with the restric- 
tion endonuclease Smal. Figure 16 
shows that rDNA is cleaved into one 
class of fragments of about 8.5 kb 
(kilobases or kilobase pairs); this class 
is heterogeneous because of length 
heterogeneity in the rDNA repeating 
units (see below). Further, there are 
discrete fragments of 2.8, 2.3 and 2.2 
kb. Since Smal sites are rare in main- 
band DNA, contaminating DNA is 
largely uncleaved and appears in the 



gel as a broad band moving more 
slowly than the 8.5-kb fragment set. 
This is easily seen in Fig. 166, which 
shows the pattern given by an Smal di- 
gest of partially purified rDNA. By 
comparison, lane c of Fig. 16 shows lit- 
tle if any contaminating material. 



Analysis of EcoRI Fragments of 
Drosophila rDNA 

The restriction endonuclease EcoRI 
cleaves rDNA into a number of frag- 
ments (Fig. 17a). Predominant bands of 
16.5 and 11 kb are seen. In addition, 
there are bands of intermediate size, 



STAIN 



rRNA 

X 



Y 





I #H KB 



Fig. 17. Electrophoretic patterns of Drosophila rDNA digested with the restriction endonuclease 
EcoRI. Lane a, from wild-type male and female flies stained with ethidium bromide. Lane b, purified 
wild-type rDNA was digested and separated by electrophoresis, and the DNA fragments were trans- 
ferred to membrane filters as described in Southern (Journal of Molecular Biology 98, 503-517, 1975), 
and hybridized with labeled rRNA. The figure shows the autoradiogram of the hybridized filter. 
Lanes c and d, autoradiograms of rDNA patterns from DNA isolated from flies that carried a bobbed 
mutation on their Y chromosome so that at least 95% of the rDNA is derived from the X chromosome. 
Lanee, rDNA pattern from flies of the genotype sc 4 -sc K /Y in which the nucleolus organizer is deleted 
from the X chromosome. 



DEPARTMENT OF EMBRYOLOGY 



33 



and many bands smaller than 11 kb, 
among which two bands at 7.4 and 5.4 
kb are relatively abundant. 

Electron microscopy was used to 
analyze the structure of EcoRI frag- 
ments of rDNA. We first summarize 
the results and then illustrate some of 
the evidence for it. The fragments of 1 1 
kb and larger are the results of a single 
EcoRI cut in the 18S RNA region of 
each rDNA repeating unit. The 11-kb 
fragments have a structure similar to 
that of Xenopus rDNA: The 18S and 
28S rRNA regions are separated by a 
short spacer segment that is likely to 
be transcribed (Spl in Fig. 18B) and by 
a longer spacer segment that must con- 
tain mostly nontranscribed sequences 
(Sp2 in Fig. 18B). 

The 16.5-kb fragments contain a 
rather unexpected type of structure: In 
addition to the regions found in the 
11-kb fragments there is a spacer seg- 
ment that is inserted within the 28S 
gene region. We call this spacer the 
"insertion." Figure 18A shows that this 
insertion is located 1.13 kb from the 
end of the 28S gene. In Drosophila as in 
other insects, 28S rRNA as isolated has 
a break about in the middle. The posi- 
tion of this break is seen in the maps of 



Fig. 18 as a gap, suggesting that the 
break is generated from the original 
continuous transcript by the removal of 
about 140 bases. It is important to note 
that the position of the spacer insertion 
and the position of the gap are sepa- 
rated by 740 base pairs. The insertion 
occurs in different size classes. The 
most abundant class is a long insertion 
that is present in the 16.5-kb type of 
repeating unit. Other insertion sizes 
account for the EcoRI fragments found 
in the gel between the 11- and 16.5-kb 
positions. Remarkably, the insertions 
occur in size classes that are multiples 
of a basic size of close to 0.5 kb. 
Whether the larger insertions are ac- 
tually repetitions of the 0.5-kb se- 
quence is not known. EcoRI fragments 
smaller than 11 kb are produced by ad- 
ditional cuts in some repeating units of 
rDNA. 

Before we discuss the implications of 
these structures we show some of the 
evidence for them in Fig. 19. The figure 
shows hybrid molecules between single 
strands of rDNA EcoRI fragments and 
rRNA. In the DNA fragments that con- 
tain insertions, one 28S rRNA 
molecule hybridizes with its com- 
plementary sequences on both sides of 



Gap 



I8S Spl 

a 



28Sb 



0.20 1.23 



■ V////////;/7 )r V/////////77777l - 
! 1.87 



1.59 



Sp 2 



I8S 
1.56 



Ins 0.5 - 6.0 




28Sb' 28Sb" 



0.74 



1.13 



-ZZZZZZZZZZZA 



Fig. 18. Models of EcoRI fragments of Drosophila rDNA. These fragments constitute complete 
repeats. The EcoRI site is in the 18S RNA region. Spl is likely to be transcribed; Sp2 is mostly 
nontranscribed. The gap in the 28S RNA region is the position where an interruption occurs in the 
28S RNA. Sp2 is shown as a broken line, since length heterogeneity occurs in this region. Model B 
shows the 11-kb fragment; the numbers are the sizes of the different regions in kb. Model A shows the 
class of fragments that contain an insertion of spacer sequences within the 28S gene region. Those 
regions not labeled in model A have the same length as in model B. The insertion occurs in size classes 
from 0.5 to 6.0 kb and in multiples of 0.5 kb. 



34 



CARNEGIE INSTITUTION 







H^j[. ' ■'•,-■ ;^;\ *:-' ; v^ 



■■••■■- •-,• ■ >.-•••.. -, •■• • ■■. .-r— -*-. ■■*■-•• ', -¥£/ H l l >. '•;->;■ ••■>■• 








"TV. 


' s P 2 -•"-; H : ^, "M- 


;•'?-' 


' .:'V'<v^'" \ '•'•"• 


• : v ' "-'i-f 




>-.■••?••;,•'•; '■■s^SvfrZ. -v. * --^A^/} ■ •.■■^■:^.i> remiss* i,> 



ge-v- 



: > :.:'/■ : - ■■'■■*■: 



M 



Sp2 







:"-. n**"-->' "^ " / '■* •• -f -.■" "■ I - w ' ■ ■'( V 1 '- ■"»'' '*•'' ^' v '■^'1 • ""'~'' ' ■ ll '''' S' " r -•^^* , *fc-tl*-> 




;<i8s;+s P i • .• ^ .''"-/?; vL ' s ^* 1 .'.'- > ' < CJj. " -■ - • 



Fig. 19. Electron micrographs of hybrid molecules between EcoRI fragments of Drosophila rDNA 
and rRNA. Hybrids were prepared for electron microscopy by the formamide modification of the 
Kleinschmidt technique. The various regions are indicated and are explained in the legend to Fig. 18. 
Insertions are indicated by arrows. (A) 11-kb EcoRI fragment. (B) EcoRI fragment with a 1.5-kb 
insertion. (C) EcoRI fragment with a 5.0-kb insertion. 



DEPARTMENT OF EMBRYOLOGY 



35 



the insertion so that the insertion 
forms a loop. Such molecules, which 
were frequent, show most clearly that 
the position of the strand interruption 
in the 28S RNA and the position of the 
insertion in the gene are distinct. In 
Fig. 19 we show one 11-kb repeat and 
two insertion repeats, one from the 
major (16.5-kb) class and another 
molecule with a 12.5-kb repeat that 
has a 1.5-kb insertion. 

These observations raise the ques- 
tion of whether, and if so how, the 
rDNA repeats with insertions are 
transcribed. To produce intact 28S 
rRNA there would have to be trans- 
cription from gene sequences on both 
sides of the insertion with subsequent 
ligation of the products by a hypotheti- 
cal RNA ligase. Or, the RNA 
polymerase would "jump" across a loop 
of DNA, as the RNA does in the hy- 
brids of Fig. 19. Such a mechanism has 
not been described in any system. Al- 
ternatively, the genes with insertions 
may not be transcribed, or transcrip- 
tion may end at the insertion point (we 
assume that transcription begins to the 
left of the 18S region and proceeds to- 
wards the 28S region. In the latter case 
RNA transcripts would arise from the 
insertion-carrying genes, but they 
could not produce complete 28S rRNA 
and would be nonfunctional. One 
would then ask why the inserted genes 
have been conserved during evolution. 
At present we cannot choose among 
these alternatives and cannot answer 
the questions raised by the observa- 
tions. 

Length Heterogeneity in the 
Spacer Region 

In addition to the length differences 
in the insertion there is also length 
heterogeneity in spacer 2 (Fig. 18), 
which corresponds to the "standard" 
nontranscribed spacer of X. laevis 
rDNA. This length heterogeneity is 
'less extreme than in Xenopus, with 
most spacers falling into a fairly nar- 
row distribution. Nevertheless, we 



have obtained unequivocal evidence for 
the existence of heterogeneity by 
examination of heteroduplex 
molecules. EcoRI fragments of rDNA 
were separated on gels, and 11-kb, 
16.5-kb, and intermediate fragments 
were isolated. These fragments were 
denatured and allowed to reassociate. 
Many of the duplex molecules that 
formed had single-stranded deletion 
loops in the spacer region. Especially in 
those molecules where only a single 
loop appeared we may conclude that 
the spacers of the two strands which 
formed the duplex differ in length. 

Drosophila rDNA has two regions in 
which length heterogeneity occurs: the 
insertion, which comes in multiples of 
0.5 kb and accounts for most of the 
heterogeneity; and the nontranscribed 
spacer, which varies in length in a nar- 
row range without detectable size clas- 
ses. 

Arrangement of Length 
Heterogeneity 

We have tried to resolve whether 
rDNA repeats without insertions and 
with insertions of different length 
occur in clusters, or whether they are 
interspersed along the chromosome. 
High molecular weight rDNA was iso- 
lated, denatured, hybridized with 
rRNA, and inspected in the electron 
microscope. The results demonstrate 
that repeats of different length are in- 
terspersed. Repeats of 11 kb may occur 
next to 11-kb, 16.5-kb, or intermediate 
length repeats, and the same is true for 
all classes of repeats. A statistical 
evaluation of 85 pairs of adjacent re- 
peats shows that the frequencies with 
which different types of repeats are 
nearest neighbors are close to those ex- 
pected from a random assortment of 
the different length classes. In some 
cases a tendency toward clustering of 
similar repeats was seen, and we can- 
not decide whether the distribution is 
fully random or not. But it is clear that 
extensive scrambling of different re- 
peat sizes occurs in Drosophila rDNA, 



36 



as it does in the rDNA of Xenopus 
{Year Book 74, p. 22). 



Distribution of rDNA Repeat 

Lengths on the X and Y 

Chromosomes of D. Melanogaster 

\nD. melanogaster the rDNA is clus- 
tered in one nucleolus organizer each 
on the X and the Y chromosome. About 
200 repeats of rDNA compose each wild 
type nucleolus organizer. To test the 
structural properties of rDNA derived 
from the two genetic loci we digested 
DNA from the appropriate stocks with 
EcoRI, separated the fragments by 
electrophoresis, and hybridized the 
fragments with rRNA by the method of 
Southern. Figure 17 shows some of the 
results. While in resolution the hy- 
bridized gels do not match the stained 
gel of purified wild type rDNA, it is 
clear that the X chromosome carries 
both major classes of repeat length (11 
kb and 16.5 kb), while the Y chromo- 
some carries the 11-kb fragment but 
not the 16.5-kb fragment. Minor frag- 
ments occur on both chromosomes but 
are not well displayed in Fig. 17 c to e. 

These observations raise some intri- 
guing questions about the evolution of 
tandem repetitive gene families and 
about the "correction mechanism" pos- 



carnegie institution 

tulated to operate during this evolution 
(see Year Book 74, p. 19). There is prior 
genetic evidence against recombina- 
tion between the X and the Y chromo- 
somes. The absence of 16.5-kb frag- 
ments on the Y chromosome supports 
this evidence: If such recombination 
occurred, one would expect the 16.5-kb 
fragments to spread to the Y chromo- 
some unless some unknown mecha- 
nism prevents these repeats from oc- 
curring on the Y chromosome. If, how- 
ever, the X and the Y are genetically 
isolated, how are the similarities be- 
tween them maintained? These 
similarities are extensive, since rRNA 
transcribed from either chromosome is 
very similar and probably identical, 
and the 11-kb fragments and some 
other features of gene organization are 
closely similar in X and Y rDNA. If 
there were no genetic exchange, one 
would have to postulate that selective 
pressures maintain all these features 
on the two separate chromosomes. It 
appears to be difficult to adhere to such 
a hypothesis in its strict form. How- 
ever, we would like to raise the possi- 
bility that selection plays a larger part 
in the maintenance of homogeneity in 
tandem gene families than had been 
believed. Drosophila rDNA seems 
highly suited for experiments that will 
aid in understanding these processes. 



STRUCTURAL AND FUNCTIONAL STUDIES OF FIBROIN GENES 

Y. Suzuki, Y. Ohshima, P. Geshelin, and P. E. Giza 



We wish to know how a tissue- 
specific gene is transcribed in that tis- 
sue at a given time. More practically, 
our long-term goal is to understand fi- 
broin gene control by reproducing its 
function in vitro with the purified gene 
and other necessary components. Each 
gene system has certain advantages 
and disadvantages for this kind of 
study. The fibroin gene system in the 
posterior silk gland of Bombyx mori 
has the advantages that (1) the coding 
sequence is extremely simple, inter- 
nally repetitious, and of high G + C con- 



tent; (2) the gene comprises a high pro- 
portion of the genome; (3) its expres- 
sion is temporally regulated, and fi- 
broin mRNA comprises an unusually 
high proportion of cellular RNA at cer- 
tain developmental stages; and (4) iso- 
lated nuclei also synthesize the mRNA 
in high proportion. Making use of these 
advantages, we have been employing 
three approaches toward our goal: (1) 
development of methods for purifying 
fibroin genes, (2) development of 
methods for cloning genes, and (3) 
transcriptional analysis of fibroin 



DEPARTMENT OF EMBRYOLOGY 



37 



genes in vivo and in vitro. Some of our 
progress is summarized below. 

Physical Purification of the 
Gene Coding for Silk Fibroin 

P. Geshelin 

In order to characterize the manner 
in which silk fibroin synthesis is con- 
trolled, we need to isolate the DNA 
that codes for the protein. Single-copy 
genes from various organisms are 
being isolated in several laboratories 
at present using one of two general 
methods. In the first, a small amount of 
whole-organism DNA is partially 
purified by proved physical methods, 
enzymatically attached to plasmid 
DNA, and grown in infected bacteria. 
After the clone(s) of bacteria that con- 
tain the sought-after sequences are 
selected (usually by hybridization to 
the corresponding messenger RNA), 
the gene is replicated many times over 
by growth of the clone(s). In the second 
method the tiny amount of desired 
DNA is isolated physically from a huge 
amount of DNA from the whole or- 
ganism by hybridizing messenger RNA 
to the sheared DNA and separating the 
DNA-RNA hybrids from the rest of the 
DNA by utilizing the physical prop- 
erties of the RNA. One makes use of 
the higher density of the RNA, com- 
pared to DNA. This density differential 
can be increased by binding substances 
to the RNA before hybridization. Al- 
ternatively, the RNA can be covalently 
bound to a solid support (i.e., agarose 
beads) before hybridization, and the 
DNA not annealed to it after hybridi- 
zation may be simply washed away. 

Attempts are under way to isolate 
silk fibroin DNA by a modification of 
these physical methods. Messenger 
RNA coding for silk fibroin will be co- 
valently bound to mercury atoms by 
procedures developed by Ward and 
co-workers (Biochemistry 14, 2447- 
2457 and 2458-2469, 1975). The mes- 
senger RNA will then be mixed with 
whole-cell DNA and placed at the melt- 
ing temperature of the DNA in high 



formamide. Because RNA-DNA dup- 
lices are more stable than DNA-DNA, 
especially in the solvent formamide, 
the RNA displaces the minus or coding 
strand of DNA from the helix over the 
region to which the RNA is homolo- 
gous, forming a triple-stranded struc- 
ture termed an R-loop. These R-loops 
will then be separated from the rest of 
the nucleic acid by affinity chromatog- 
raphy on sulhydryl agarose, which 
selectively binds the mercury atoms. 

This new method could have several 
advantages over previous procedures 
for purifying single-copy genes. First, 
one will get both strands of the silk fi- 
broin DNA, in contrast to the methods 
involving melting and annealing, 
which yield only the coding strand. 
Second, affinity chromatography 
theoretically offers much greater reso- 
lution between mercurated and non- 
mercurated nucleic acids than does 
density centrifugation between DNA 
and RNA. Finally, since the resolution 
of the separation technique does not 
depend on DNA size, one can strive to 
isolate whole-gene DNA with large 
flanking sequences thought to contain 
important control sequences. 

Cloning of Fibroin Gene Plasmid 

Y. Ohshirna and Y. Suzuki 

The fibroin gene exists as a single 
copy per haploid complement. How- 
ever, the gene is so large (about 12-14 
x 10 6 daltons) that it comprises 0.004% 
of the B. mori genome. In order to be 
able to carry out the structural and 
functional analyses of the gene, we 
need to have large amounts of it in a 
pure form. Therefore, we are trying to 
obtain an E. coli clone that carries a 
recombinant plasmid including the 
whole fibroin gene with some adjacent 
DNA. Such plasmid DNA, when it is 
isolated, can be produced in milligram 
quantities and will be of crucial impor- 
tance for our experiments. 

The plasmid we have chosen is 
tetracycline-resistant Col El type 
plasmid PMB9. It has a molecular 



38 



CARNEGIE INSTITUTION 



weight of 3.5 x 10 6 and carries a repli- 
cation function in the presence of 
chloramphenicol and one restriction 
site each for the endonucleases EcoRI 
and Hindlll. In order to recombine this 
plasmid with a large DNA molecule 
containing the full-size fibroin gene 
and some extra sequences (about 25 x 
10 H daltons) we employ the dA-dT join- 
ing method. Covalently closed DNA of 
PMB9 was isolated and made into 
linear forms by EcoRI. For poly(dA) or 
poly(dT) addition we use terminal 
deoxynucleotidyl-transferase from calf 
thymus in the presence of 1 mM CoCl 2 
(in either the presence or the absence of 
MgCl 2 ). We have succeeded in adding 
the single-stranded tail of poly(dA) or 
poly(dT) directly to EcoRI ends of the 
plasmid DNA without pretreatment of 
the DNA by lambda exonuclease. Most 
of the dA or dT incorporation was found 
co-sedimenting with the plasmid DNA 
in a neutral or an alkaline sucrose gra- 
dient. When observed under the elec- 
tron microscope, 80-90% of the ends of 
the plasmid DNA have a single- 
stranded tail. 

For the fibroin gene part, in order to 
obtain the adjacent control regions as 
well as the whole coding sequence in a 
plasmid we must start with a high 
molecular weight preparation of B. 
mori DNA, since the gene is extraordi- 
narily large. We isolated a large quan- 
tity of DNA with a molecular weight of 
15-30 x 10 6 from frozen silk glands. 
After two successive CsCl-gradient 
bandings in the presence of actinomy- 
cin D, we obtained a DNA preparation 
in which fibroin gene DNA assayed by 
hybridization with 125 I-mRNA was en- 
riched by 40-fold (the gene concentra- 
tion is about 0.2%). With this DNA we 
propose to make poly(dT) tail additions 
to the poly(dA)-tailed PMB9 DNA fol- 
lowed by transformation of E. coli. E. 
coli clones carrying the fibroin gene- 
PMB9 plasmid will be selected by tet- 
racycline resistance and hybridizabil- 
ity with 125 I-mRNA of high specific ac- 
tivity (about 1 x 10 4 cpm/ng. 



Accentuated Expression of 

Fibroin Genes in vivo 

and in vitro 

Y. Suzuki and P. E. Giza 

We have developed a direct chemical 
method for quantitating fibroin mRNA 
in RNA mixtures {Year Book 73, p. 20). 
Using this method we have measured 
the amount of fibroin mRNA in the 
posterior silk gland of B. mori. Fibroin 
mRNA accumulates throughout the 
fifth instar, ultimately comprising as 
much as 4.4% of the total cellular RNA 
toward the end of the instar. The 
number of fibroin mRNA molecules ac- 
cumulated by the end of the fifth instar 
was 5.8 x 10 4 molecules/gene, about 
5-fold higher than the number of rRNA 
molecules (1.1 x 10 4 molecules/gene). 
This observation has led us to estimate 
the rates of fibroin mRNA and rRNA 
accumulation at various stages of the 
instar. As shown in Table 2, it was 
found that (1) the rate of mRNA ac- 
cumulation is almost constant 
throughout the stage, whereas that of 
rRNA changes as much as 30-fold, and 
(2) mRNA accumulation relative to 
gene number is proceeding at a higher 

TABLE 2. Rates of Accumulation of Fibroin 
mRNA and rRNA during the Fifth Instar 





rRNA 


Fibroin mRNA 


Time Intervals* 


molecules/gene/min 


Ecdysis - 1st day 
1st day - 2nd day 
2nd day - 4th day 
4th day - 6th day 
6th day - 7th day 


1.8 
6.6 
2.6 
0.3 
0.2 


< 1.2 

10 

7.9 

7.4 

9.3 



* Every 24-hr period after the fourth ecdysis 
was designated as 1st day, 2nd day, etc. The 
quantitation of the RNAs was done at ecdysis 
and at the end of 1st, 2nd, 4th, 6th and 7th days. 
The difference in amount of RNA between two 
time points was divided by the total number of 
genes and the time elapsed (24 hrs or 48 hrs in 
minutes). The number of genes was calculated 
from the amount of DNA present at the middle of 
the two time points, with the further assumption 
that each haploid complement of DNA contains 
240 rRNA genes and 1 fibroin gene. 



DEPARTMENT OF EMBRYOLOGY 



39 



rate than that of rRNA not only at 
later stages of the instar but through- 
out the instar except on the 1st day. 
The most exaggerated situation is seen 
on the 7th day of the instar. At this 
stage new rRNA synthesis almost 
ceases while mRNA accumulation con- 
tinues actively. These results as well as 
the consideration of the high propor- 
tion of fibroin genes in the genome 
have prompted us to try to detect fib- 
roin mRNA in pulse-labeled RNAs by 
our direct chemical method. 

The calculated proportion of fibroin 
mRNA synthesized and accumulated 
for different labeling periods of 10 to 
720 min at various stages of the fifth 
instar and the preceding fourth moult- 
ing stage are summarized in Table 3. 
As expected from the exaggerated situ- 
ation found on the 7th day of the instar, 
fibroin mRNA was detectable even by a 
10-min pulse, and occupied as much as 
about 4% and 6% of the total cellular 
RNA pulse-labeled on the 7th and 8th 
day of the instar, respectively (Table 
3). These are the highest proportions of 
transcription ever found for a single 
mRNA species. The proportion of 
mRNA becomes even greater with 
longer pulses, and reaches as high as 
24% of the total RNA labeled in a 
240-min pulse, indicating that fibroin 



mRNA is much more stable than other 
heterogeneous RNAs. When 30-min 
pulses were given at various stages of 
the fifth instar as well as the fourth 
moulting stage, it was found that fi- 
broin mRNA was not detectable during 
the moulting stage, but in contrast, as 
the fifth instar proceeded the propor- 
tion of mRNA increased from 1.2% up 
to 7.5% (Table 3). These observations 
indicate that the synthesis of RNAs of 
non-fibroin mRNA type is active at the 
earlier stages, decreasing drastically 
toward the end of instar, since it was 
already shown that the rate of fibroin 
mRNA accumulation is constant 
throughout the stage (Table 2). As for 
the moulting stage, we cannot deter- 
mine whether fibroin genes are inac- 
tive or whether their transcription is 
not detectable simply because of a 
rapid and selective degradation of fi- 
broin mRNA. 

From the proportion of fibroin 
mRNA (Table 3) and rRNA (not shown) 
synthesized in a 4-hr and a 12-hr pulse 
in the 7th day we calculated rates of 
each RNA synthesis per gene. The 
ratio of the rates of fibroin mRNA syn- 
thesis to that of rRNA synthesis was 
about 50 to 1 both for the 4-hr and 
12-hr pulses. This result indicates that 
fibroin mRNA is being synthesized 



TABLE 3. Proportion of Fibroin mRNA to Total Cellular RNA 
i Synthesized during Short-Pulse Periods* 







Time of Pulse Labeling 




10 min 


30 min 


90 min 240 min 


720 min 


Staget 






Proportion (%) 




Fourth moulting 


...* 


<1.7 




<0.5 


Fifth instar 










1st day 








0.6 


2nd day 




1.2 




1.8 


3rd day 




2.0 






4th day 




3.0 




4.6 


6th day 




4.8 




10 


7th day 


3.9 


7.5 


12 24 


27 


9th day 


5.9 









* Pulse-labeling experiments were done in the middle of the day. 
t Defined as in Table 1. 
tNot determined. 



40 



CARNEGIE INSTITUTION 



about 50 times more rapidly than 
rRNA. Because the ratios are the same 
between the two time points, we infer 
that rRNA and fibroin mRNA have 
similar stabilities during these label- 
ing periods and that the ratio of trans- 
cription for these genes would also be 
near 50 during shorter labeling periods 
at this stage. 

Because of this accentuated expres- 
sion of fibroin genes in vivo, we have 
tried to isolate and characterize silk 
gland nuclei aiming at transcription in 
vitro (Year Book 74, p. 36). The nuclear 
isolation method has been applied to 
the determination of the distribution of 
unlabeled and newly synthesized fi- 
broin mRNA in the cell. The results are 
shown in Table 4. More than 80% of the 
long lived mRNA which was identified 
by A 2 6 o oligonucleotide profile after 
T , RNase digestion was found in the 
cytoplasm, and only 8% in the nuclei. 
With our current nuclear isolation 
method we must admit the possibility 
of contamination of cytoplasmic RNA 
in our nuclear preparations. Therefore, 
we conclude that most, if not all, of the 
long-lived mRNA is localized in the 
cytoplasmic fraction. The result also 
indicates that with our current method 
we can eliminate about 90% of un- 
labeled fibroin mRNA from the nuclear 
preparation. The newly synthesized 
fibroin mRNA (in 30 min) was found 
about equally in the cytoplasmic frac- 
tion (47%) and the nuclear fraction 



(51%). This indicates that it takes less 
than 30 min to transport fibroin mRNA 
from the nucleus to the cytoplasm. 

RNA synthesis with disrupted and 
isolated nuclei using endogenous RNA 
polymerase was carried out at 25 °C. 
The incorporation of 3 H-UTP or a- 32 P- 
UTO was almost linear for 30 min, and 
then slowed, but a slight increase was 
still observed at 120 min. The labeled 
RNA was usually extracted after 30 
min incubation for characterization. 
RNA transcribed in vitro was hybri- 
dized to B. mori DNA fractionated by 
Act D/CsCl (Fig. 20). The hybridization 
conditions were such that only trans- 
cripts of repetitive sequences, which 
include fibroin mRNA because of its 
internal repetitive nature, hybridized. 
As shown in Figure 20, a prominent 
hybridization peak is seen at fractions 
9 to 11. This peak is presumed to be 
rRNA since it was competed for by cold 
rRNA. A small and heterogeneous 
peak at fractions 16 to 19, which is the 
region for fibroin genes, was competed 
for by unlabeled pure fibroin mRNA 
(see Table 5). It was also observed that 
virtually all of the counts hybridized 
were competed for by total posterior 
gland RNA. These results indicate that 
similar repetitive sequences are trans- 
cribed both in vivo and in vitro. 

The RNA labeled in vitro for 30 min 
was fractionated by a Bio-Gel A-50m 
column (Fig. 21a) and compared with 
the RNA labeled in vivo for 30 min 



TABLE 4. Localization of Fibroin mRNA in Cytoplasmic and Nuclear Fractions 



Fractions 



Newly Synthesized 

mRNA* 

(%) 



Long-Lived, 

Unlabeled mRNAt 

(%) 



Cytoplasmic sup (a) 
Sucrose sup (b) 
Nuclear pellet (c) 



47 

2 

51 



84 



Homogenate of posterior silk glands from the 7th day of the fifth instar was washed and centrifuged 
three times in the presence of NP40, and the combined supernates were designated as cytoplasmic sup 
(a). Crude pellet thus obtained was centrifuged through 2.2 M sucrose, and the supernate (b) and 
nuclear pellet (c) were obtained. 

* Posterior silk glands were labeled in vivo for 30 min with :i H-uridine and subjected to the 
fractionation mentioned above, and radioactive fibroin mRNA was quantitated by T, RNase 
fingerprint method. 

tThe long-lived, unlabeled mRNA was quantitated by A 260 oligonucleotide profile after T, RNase 
digestion. 



DEPARTMENT OF EMBRYOLOGY 



41 



ro 
i 
O 



u 




FRACTION NUMBER 

Fig. 20. Hybridization of RNA synthesized in 
vitro to B. mori DNA fractionated by Act D/CsCl 
and competition of the hybridization by un- 
labeled posterior silk gland RNA. The RNA was 
labeled with 500 /xCi of a- i2 P-UTP (23.7 Ci/ 
mmole) in vitro for 30 min in disrupted nuclei 
(170 jug of DNA). Aliquots equivalent to 5% of 
whole nuclear preparation from a pair of pos- 
terior silk glands were hybridized toS. mori DNA 
(200 yu.g/gradient) in the presence or absence of 
competitor at 50°C for 17 hr in 50% formamide-4 
x SET. No competitor added (•); competitor RNA 
equivalent to 20-fold excess of posterior-gland 

nuclei added (o o); competitor equivalent to 

200-fold excess of posterior-gland nuclei added 
(A A). 



(Fig. 21b). Comparison of Fig. 21a and 
Fig. 21b reveals that the radioactivity- 
profiles from Bio-Gel columns are simi- 
lar for RNA labeled in vitro and in 



vivo. However, there is a higher pro- 
portion of lower molecular weight 
RNAs labeled in vitro than in vivo. 
More than 55% of the radioactivity of 
the RNA in vitro was found in the re- 
gion larger than 28S (Fig. 21a). 

The void volume fraction shown in 
Fig. 21a, which contained 14% of the 
total radioactivity, was recovered and 
an aliquot was subjected to RNase T 1 
digestion for characterization. The 
sample was estimated to contain about 
15% fibroin mRNA (Fig. 22c). There- 
fore, we conclude that about 2.1% of the 
total transcripts synthesized in vitro by 
disrupted nuclei in 30 min was fibroin 
mRNA of apparent full-size length. 
This is the highest value ever found for 
a single mRNA species and is about 
two orders of magnitude higher than 
that of globin mRNA sequences syn- 
thesized in vitro. The hybridization 
assay was used to confirm the presence 
of mRNA sequences. An aliquot of the 
void volume fraction of Fig. 21a was 
hybridized to DNA in the fibroin gene 
region (Fig. 23). The hybrid was recov- 
ered from filters 10 to 15, digested with 
T j RNase, and the digest was subjected 
to a DEAE-Sephadex A25 column (Fig. 
22d). Figure 22d demonstrates the 
existence of fibroin mRNA sequences 
in the RNA in vitro, and a comparison 
of Fig. 22c (15% pure fibroin mRNA) 
with Fig. 22d (40% pure) indicates that 
mRNA was selected by the hybridiza- 
tion procedure. 

Nucleoplasmic RNA polymerase II is 
known to be sensitive to low concentra- 
tions of cv-amanitin and is possibly the 



TABLE 5. Inhibition of Fibroin mRNA Synthesized in vitro by Low Dose of a-Amanitin 



System of RNA Synthesis 



Hybridization to Partially Purified Fibroin Genes 



Without Cold Fibroin mRNA 



With Cold Fibroin mRNA 



Complete (a) 

Complete + a-amanitin (b) 



474 cpm 
156 cpm 



124 cpm 
112 cpm 



Nuclei containing 150 fj.g of DNA were incubated with 80 /iiCi of :! H-UTP for 30 min in the absence 
(a) or presence (b) of 0.2 /u.g/ml of a-amanitin. For assay 2.0 mg of 5. mori DNA was fractionated by 
Act D/CsCl, and 10% aliquots of the fractions containing fibroin genes were used for hybridization. 
For the hybridization 2.5% aliquots of each reaction mixture (a or b) were used in the absence or 
presence of unlabeled fibroin mRNA (5.1 /i.g/ml). Hybridization was done at 50°C for 2.5 hr in 5 ml of 
50% formamide-4 x SET. 



42 



CARNEGIE INSTITUTION 



O 



X 



o 




number 



Fig. 21. Bio-Gel fractionation of the RNAs synthesized in vitro by disrupted nuclei (a) and in vivo 
(b) in 30 min. (a) On the 7th day of the fifth instar disrupted nuclei were isolated from the posterior 
silk glands. An aliquot of the nuclei containing about 200 /xg of DNA was incubated with 250 /^Ci of 
a- ,2 P-UTP (10 Ci/mmole) and other ingredients in 1.0 ml volume at 25°C for 30 min. The RNA 
products were applied to a 2.6 x 66 cm column of Bio-Gel A-50m, 5.5-ml fractions were collected, and 
0.5-ml aliquots were counted after TCA precipitation, (b) A larva was injected with 2.5 mCi of 
'H-undine on the 7th day of the fifth instar and incubated at 25°C for 30 min. The RNA was extracted 

from the posterior gland and fractionated as in (a). cts/min/0.5ml (• •); A, (i „ monitored by a Gilson 

spectrophotometer ( ). 



polymerase form that is involved in 
transcription of the nuclear precursors 
to mRNAs. Therefore, we wondered 
whether fibroin mRNA synthesis in 
disrupted nuclei is sensitive to low 
doses of a-amanitin. RNA synthesis 
was carried out for 30 min in the pre- 
sence or absence of 0.2 fxg/ml of 
a-amanitin. The drug inhibited 21% of 
RNA synthesis in vitro. Each product 
was hybridized to partially purified 
fibroin genes in the presence or ab- 
sence of unlabeled pure fibroin mRNA 
as a competitor (Table 5). Fibroin 
mRNA, as defined by labeled RNA that 
hybridized to fibroin DNA and was 
competed for by excess cold fibroin 
mRNA, was not synthesized in the pre- 
sence of a-amanitin, thus providing the 
first direct evidence that RNA 
polymerase II is the form that trans- 
cribes structural genes in the living 
cell. 



We wanted to test the ability of the 
disrupted nuclei to initiate RNA syn- 
thesis in vitro. Incorporation of y- 32 P- 
ATP was carried out for 30 min. The 
RNA was fractionated on a Bio-Gel 
A-50m column (Fig. 24). Aliquots of 
fractions 14 to 19 were pooled and sub- 
jected to the following analysis. The 
sample was digested with RNase T t 
and the digest was divided into two. 
One part was directly fractionated by a 
DEAE-Sephadex column (Fig. 25a). 
The other was digested further with 
alkaline phosphatase and then sub- 
jected to a DEAE-Sephadex column 
(Fig. 25b). In Fig. 25a, digests having 
negative charges of 5.5, 6.5, 7.5 and so 
on were observed. These are tenta- 
tively identified as 32 ppp A p G p, 
32 ppp A p x p G p, 32 ppp A p x p x p G p and so on. 
The 32 P-radioactivity shown in Fig. 25a 
was mostly sensitive to alkaline phos- 
phatase (Fig. 25b). This result excludes 



DEPARTMENT OF EMBRYOLOGY 



43 



e 


I H m EY33 


a 


M 

2 4 

X 


- 


' 


, 




c 
E 

£ 2 
(J 


J 


y 


L 


Wvy 


-v^. 





rf y 


d 


50 


"1 


25 




1J 


ft^V\ 





*£. 


Wf 


v \ 



20 



40 



Fraction number 



Fig. 22. RNase T, "fingerprints" of various RNAs labeled with a-' 2 P-UTP. (a) Pure fibroin mRNA 
from the posterior glands labeled in vivo for 24 hr during the 6th day of the fifth instar. (b) High 
molecular weight RNA (void volume fraction of a Bio-Gel A-50m column) from animal's carcass 
labeled in vivo for 24 hr during the 6th day. (c) An aliquot of crude fibroin mRNA fraction (the void 
volume fraction shown by the bracket in Fig. 21) synthesized in vitro in disrupted nuclei, (d) Other 
aliquot of the void volume fraction of Fig. 21 was hybridized to fibroin genes (Fig. 23), and purified. 32 P 
cts/min (• -•). 



the unlikely possibility that a strange 
structure like A(5')ppp 32 (5')G p or 
A(5')pp P 32(5')X p G p is the product. Frac- 
tions 20 to 54 were also pooled, and 
analyzed. Hydrolysis of an aliquot of 
the sample by 0.3N KOH gave a single 
peak at a negative charge of 4.3 (Fig. 
25c), which we tentatively identify as 
32 ppp A ppp. Therefore, 32 P was not in- 
corporated into internal positions. Di- 
gests having negative charges of 5, 6, 7, 
etc., were obtained by pancreatic 
RNase digestion (Fig. 25d) together 
with an unidentified peak having -3.8 
charge. By T 1 RNase digestion we 
found digests having negative charges 
of 5.5, 6.5, 7.5, etc. (Fig. 25). There was 



a small peak cochromatographing with 
5'-ATP marker and a large peak hav- 
ing -4.3 charge which we could not 
identify (Fig. 25e). Although there are 
some unidentified products in the di- 
gests, we conclude that most of the 
y- 32 P-ATP was incorporated at the 5' 
end of RNA molecules in the void col- 
umn fraction, which contained 15% fi- 
broin mRNA, as well as in the lower 
molecular weight region of the Bio-Gel 
column (Fig. 24). These observations 
that the disrupted nuclei retain their 
ability to reinitiate RNA synthesis 
strengthen the usefulness of the sys- 
tem for the study of fibroin gene regu- 
lation in vitro. 



44 



CARNEGIE INSTITUTION 



cvj 

i 

O 



u 




5 10 15 

FRACTION NUMBER 



20 




20 40 60 

FRACTION NUMBER 



Fig. 23. Hybridization of crude fibroin mRNA 
synthesized in vitro to partially purified Fibroin 
genes. B. mori DNA (2.0 mg) was fully sheared, 
actinomycin D (1.0 mg) was added, and the re- 
fractive index was adjusted to 1.3842. The mix- 
ture was centrifuged at 32,000 rev/min at 20°C 
for 66 hr in a Spinco 50.2 rotor, and fractions 
were trapped on filters. The void volume fraction 
of Fig. 21a was hybridized to the DNA filters in 
50% formamide-4 x SET at 50°C for 20 hr, and 
the RNA hybridized to fibroin gene region shown 
by the bracket was recovered for RNase T, 
"fingerprint" analysis (Fig. 22d). In order to ob- 
tain enough counts the hybridization and elution 
of the hybrid RNA was repeated, cts/min 
(• •); A, 60 ( ). 



Fig. 24. Incorporation of y- i2 P-ATP into RNA 
synthesized in vitro by disrupted nuclei. Nuclei 
containing 150 /xg of DNA were incubated with 
500 fid of y- :i2 P-ATP (18.7 Ci/mmole) and other 
ingredients in 1.0 ml volume at 25 C C for 30 min. 
The RNA was phenol-extracted, precipitated 
twice with ethanol, passed through a 0.9 x 66 cm 
column of Sephadex G25, precipitated again 
twice with ethanol, and fractionated by a 2.6 x 
66 cm column of Bio-Gel A-50m. Out of 5.5-ml 
fractions each 1.0-ml aliquot was counted after 
TCA precipitation. Fractions shown by the brac- 
kets were pooled and subjected to analyses shown 
in Fig. 25. 






DEPARTMENT OF EMBRYOLOGY 



45 




Fraction number 

Fig. 25. Characterization of the RNA synthesized in vitro labeled with y- 32 P-ATP. Aliquots of 
fractions 14-19 of Fig. 24 were pooled; E. coli tRNA was then added as a carrier, precipitated with 
ethanol twice, and the precipitate digested with T, RNase and divided into two. One was applied 
directly to a DEAE-Sephadex column (a), and the other was digested further with alkaline phos- 
phatase (b). Aliquots of fractions 20-54 of Fig. 24 were pooled, and E. coli tRNA was added and 
precipitated with ethanol twice. Aliquots were subjected to digestion with 0.3 A KOH (c), pancreatic 
RNase (d), and T, RNase (e). All the digests were fractionated on DEAE-Sephadex A25 columns with 
an NaCl gradient 0.10-0.40 M in the presence of 1M urea. Before application to the column the digest 
for (c) was neutralized' and carrier oligonucleotides were added. Cold 5'-ATP was added as marker to 
the digest for (e). i2 P cts/min (• • ); A, (1 „ ( ); [MJNaCl (- - -). 



BIOGENESIS OF MITOCHONDRIA 

/. B. Dawid, E. A. Godwin, C. K. Klukas, J. L. Ramirez, S. Ohi, and W. B. Upholt 
with the assistance ofM. Rebbert 



We have continued our studies on 
the structure of mitochondrial DNA 
(mtDNA) in several animal cells with 
the aim of obtaining comparative maps 
of different mtDNAs (see Year Book 74, 
p. 46). Ultimately, we would like to 
know what functions are coded for by 
mtDNA; how these functions are ar- 
ranged on the DNA; how the primary 



sequence and the overall anatomy of 
mtDNA changes in evolution; and 
what kind of evolutionary forces and 
mechanisms operate on this DNA. 

MtDNA codes for two specific 
ribosomal RNAs (rRNAs), a set of 
transfer RNAs (tRNAs), and several 
poly(A)-containing RNAs that may be 
messenger RNAs. The rRNAs and 



46 



CARNEGIE INSTITUTION 



tRNAs have been well characterized in 
a number of organisms. Our aim has 
been to map the location of the regions 
coding for these RNAs in the mtDNA of 
Xenopus laevis and X. mulleri. We 
have also studied the location of the 
D-loop, which marks the replication 
origin. Experiments have been ini- 
tiated to study nonribosomal RNAs in 
Xenopus mitochondria. A mapping 
study of Drosophila melanogaster 
mtDNA has been completed during 
this year. A detailed comparison of the 
mtDNAs of the goat and sheep has 
been carried out. 

The major results of these experi- 
ments are as follows. The mt-rRNA 
genes in Xenopus and Drosophila as 
well as in Hela mtDNA are located 
next to each other on the H-strand with 
a gap of 120 to 160 nucleotides between 
them. In X. laevis and Hela this gap 
carries the gene sequence for a 
mitochondrial 4S RNA, and one 4S 
RNA gene is located immediately be- 
yond the ends of the rRNA genes. A 
total of 15 to 16 genes for 4S RNA are 
distributed widely over the H-strand. 
These features are again similar to the 
situation in Hela mtDNA as reported 
by Wu and her colleagues {Journal of 
Molecular Biology, 71, 81-93, 1972). 
We further determined the polarity of 
the mt-rRNA in Drosophila and 
Xenopus: transcription proceeds from 
the smaller to the larger RNA. Trans- 
cription in the direction small-to-large 
rRNA is now recognized to be generally 
true for rRNA genes. In both X. laevis 
and X. mulleri the D-loop begins at a 
distance of about 800 base pairs from 
the end of the rRNA genes and replica- 
tion is directly away from the rRNA 
genes. 

We reported previously {Year Book 
71, p. 22; Year Book 70, p.44) that the 
primary sequence of mtDNA changes 
quite rapidly in evolution. Our recent 
observations show that the overall 
anatomy of the mtDNA molecule is 
highly conserved over long evolu- 
tionary distances. 



Cloning OFXenopus laevis 
Mitochondrial DNA 

/. B. Dawid, W. B. Upholt, and J. L. Ramirez 
with the assistance ofM. Rebbert 

To aid structural studies of X. laevis 
mtDNA, we prepared hybrid plasmids 
between mtDNA and suitable vectors 
and grew these plasmids in E. coli (see 
Year Book 74, pp. 16 and 20). In this 
work we adhered to the safety stan- 
dards proposed by the Asilomar Con- 
ference on Recombinant DNA. The re- 
striction endonuclease EcoRI cleaves 
X. laevis mtDNA into two fragments, 
one of 15 kb and one of 2 kb (kb is 
kilobases or kilobase pairs). The small 
fragment was linked to the 
kanamycin-resistant vector PCR11 
(PCR11 is 12.6 kb); the larger fragment 
was linked to the tetracycline-resistant 
vector PMB9 (PMB9 is 5.3 kb). After 
growth in E. coli the recombinant 
plasmids were isolated, digested with 
EcoRI, and the mitochondrial frag- 
ments were purified. Both cloned 
fragments were shown to be identical 
with the EcoRI fragments of frog 
mtDNA by gel electrophoretic analysis 
of the patterns obtained by digestion 
with other restriction enzymes. These 
cloned mtDNA fragments were then 
used in mapping studies in addition to 
fragments isolated directly from frog 
mtDNA. 



Maps of X. laevis and X. mulleri 

mtDNA: Restriction Sites, the 

D-loop, and the rRNA Genes 

J. L. Ramirez and I. B. Dawid 



In Year Book 74, pp. 46-47, we 
showed maps for these two mtDNAs 
which contain the locations of EcoRI 
sites, the Smal site in X. mulleri 
mtDNA, and the D-loop positions. We 
now add the positions and the polarity 
of the rRNA genes (Fig. 26). 



DEPARTMENT OF EMBRYOLOGY 



47 




Bam \ 




Fig. 26. Maps ofX. laeuis andX. mulleri mtDNA. EcoRI sites are numbered in italics inside the 
circles. Other restriction sites are labeled. The rRNA genes and the D-loops are shown, and the 
polarity of transcription and replication is indicated with an arrow pointing in the 5'-to-3' direction 
on the growing RNA or DNA strand. Distances are given as percent of total genome, which is 17,100 
base pairs. The H-strand is shown on the outside, the L-strand on the inside. 



The mapping of the rRNA genes was 
done in the following way. We know 
that the two genes are located next to 
each other with a gap of about 120 
bases separating them {Year Book 72, 
p. 45). EcoRI fragments of X. laevis 
mtDNA were annealed with mt-rRNA, 
and the hybrid molecules were in- 
spected in the electron microscope. The 
rRNA/DNA hybrid regions were close 
to the end of the larger EcoRI frag- 
ment, with the small mt-rRNA located 
at the end. This position for the rRNAs 
was confirmed by "R-loop mapping," a 
method developed by White and Hog- 
ness of Stanford University. In this 
method, native DNA and RNA are an- 
nealed under conditions in which the 
RNA invades the DNA duplex and dis- 
places one of the strands. A loop is 
clearly visible in the electron micro- 
scope. To determine the relation of the 



rRNA genes and the D-loop, the DNA 
is fixed with glyoxal to preserve the 
D-loop, then digested with EcoRI, and 
finally hybridized with large rRNA be- 
fore examination in the electron micro- 
scope. Figure 27 shows such a molecule 
which established the relative posi- 
tions of the D-loop and the rRNA genes 
in X. laevis mtDNA. The X. mulleri 
map was derived in an analogous way 
and is also shown in Fig. 26. 

The polarity of the mt-rRNA was de- 
termined by digestion of EcoRI frag- 
ments of the DNA with specific exonu- 
cleases, followed by hybridization of the 
resulting single-stranded tails with 
rRNA. The result is incorporated into 
the models of Fig. 26. The direction of 
replication was deduced from the ob- 
servation of a number of "expanded 
D-loop molecules" in X. mulleri 
mtDNA, and is also shown in the maps. 



48 



CARNEGIE INSTITUTION 



■ v "'>^ ^ 







Fig. 27. Electron micrograph and tracing of an X. laeuis mtDNA molecule with the D-loop in the 
"H-form" ID) and an R-loop formed by the large rRNA (R). Note that the R-loop has a "whisker," i.e., a 
portion of the RNA is displaced by the DNA strand and forms a single-stranded tail. 



Mapping of 4S RNA Genes on 
X. laeuis mtDNA 

S. Ohi, W. B. Upholt and I. B. Dawid 

In Year Book 74, p. 47, we described 
our initial experiments on the mapping 
of mitochondrial 4S RNA genes of X. 
laeuis. The covalent attachment of fer- 
ritin to 4S RNA was achieved by the 
method of Wu and Davidson (Journal 
of Molecular Biology 78, 1, 1973); 
ferritin-4S RNA complexes were hy- 
bridized to separated strands of 
mtDNA, and the hybrids were 
analyzed in the electron microscope. 

The H-strand of mtDNA carries the 
genes for the mt-rRNAs and a majority 
of the 4S RNA genes. The rRNA re- 
gions were used as reference points in 
the mapping. The result of the meas- 
urements of these hybrid molecules is 
shown in Fig. 28. At least 15 and possi- 



bly 16 sites for 4S RNA were found on 
the H-strand. The more remarkable 
positions are those between and im- 
mediately flanking the rRNA genes: 
Hela cell mtDNA has 4S RNA posi- 
tions at the analogous points, as de- 
termined by Wu and her colleagues (J. 
Mol. Biol. 71, 71-93,1972). The remain- 
ing 4S RNA sites are distributed 
widely over the H-strand. This wide 
distribution is also similar to the situa- 
tion in Hela mtDNA. 

Our results with the L-strand of X. 
laeuis mtDNA are incomplete. There is, 
however, tentative evidence for 5 or 6 
sites on this strand. If these results can 
be confirmed in future experiments, we 
would have evidence for 20 to 22 sites 
for 4S RNA on Xenopus mtDNA. This 
number would come close to the ex- 
pected number of tRNAs required for a 
complete protein synthesizing "ma- 
chine" that depends fully on endogen- 
ous tRNAs. 



DEPARTMENT OF EMBRYOLOGY 



49 




Fig. 28. Map of 4S RNA sites on the H-strand of X. laevis mtDNA. Closely adjacent sites are only 
then considered to represent two sites if both sites were seen repeatedly to be occupied by ferritin on 
the same DNA strand. Site 10 may be a doublet site, but it has been observed only twice out of 158 
molecules examined as a doublet and is not considered certain. Comparison with Fig. 26 shows that the 
D-loop position is from about 0.82 to 0.90 on this map. Thus, position H14 may be within the D-loop, 
but this is not certain. 



Mapping Studies with mtDNA 
from Drosophila melanogaster 

C. K. Klukas and I. B. Dawid 

mtDNA and mt-rRNA from 
Drosophila have been isolated and 
characterized {Year Book 74, pp. 49- 
53). The circular mtDNA molecule has 
a molecular weight of 12 x 10 6 , i.e., it 
consists of 18,400 base pairs. About 
25% of this molecule has a very high 
content of adenylic and thymidylic 
acids and can be visualized as an early 
melting region by electron microscopy. 
The mt-rRNAs have molecular weights 
of about 300,000 and 530,000. 

We have mapped the positions of the 
cleavage sites of the restriction en- 
donucleases Haelll and Hindlll rela- 
tive to each other and relative to the 
early melting region. The map was 
constructed from electron microscopic 
measurements of partially denatured 



total and partial restriction digestion 
of the DNA, and from gel electrophore- 
tic analysis of the DNA with each en- 
zyme separately and both enzymes to- 
gether. Figure 29 shows the results. 

We next mapped the location of the 
rRNA genes relative to restriction sites 
by electron microscopy of RNA/DNA 
hybrids. The assignment of positions to 
the rRNA genes was confirmed and re- 
fined by a quantitative hybridization 
experiment in which the small and the 
large rRNA were separately hybridized 
to Hindlll and Haelll fragments of 
mtDNA. The resulting map shows the 
two rRNA genes located in close prox- 
imity and separated by a gap of 160 
nucleotides. 

The polarity of the rRNA was deter- 
mined in an analogous way to the 
method outlined forX. laevis mt-rRNA. 
As in the case of Xenopus the direction 
of transcription in Drosophila mtDNA 
is from the smaller to the larger rRNA. 



50 



Both the close proximity of the two 
rRNA genes and the polarity of the 
RNA are highly conserved in the evo- 
lution of animal mtDNAs, since the 
identical arrangement is found in frogs 
and fruit flies. 




280 



Fig. 29. Map of D. melanogaster mtDNA. The 
cleavage sites of Haelll and Hindlll are indi- 
cated by arrows, and the early melting region is 
indicated by an open section of the circle. The 
rRNA genes are indicated and are separated by a 
gap of 160 nucleotides. The direction of trans- 
cription, i.e., 5' to 3' polarity of the RNA, is 
shown. Distances are given in base pairs. The 
total genome size is 18,400 base pairs. 



Mapping and Sequence 

Comparisons of Goat and 

Sheep mt DNAs 



W.B. Upholt 



A detailed comparison of sheep and 
goat mtDNA was initiated with the 
aim of observing the differences in the 
mtDNAs of closely related animals 
(Year Book 74, p.48). This study should 
give information about the least con- 
served regions of the mtDNA rather 
than the most conserved ones, which 



CARNEGIE INSTITUTION 

are best studied in more distantly re- 
lated species (see Year Book 70, p. 44). 

The analysis of restriction fragments 
of sheep and goat mtDNA was carried 
further. Endonuclease Haelll yields 27 
to 30 fragments from these mtDNAs. 
From a consideration of conserved 
fragments it follows that sheep and 
goat mtDNAs have diverged to the ex- 
tent of 7% to 11% of their sequences. In 
the course of these studies the interest- 
ing fact was observed that the mtDNAs 
from individual sheep and goats dif- 
fered from each other with respect to 
the Haelll fragment pattern and there- 
fore with respect to primary sequence. 
Figure 30 shows the electrophoretic 
separation of these fragments from the 
mtDNA of two goats and three sheep. 
No two patterns are exactly alike. The 
sheep patterns are all rather similar to 
each other, the two goats differ sub- 
stantially, and the sheep and goat pat- 
terns are very different. On the basis of 
these experiments and with certain as- 
sumptions, one may calculate a se- 
quence divergence of 0.5% to 1% among 
the sheep mtDNAs and about 2% be- 
tween the DNAs of the two goats. 

Detailed maps were constructed for 
the cleavage sites of endonucleases 
EcoRI and Hindlll on both sheep and 
goat mtDNAs. The two goats differed 
with respect to EcoRI and Hindlll sites, 
a fact that emphasizes the considerable 
difference in sequences of the two 
DNAs. The position and lengths of the 
goat and sheep D-loops were deter- 
mined and are also shown on the maps 
of Fig. 31. Several EcoRI Hindlll sites 
were conserved between sheep and 
goat, but some of the sites differ. The 
D-loops are at analogous positions on 
the two mtDNAs, but the lengths of the 
loops are different. We reported last 
year that a region of high evolutionary 
change overlaps one end of the D-loop 
(the left end in Fig. 31). This region 
where the two D-loops differ in length 
and the area immediately beyond it 
have been studied in more detail by 
electron microscopic heteroduplex 
mapping. We confirm the earlier con- 
clusion that this region shows a high 



DEPARTMENT OF EMBRYOLOGY 



51 



— G— 



P 



t- ■ » i 




I 







^ „ 



•*>,-■.->, 







Fig. 30. Haelll fragments of sheep and goat mtDNA. The direction of electrophoresis is from top to 
bottom. Fragment sizes range from just under 2000 to 200 base pairs. Smaller fragments occur but are 
not shown in this figure. mtDNA fragments from three individual sheep (S) and two goats (G) are 
displayed; the unmarked lane contains Haelll fragments of the plasmid PSC101. Differences in the 
band patterns of individual sheep and goats are apparent. 



rate of evolutionary change and 
further suggest that it contains a dup- 
lication of about 150 base pairs. Thus, 
the segment of DNA overlapping and 



adjacent to the D-loop may show a high 
rate of base mutation and also a ten- 
dency to rearrangements on a larger 
scale. 



52 



CARNEGIE INSTITUTION 




Fig. 31. Maps for sheep (S) and goat (G) mtDNAs. EcoRI sites are labeled/? and numbered; Hindlll 
sites are labeled//; fragments are labeled with capital letters. Subscripts refer to sites or fragments 
that are present in one of the two goat mtDNAs only. The position and length of each D-loop are 
indicated. The lengths of the different regions in the molecules are shown on the outer circles in base 
pairs. 



PHOSPHOLIPID-CHOLESTEROL DYNAMICS IN MAMMALIAN 

CELLS 

R. E. Pagano, L. Huang, K. Ozato and M. Takeichi 
with the technical assistance ofE. Asch, E. Someruille and D. Duncan 



Work in this laboratory is concerned 
with the possible dynamic role that 
membrane phospholipids and choles- 
terol may play in the regulation of cell 
behavior. During the past several 
years we have been developing a tech- 
nique utilizing artificially generated 
lipid vesicles, which permits a con- 
trolled and systematic perturbation of 
these membrane components in living 
cells. In the present report we empha- 
size our recent findings on the interac- 
tions of phosphatidyl choline vesicles 
with murine lymphocytes and the 
physiological consequences of these in- 
teractions. We show that the pathways 
of vesicle-cell interactions, enumerated 
in previous reports (vesicle-cell fusion, 
adsorption, and lipid exchange) are ex- 



quisitely sensitive to the molecular 
composition and physical properties of 
the artificial vesicles. When lympho- 
cytes are the "recipient" cells, the re- 
sulting membrane perturbations lead 
to altered surface properties and an 
enhanced proliferative activity of these 
cells, when stimulated by various 
mitogens and antigens. 

In a separate but related line of re- 
search, Takeichi has explored some of 
the adhesive properties of a fibroblastic 
cell line and reports on attempts to 
identify and characterize membrane 
proteins that participate in this 
phenomenon. Parallel studies on the 
adherence of lipid vesicles of known 
composition to cells are also discussed. 

Finally, a critique of the use of small 



DEPARTMENT OF EMBRYOLOGY 



53 



hydrophobic fluorescent probes to 
monitor changes in the physical state 
of the lipids in natural membranes is 
presented. 

Details of our studies on each of 
these topics are presented in separate 
sections below. 

Interactions of Phospholipid 

Vesicles with Murine 

Lymphocytes: Vesicle-Cell 

Adsorption and Fusion as 

Alternative Pathways of 

Uptake 

L. Huang, K. Ozato, and R. E. Pagano 

The various mechanisms by which 
unilamellar lipid vesicles interact with 
cells and the subsequent modification 
of cell behavior that would result have 
been the subject of recent investiga- 
tions from this and other laboratories. 
In Year Book 74 (p. 53) we showed that 
the incorporation of lecithin into 
Chinese hamster fibroblasts from 
dioleyl lecithin (DOL) vesicles were 
primarily via vesicle to cell fusion and 
lipid exchange processes. The interac- 
tion of dimyristoyl lecithin (DML) vesi- 
cles with these cells was also studied, 
and exhibited an enhanced uptake 
when the temperature of vesicle-cell 
incubation was below the thermotropic 



phase transition temperature (T c — 
25 °C) of DML. While the detailed 
mechanisms for DML uptake were not 
explored, this peculiar temperature 
dependence suggested an additional 
pathway for vesicle incorporation. In 
the present study with mouse thymic 
lymphocytes, we identify this pathway 
as vesicle-cell adsorption. Further- 
more, in a survey of the uptake of vari- 
ous lecithin vesicles by thymocytes, it 
is shown that these lipid vesicles can be 
classified according to whether they 
are incorporated principally by vesicle 
to cell fusion or a stable vesicle to cell 
adsorption. 

Evidence for a Stable Vesicle-Cell 
Adsorption 

In an EM autoradiographic survey of 
thymocytes treated with a variety of 
[ 3 H] lecithin vesicles, we observed two 
distinct patterns of silver grain distri- 
butions in treated cells. In the case of 
EYL (egg yolk lecithin) or DOL vesi- 
cles, most of the exogenous lipid label 
appeared inside the cell, while for DML 
or DPL (dipalmitoyl lecithin) vesicle- 
treated cells, relatively large amounts 
of radioactivity appeared to reside at 
the cell surface. Typical examples of 
such autoradiograms are shown in Fig. 
32 for cells treated with EYL vesicles 







at 



3 



Kf> & 



a 






A* 



^ 



%j£ 



•4*.™ 



4 



& 



J 






«>' 



S* 



* 



Fig. 32. Electron microscope autoradiographs of mouse thymocytes treated with (a) [ 3 H] DML 
vesicles (2°C, 60 min), x 14,000 and (b) [ :i Hj EYL vesicles (22-23°C, 30 min), x 7,700. Bar is 0.1 /xm. 



54 



CARNEGIE INSTITUTION 



(30 min at 22-23 °C) and DML vesicles 
(1 hr at 2 °C). Similar auto-radiograms 
were also obtained for thymocytes 
treated with these lipids at 37 °C. Thin 
layer chromatographic (TLC) analysis 
of the extracted lipids from these cells 
revealed that most (—90%) of the 
radiolabel was still in lecithin, indicat- 
ing that the grains seen in the au- 
toradiograms represent the approxi- 
mate location of the exogenously intro- 
duced lecithin and not some degrada- 
tion product. This qualitative observa- 
tion of two classes of grain distribution 
was confirmed in a statistical analysis 
of a large number of autoradiograms of 
treated cells and is summarized in 
Table 6. The accumulation of silver 
grains at the surfaces of DPL or DML 
(vs. EYL or DOL) vesicle-treated cells 
is readily apparent, both from calcula- 
tions of the percentage of grains found 
near the cell surface and from the ratio 
of the normalized grain densities. 

It should be noted that all vesicle-cell 
incubations were carried out at 22- 
23 °C except in the case of DML. In this 
instance, incubations were carried out 
at 2 °C to avoid complications which 
might arise from the phase transition 
of the DML vesicles. A suggestion of 
the complex uptake behavior which re- 
sults from this transition is seen in Fig. 
33, where the temperature dependence 
of DML and EYL uptake by thymo- 
cytes is compared. Each data point in 
this figure represents the lipid uptake 
by cells following a 60-min incubation 
at a given temperature. For EYL vesi- 
cles, the lipid uptake increased mono- 



tonically with temperature between 
0°C and 37°C. In striking contrast, the 
lipid uptake from DML vesicles showed 
a peculiar temperature dependence, 
exhibiting a sharp maximum at about 
20°C and minima at about 15° and 
24°C. Furthermore, the uptake at 0°C 
was greater than at 37°C. DML uptake 
was always larger than EYL uptake; 
for example, about 1.5 ^ig of EYL vs. 4 
/u,g of DML were taken up by 10 7 cells 
at 37°C. 




10 20 30 40 

Temperature (°C) 

Fig. 33. Temperature dependence of J :i H] EYL 
(1 mg/ml, 1 x 10" cpm/mg) and [ 14 Cj DML (1 
mg/ml, 4 x 10 s cpm/mg) uptake by mouse thymo- 
cytes. 10 7 cells/ml were incubated 60 min with 
vesicles in Gey's balanced salt solution. 

The existence of two types of lecithin 
vesicle-cell interactions suggested by 
the data above was confirmed in ul- 



TABLE 6. Distribution of Autoradiographic Grains over Mouse Thymocytes 
Treated with [ :! H] Lecithin Vesicles 













(Surface Grain* 






Treatment 




Percent Grains 
in Surface 


Density) 


. + SEM 




Time 


Temp. 


Total Silver 


(Interior Grain 




Lipid Type 


(min) 


CO 


Grains Counted 


Compartment* 


Density) 




:! H DML 


60 


2 


565 


69 


8.1 ± 1.0 




<H DPL 


30 


22-23 


342 


71 


8.6 ± 1.2 




'H EYL 


30 


22-23 


348 


27 


1.5 ± 0.2 




:! H DOL 


30 


22-23 


133 


25 


1.4 ± 0.2 





'See Pagano, Ozato, and Ruysschaert, elsewhere in this report, for description of analysis. 



DEPARTMENT OF EMBRYOLOGY 



55 



trastructural studies of vesicle-treated 
cells. Thymocytes incubated with EYL 
or DOL vesicles at temperatures be- 
tween 2 ° and 37 °C showed smooth cell 
surfaces identical to those of untreated 
control cells. A typical example of the 
surface of an EYL-treated cell is shown 
in Fig. 34a. In the DPL-treated cells 
(Fig. 34b), many of the intact vesicles 
had apparently adsorbed to the cell 
surface. These vesicles ranged in size 
from about 250 A to about 650 A in 
diameter, possibly reflecting heteroge- 
neity in the applied vesicle population. 
Furthermore, the distribution of ad- 
sorbed vesicles on the cell surface was 
not uniform. Micrographs obtained 
from cells treated with DML vesicles 
also showed evidence of adsorbed ma- 
terial at their surfaces. However, the 
DML vesicles were apparently rup- 
tured or collapsed in the 2 °C treat- 
ments (Fig. 34c), while large masses of 
amorphous material were seen ad- 



sorbed to cells treated at 37 °C (Fig. 
34d). From the ultrastructural studies 
shown in Fig. 34 and the autoradio- 
graphic data (Fig. 32; Table 6) showing 
an accumulation of [ [6 H] DPL or [ 3 H] 
DML at the surfaces of treated cells, it 
is concluded that the adsorbed mate- 
rials seen in Fig. 34b, c, d actually rep- 
resent native or modified applied vesi- 
cles. In the case of DML, intact vesicles 
were rarely seen adsorbed at the sur- 
face of treated cells. The presence of 
collapsed or ruptured vesicles and 
amorphous material suggests a reor- 
ganization of the vesicle membranes. 
This modification of vesicle morphol- 
ogy could be a consequence of an inher- 
ent bilayer instability in this relatively 
short chain lecithin. Our findings of an 
enhanced uptake of DML by thymo- 
cytes near or below the phase transi- 
tion temperature (T c ) or DML (Fig. 33) 
and the ultrastructural evidence pre- 
sented in Fig. 34c and d suggest an en- 




Fig. 34. Transmission electron micrographs of mouse thymocytes treated for 30 min with vesicles 
comprised of (a) EYL (22-23°C), (b) DPL (22-23°C), (c) DML (2°C) and (d) DML (37°C) x 78,000. Bar is 
1 pirn. Intact vesicles (b), collapsed or ruptured vesicles (c), and amorphous material (d) are seen at 
arrows. 



56 



CARNEGIE INSTITUTION 



hanced adsorption of DML at these 
temperatures. Such an enhancement 
could be due to the increase in bilayer 
thickness below the T c of the lipid. 

Vesicle-Cell Fusion and Lipid 
Exchange 

Since the EM autoradiographic data 
as well as the micrographs presented in 
Fig. 34a suggested an absence of 
vesicle-cell adsorption for DOL and 
EYL vesicles, other possible mecha- 
nisms for the uptake of these vesicles 
have been investigated. In Year Book 
74 (p. 62) we suggested that EYL vesi- 
cles were incorporated by thymocytes 
principally by a vesicle-cell fusion 
mechanism. This suggestion was based 
in part on the finding that when 
thymocytes were treated with [ 14 C] 
EYL vesicles containing trapped [ 3 H] 
inulin, both components became cell 
associated at identical rates, consistent 
with the uptake of "intact" vesicles. 
Additionally, the uptake of EYL vesi- 
cles was insensitive to NaN 3 and 
cytochalasin-B, two known inhibitors 
of endocytosis in other cell types. Dur- 
ing the past year, further direct evi- 
dence for the fusion of EYL vesicles 
with thymocytes was obtained. Cells 



were treated with EYL vesicles con- 
taining high specific activity [ 3 H] in- 
ulin under various conditions. EM au- 
toradiography was then used to 
examine the intracellular distribution 
of the vesicle marker in treated cells. 
Statistical analysis of a large number 
of autoradiograms showed approxi- 
mately 75% of the trapped PH] inulin 
was randomly distributed in the cyto- 
plasm and nucleoplasm of the treated 
cells, suggesting that the contents of 
EYL vesicles were released into the 
cells by fusion. 

In order to explore the possibility of 
lipid exchange between vesicles and 
cells, thymocytes with endogenously 
( 3 H-palmitate) labeled lipids were in- 
cubated with [ 14 C] EYL unilamellar 
vesicles for 1 hr at 37 °C. Following 
this incubation, the vesicles were sepa- 
rated from the cells and subjected to 
molecular sieve chromatography on 
Sepharose 4B. Each fraction was 
examined for both 14 C- and 3 H-cpm. 
The results of such an experiment are 
given in Fig. 35. The 14 C-cpm showed 
one broad symmetrical peak, typical of 
unilamellar lipid vesicles. Three 
3 H-cpm peaks were observed. The first 
peak eluted at the exclusion volume of 
the column and probably represents 




20 30 40 

FRACTION NUMBER 



Fig. .'35. Elution profile of ["C] EYL vesicles on Sepharose 4B following incubation (1 hr at 37°C) 
with mouse thymocytes. Endogenous cell lipids were labeled with [ :i Hj palmitic acid. Fractions 
between arrows were analyzed for lipid composition. 



DEPARTMENT OF EMBRYOLOGY 



57 



unremoved whole cells or fragments. 
The third and major 3 H-cpm peak 
eluted at the end of the column volume; 
its radioactivity was chloroform- 
methanol nonextractable and presum- 
ably represents some secreted 
metabolic products of small molecular 
weights. The second 3 H-cpm peak pre- 
cisely cochromatographed with the un- 
ilamellar 14 C-vesicle peak. Between 
fractions 15 and 27 (Fig. 35, arrows) a 
total of 3 x 10 3 3 H-cpm was found. In 
control experiments in which the [ 14 C] 
EYL vesicles were incubated with a 
balanced salt solution previously in 
contact with the 3 H labeled cells, a 
total of only 5 x 10 2 3 H-cpm was found 
in these fractions. Practically all 
(-90%) of both the 3 H-cpm and 14 C-cpm 
in these fractions was chloroform- 
methanol extractable; TLC analysis 
shows more than 80% of the 3 H-cpm 
was lecithin. These results demon- 
strate that transfer of cellular lipid to 
the vesicles, always present in excess, 
can occur in addition to EYL vesicle- 
cell fusion. In this experiment, al- 
though lecithin was a minor lipid 
(—30%) in the endogenously labeled 
thymocytes, it constituted a major frac- 
tion (—80%) of the cell lipids trans- 
ferred to the vesicles. This indicates an 
apparent specificity in the vesicle-cell 
lipid transfer process. From the data in 
Fig. 35, the number of lecithin 
molecules per EYL vesicle, and a lower 
limit for the specific activity of the cel- 
lular lecithin, it was calculated that no 
more than two cellular lecithin 
molecules were transferred per 100 
EYL vesicles. If it is assumed that the 
transfer of cell lipids to vesicles is ac- 
companied by an equal amount of 
transfer of vesicle lipids to the cells, it 
is further calculated that only 1-2% of 
the total EYL uptake (37°C) is due to 
the lipid exchange process. 

Thus we conclude that the major 
pathways of lecithin vesicle uptake by 
mouse thymocytes are (1) a stable vesi- 
cle to cell adsorption in the cases of 
DML or DPL and (2) vesicle-to-cell fu- 
sion for EYL (or DOL) with a minor 
contribution due to lipid exchange. 



Since all these processes require an ini- 
tial vesicle-to-cell contact, it follows 
that for DML and DPL this contact is 
further developed into a state of stable 
adsorption, while for EYL or DOL it 
leads to subsequent fusion. We specu- 
late that the physical properties of the 
lipid vesicles, dictated by the fatty acyl 
composition of the vesicle lecithin, ul- 
timately determine the predominant 
pathway for vesicle uptake. 

Interaction of Phospholipid 

Vesicles with Murine 

Lymphocytes: Vesicle-Cell 

Adsorption Leads to Altered 

Surface Properties and 

Enhanced Proliferative 

Response 

K. Ozato, L. Huang, and R. E. Pagano 

In order to delineate the relationship 
between cell surface properties and 
membrane lipid composition, we have 
explored the use of unilamellar lipid 
vesicles to perturb the surface of mouse 
lymphocytes, stimulated by various 
mitogens to undergo blast transforma- 
tion. This approach seemed particu- 
larly hopeful, since previous studies 
from this and other laboratories using 
other cell types had demonstrated that 
vesicle lipids could be incorporated into 
cells by vesicle-to-cell fusion and lipid 
exchange processes. We first made a 
detailed study of the interactions of a 
variety of lecithin vesicle types with 
mouse lymphocytes and confirmed the 
existence of these pathways of vesicle 
uptake (Huang, Ozato, and Pagano; 
this report). Surprisingly, in certain 
types of lipid vesicles, a previously un- 
reported mechanism of vesicle uptake 
was also found, namely, a stable 
vesicle-to-cell adsorption. We report 
here on the ability of lipid vesicles to 
modify the agglutinability and lectin 
binding of lymphocytes. Most signifi- 
cantly it is shown that vesicle pre- 
treatment can also enhance .the pro- 
liferative activity of stimulated cells. 
The extent of these modifications of cell 



58 



CARNEGIE INSTITUTION 



behavior depends on the lipid vesicle 
type and is correlated directly with the 
ability of vesicles to adsorb to the cell 
surface. 

Enhancement of Proliferation by Lipid 
Vesicle Pretreatment 

Cortisone-resistant thymocytes 
(CRT) were briefly (30 min) treated 
with unilamellar lipid (lecithins or 1:2 
lecithin: cholesterol) vesicles and sub- 
sequently cultured in a serum- and 
lipid-free medium containing a 
mitogenic dose of Con A (1 fxg/ml). The 
proliferative activity of these cultures 
was periodically examined using the 
criteria of increased PH] TdR incorpo- 
ration and increased number of total 
cells and lymphoblasts. Figure 36 
shows the time course of DNA synthe- 
sis in CRT, activated by Con A, for both 
DML vesicle pretreated and, for con- 
trol, untreated cells. In both cultures 
[ 3 H ] TdR incorporation reached a peak 
on day 3. The pretreated cells showed 



obviously higher DNA synthesis than 
control at every time point tested, with 
the largest differences (2-3 x ) occurring 
at days 3 and 4 in culture. By the fifth 
day, DNA synthesis in both cultures 
had practically ceased. The degree of 
enhancement of the mitogenic response 
by DML varies from experiment to ex- 
periment (2-5 x control) but was always 
the largest of the lipid vesicles tested. 
The level of the DML effect was depen- 
dent on the temperature of the vesicle 
pretreatment with pretreatment at 
2 °C giving about a twofold larger ef- 
fect than vesicle-cell incubations car- 
ried out at 22-23 °C. Table 7 compares 
the effect of different lipid vesicle pre- 
treatments on the level of DNA syn- 
thesis in Con A-stimulated CRT. Each 
of the vesicle pretreatments also re- 
sulted in an increase in total cells as 
well as lymphoblasts in the culture. 
The degree of enhanced proliferation 
depended on the type of lipid vesicle 
used, with DML always giving the 
largest stimulation and EYL the least. 



▲ DML PRETREATMENT 
• CONTROL 




12 3 4 5 6 

DAYS IN CULTURE 
Fig. 36. Time course of DNA synthesis in cortisone-resistant thymocytes stimulated by Con A (1 
/xg/ml). 






DEPARTMENT OF EMBRYOLOGY 



59 



TABLE 7. Effects of Lipid- Vesicle Pretre'atments on Cortisone-Resistant 
Thymocytes Stimulated with Con A 





Temperature of 


Typical Experiment 


Total/ 


Lymphoblast/ 


Lipid Vesicle 


Pretreatment 


(cpm/culture 


Culture 


Culture 


Pretreatment* 


(0°) 


x 10- ;! t 


x 10"" 


x 10- 


EYL 


22-23 


11.5 ± 0.9 


3.0 ± 0.1 


6.1 ± 0.2 


DOL 


22-23 


11.4 ± 0.8 


3.0 ± 0.2 


6.0 ± 0.3 


1:1 EYL:Chol 


22-23 


14.6 ± 1.2 


3.1 ± 0.2 


6.3 ± 0.3 


DPL 


22-23 


29.0 ± 1.5 


3.7 ± 0.3 


9.0 ± 0.5 


DML 


22-23 


18.3 ± 1.1 


3.6 ± 0.3 


8.7 ± 0.4 


DML 


2 


37.8 ± 1.3 


4.4 ± 0.3 


14.3 ± 0.8 


Control: no vesicles 




10.5 ± 0.4 


2.8 ± 0.1 


5.2 ± 0.3 


Control: no mitogen 




0.4 ± 0.0 


1.7 ± 0.0 


0.0 


no vesicles 










Control: DML or DPL 


22-23 


0.9 ± 0.0 


1.6 ± 0.0 


0.0 


vesicles; 










no mitogen 











*Optimum lipid concentration of vesicle suspension was 0.5 mg total lipid/ml cell suspension, except 
for 1:1 EYL:Chol vesicles (1.0 mg/ml). 
tMean of triplicate cultures ± standard deviation. 



The observed order was DML (2-5 x ) > 
DPL (2-3 x )> 1: 1 EYL:Chol ( 1.5-2.5 x ) 
> EYL = DOL (1.2-2 x) > untreated 
cells. 

Cells pretreated with lipid vesicles 
and subsequently cultured in the ab- 
sence of mitogen did not exhibit signif- 
icant DNA synthesis 72 hr after cul- 
ture. Additional experiments in which 
cells were continuously cultured in the 
presence of lipid vesicles at sufficiently 
low concentrations (0.1 mg/ml) to pre- 
vent toxicity even for periods of incuba- 
tion up to 72 hr, also showed no signifi- 
cant enhancement of DNA synthesis 
levels. Thus it is concluded that none of 
the lipid vesicles tested were them- 
selves mitogenic for T-cells. 

The effects of lipid vesicle pretreat- 
ment on spleen cell cultures stimulated 
by a B-cell mitogen, bacterial 
lipopolysaccharide (LPS), were also 
studied. In nearly every case lipid- 
vesicle pretreatment of B-cells resulted 
in a significant enhancement of their 
proliferative activity when stimulated 
by LPS. Again, a graded enhancement 
in DNA synthesis, depending on the 
vesicle lipids used, was observed in the 
same order as that found in T-cells. 
Lipid vesicle pretreatment of spleen 
cell cultures was not itself mitogenic. 



Alterations in Lectin-Induced 

Agglutinability and Lectin-Binding in 

Vesicle-Treated Cells 

The lectin-induced agglutinability of 
normal thymocytes pretreated with 
lipid vesicles was assayed quantita- 
tively by measurements of the total 
particle number in cell suspensions in- 
cubated with and without Con A. As 
shown in Table 8 pretreatment of 
thymocytes with lipid vesicles prior to 
incubation with Con A significantly 
enhanced the agglutinability of the 
treated cells. Furthermore, the degree 
of enhanced agglutinability was de- 
pendent on the vesicle type and showed 
exactly the same order as that found in 
the proliferation studies, that is, DML 

> DPL > 1:1 EYL:Chol > EYL ~ DOL 

> no treatment. Vesicle pretreatments 
themselves did not induce thymocyte 
agglutination in the absence of lectin. 
In control and EYL or 1:1 EYL:Chol 
vesicle pretreated cells, a-MM (but not 
galactose) could almost completely re- 
verse Con A-induced agglutination. 
However, the agglutination was only 
partially reversed for DML or DPL pre- 
treatments. These effects of vesicle pre- 
treatments on lectin induced aggluti- 
nation were consistently observed for 



60 



CARNEGIE INSTITUTION 



different lectin concentrations (5-50 
ixg/m\). The effect of vesicle pretreat- 
ments on the binding of radiolabeled 
Con A to normal thymocytes was also 
studied. In Fig. 37, the binding of [ 125 I] 
Con A (1 /Ltg/ml; 60 min at 2 °C) to nor- 
mal thymocytes pretreated with vari- 
ous lipid vesicles is compared. The 
binding of [ 125 I] Con A to 1:1 
EYL:Chol, EYL or DOL vesicle pre- 
treated cells was indistinguishable 
from that of untreated control cells, 
both in terms of the total as well as the 
«-MM inhibitable portion of the bind- 
ing. In the case of DML or DPL vesicle 
pretreatments, however, the binding 
was significantly enhanced. Cells pre- 
treated with DPL vesicles always 
showed greater total [ 125 I] Con A bind- 
ing than DML-pretreated cells, while 
the opposite order was found when the 
a-MM inhibitable portions were com- 
pared. Binding studies carried out at 
37 °C showed increased lectin binding 
and an identical trend with respect to 
the ability of different vesicle types to 
modify that binding. The binding of 
PHI Con A to vesicle-treated cells 
studied at higher lectin concentrations 
(50 /xg/ml) also showed a striking en- 
hancement of overall and a-MM in- 
hibitable binding of DML and DPL 
vesicle-pretreated cells. 



TABLE 8. Effect of Lipid- Vesicle Pretreatment 

on Lectin-Induced Agglutinability of 

Thymocytes 



Agglutination Index* 
Con A (10 pig/ml) 



Vesicle Pretreatment 



-MM 



-MM 



None 

DML(2°C) 
DML(22-23°C) 
DPL(22-23°C) 
1:1 EYL:Chol 

(22-23°C) 
DOL(22-23°C) 
EYL(22-23°C) 



76.5 ± 1.7t 1.0 ± 0.0 

93.0 ± 2.4 25.8 ± 0.2 

91.8 ± 1.5 12.3 ± 0.1 

89.7 ± 1.8 31.6 ± 0.1 

87.7 ± 2.0 2.4 ± 0.0 

85.0 ± 0.3 NDt 

84.4 ± 0.8 1.2 ± 0.0 



* Defined as 



Particle no. in presence of Con A 



1- 



_ Particle no. in presence of Con A 1 

tMean ± standard deviation. 
tNot determined. 



Alteration of Cell Surface Properties by 
Vesicle Adsorption 

In the foregoing studies (Huang, 
Ozato, and Pagano) on the mechanism 
of lecithin vesicle uptake by lympho- 
cytes, it was shown that DML or DPL, 
but not EYL or DOL, vesicles become 
associated with lymphocytes princi- 
pally through a stable vesicle-to-cell 
adsorption process. Thus, those lipid 
vesicles which have the most signifi- 
cant effect on the mitogenic response 
and on lectin binding and agglutina- 
tion are those which adsorb to the cell 
surface. This correlation suggests that 
the alteration of lymphocyte surface 
properties may be a direct consequence 
of vesicle-to-cell adsorption. 

The stable adsorption of vesicles to 
cells could lead to an enhanced binding 
of mitogen to vesicle-treated cells, in 
those cases in which mitogen binds di- 
rectly to lipid. Thus, the enhanced Con 
A binding observed in DML or DPL 
pretreated cells may be a result of di- 
rect interaction between Con A and ad- 
sorbed vesicles. The finding that a sig- 
nificant fraction of Con A binding is 
noninhibitable by «-MM, particularly 
in DPL treated cells, supports the idea 
that some fraction of the binding might 
represent a direct association of lipid 
with Con A. In further support of this 
notion is a recent study (van der Bosch 
and McConnell, Proc. Nat. Acad. Sci. 
U.S.A., 72, 4409-4413, 1975) demon- 
strating a strong interaction of Con A 
with DPL vesicles, which results in 
vesicle- vesicle fusion. 

The adsorption of vesicles to cells, as 
well as the postulated vesicle-lectin in- 
teractions, can also be used to explain 
the differential effects of vesicle treat- 
ments on lectin-induced agglutinabil- 
ity. Thus, in the presence of Con A, 
agglutination could be mediated both 
by cell-bound Con A and Con A bound 
to cell-associated vesicles. In this re- 
gard it is interesting to note that the 
agglutination of DPL or DML vesicle 
pretreated thymocytes could not be 
fully reversed by a-MM (Table 8). This 



DEPARTMENT OF EMBRYOLOGY 



61 



may be due to the large residual Con A 
binding found in cells treated with 
these vesicles (Fig. 37). 

The present findings demonstrate an 
enhanced proliferation in stimulated T 
cells and B cells following incubation 
with various unilamellar lipid vesicles. 
This observation is particularly signif- 
icant, since T and B cells respond to 
different specific mitogens and may 
have different triggering mechanisms. 
The stable adsorption of lipid vesicles 
to lymphocytes may provide a common 
element by which the enhancement in 
these different cell types can take 
place. Thus, the enhancement of cell 
proliferation following vesicle treat- 
ment may be due to the increased 
mitogen binding resulting from 
vesicle-to-cell adsorption and the sub- 
sequent utilization of mitogen in 
vesicle-treated cells. 

In conclusion, our results suggest a 
direct correlation between the surface 



localization of exogenous lipids result- 
ing from vesicle to cell adsorption and 
their ability to modify lymphocyte cell 
surface properties such as agglutina- 
bility and lectin binding. It is further 
suggested that vesicle-to-cell adsorp- 
tion can have profound physiological 
consequences in treated cells — namely, 
enhancement of the proliferative activ- 
ity of stimulated lymphocytes. 

Studies of Membrane Adhesion 

M. Takeichi and R. E. Pagano 
with the assistance ofW. Duncan 

The adhesive properties of cells are 
among the more important factors con- 
trolling cell behavior and mor- 
phogenesis in multicellular organisms. 
While many investigators have at- 
tempted to elucidate the mechanisms 
of cell adhesion, the molecular basis of 
this phenomenon remains unclear. In 



4- 



2- 



I I Total Binding 

Binding Inhibitable by a _ MM 





DML 



DPL 



EYDChol 
I'l 



EYL 



DOL 



Cont. 



Fig. 37. Effect of different lipid vesicle pretreatments on the binding (1 hr at 2°C) of [ 125 l] Con A ( 1 
fig/ml) to normal thymocytes. 



62 



CARNEGIE INSTITUTION 



the present study of membrane- 
membrane interactions, we describe 
the adhesive properties of Chinese 
hamster V79 fibroblasts to each other 
as well as to artificial lipid vesicles, 
and we present a preliminary charac- 
terization of some of the cell surface 
molecules that may play a role in the 
adhesion process. 

Vesicle to Cell Adhesion 

Elsewhere in this report we have 
documented the formation of a stable 
adsorption between unilamellar phos- 
phatidyl choline vesicles comprised of 
dimyristoyl lecithin (DML) or dipal- 
mitoyl lecithin (DPL) in vesicle-treated 
lymphocytes. Such a stable vesicle-to- 
cell adhesion appears to be a general 
phenomenon and has also been found 
to occur in the V79 fibroblast cell line. 
The characteristics of this vesicle-to- 



cell adhesion phenomenon are de- 
scribed below. 

Monolayer cultures of V79 cells were 
dissociated into single cells with 
chelating agents (EDTA; E cells) or a 
combination of EDTA followed by tryp- 
sin treatment (ET cells). The dispersed 
cells were then incubated with [ 3 H] 
DML vesicles for 1 hr at 2 °C, washed 
and analyzed for the incorporation of 
radioactive phospholipid. Figure 38 
shows the dependence of DML uptake 
on vesicle concentration. Between con- 
centrations of 0.005 and 5.0 mg/ml of 
applied DML vesicles, the DML uptake 
increased monotonically from about 0.1 
to 8.7 x 10 9 molec DML/cell. Further- 
more, the technique used to dissociate 
the cells from the culture dish prior to 
vesicle-cell incubation had a marked 
effect on the amount of DML which be- 
came cell associated, with E cells tak- 
ing up about an order of magnitude 



9.0 



<u 5.0- 



Q 4.0 



E 3.0 




0.00 



10.0 



(ng. DML/cell) applied 



Fig. 38. Concentration dependence of DML uptake by V79 cells in suspension. Cells were dis- 
sociated with EDTA (o o) and subsequently treated with trypsin (x x) prior to incubation with 

vesicles. 



DEPARTMENT OF EMBRYOLOGY 



63 



more exogenous phospholipid than ET 
cells. The presence of the divalent cat- 
ions (Ca ++ or Mg ++ ) in the vesicle sus- 
pension had no effect on the amount of 
DML uptake in E or ET cells. 

EM autoradiograms of E cells and 
ET cells (Fig. 39) treated with [ 3 H] 
DML vesicles (1 mg/ml) for 1 hr at 2 °C 
clearly show that the radiolabeled 
DML is confined primarily to the cell 
surface, consistent with the stable 
adhesion of this vesicle type of cell. Ad- 
ditionally, it is seen that the amount of 
radiolabel found associated with ET 
cells is considerably less than for E 
cells, in agreement with the data in 
Fig. 38. In preliminary experiments, a 
protein homogenate of E and ET cells 
incubated with [ 3 H] DML vesicles was 
subjected to SDS-polyacrylamide gel 
electrophoresis. The distribution of 
[ 3 H] DML in the gel suggests that a 
few of the protein bands had a high af- 
finity for the radiolabeled DML. Thus, 
it is concluded that the adsorption of 
intact DML vesicles may involve some 
trypsin-sensitive material at the cell 
surface. A further characterization of 
this material is in progress. 



Cell to Cell Adhesion 

Monolayers of V79 cells were dis- 
sociated into single cells with chelating 
agents or proteases, and the ability of 
these cells to reaggregate was deter- 
mined. Cells dispersed with EDTA (E 
cells) possess strong reaggregability in 
the absence of divalent cations, which 
was enhanced in the presence of CA 2+ 
(Fig. 40a). Cells dispersed with crystal- 
line trypsin in Ca 2+ - and Mg 2+ -free 
medium or with trypsin in the presence 
of EDTA (TE cells) had no ability to 
reaggregate in any medium tested 
(Fig. 40c). Cells dispersed with crystal- 
line trypsin in the presence of Ca 2+ (TC 
cells), however, totally retained the 
reaggregability in a Ca 2+ containing 
medium but showed no indication 
that there may be two pathways for 
cell-to-cell aggregation, a Ca 2 + - 
independent and a Ca 2+ -dependent one. 
The function of Ca 2+ -dependent adhe- 
sion sites seems to be protected from 
trypsin digestion by the presence of 
Ca 2+ (Fig. 40b vs. 40c). Mg* + at physio- 
logical concentration could not substi- 



¥ 







Fig. 39. EM autoradiographs of V79 cells incubated with [ :! H] DML vesicles (1 hr at 2°C). (a) 
EDTA-dissociated cell, (b) EDTA-dissociated and trypsin-treated cell. 



64 



CARNEGIE INSTITUTION 



Nt 
No 



1.0 








— O O Q J 

-Ca 


5 9 e 

-Ca or + Ca 


— ©■ — 


— © 


0.8 


- \ 


Nq -Ca 












0.6 




N. ° 












0.4 








\ + Ca 


- 






0.2 


a 


+ Ca ^^^ 

i i 


b 


i i i 


» C 


i 





30 45 



30 45 
M I N 



15 30 



45 



Fig. 40. Reaggregation of cells dissociated with 1 mM EDTA (a), 0.01% trypsin + 0.1 mM Ca 2+ (b), 
and 0.017r trypsin + 1 mM EDTA (c) in the medium with or without 1 mM Ca 2 + . The extent of 
aggregation was represented by an index Nt/No, where No is the initial particle number in cell 
suspension and Nt is the total particle number at the time t,. of incubation. 



tute for Ca 2+ in the phenomenon de- 
scribed above. 

Additional experiments were carried 
out to compare the adhesive properties 
of cells to a noncellular substrate with 
the cell-to-cell adhesion described 
above. Ca 2+ (but not Mg 24 " or Mn 2+ ) was 
totally ineffective in promoting the 
adhesion of cells to gelatin-coated plas- 
tic substrate. Furthermore, reagents 
that inhibit cell-to-substrate adhesion 
(e.g., cytochalasin B, colcemid, and tet- 
racaine) did not significantly affect the 
Ca 2+ -dependent cell-to-cell adhesion. 
TC and TE cells were not markedly dif- 
ferent in their adhesiveness to noncell- 
ular substrates. These observations 
suggest that a Ca 2+ -dependent mecha- 
nism observed for cell-to-cell adhesion 
is absent in cell-to-substrate adhesion. 

In order to explore the possibility 
that changes in cell-to-cell adhesive- 
ness correlate with modifications of cell 
surface proteins, the surface proteins of 
intact fibroblasts were labeled with 125 I 
using lactoperoxidase, and the protein 
species subsequently examined by 
SDS-polyacrylamide gel electro- 



phoresis. While a number of proteins 
are labeled with 125 I in all three cell 
types (Fig. 41a), several iodinated 
bands were more intensely labeled in E 
cells than the others, suggesting that 
one of the cell surface proteins could be 
related to the Ca 2+ -independent adhe- 
siveness of cells. The difference in iodi- 
nated bands between TC and TE cells, 
however, was clear and simple. One 
protein whose molecular weight (MW) 
is in a range of 140K to 150K was pre- 
dominantly found in TC cells. No other 
qualitative difference in iodinated 
bands was observed between these two 
types of cells. The above 140-150 MW 
band was also present in E cells. 

The correlation of the presence of 
140-150 MW protein with the ability 
of cells to aggregate in Ca 2+ was also 
found in another experiment. TE cells 
did recover the Ca 2+ -dependent adhe- 
siveness after several hours of incuba- 
tion in normal cell culture medium. 
This recovery could be completely in- 
hibited by the addition of cy- 
cloheximide to the medium. Interest- 
ingly, the recovery of cell adhesiveness 



DEPARTMENT OF EMBRYOLOGY 

a b m 



65 




ft 



1 2 3 



Fig. 41. Autoradiograph of [ 125 I]-labled cell- 
surface proteins electrophoresed in 7.57c SDS- 
polyacrylamide gel. (a) E cells (1), TC cells (2), 
and TE cells (3). (b) TE cells cultured for 6 hr in 
the medium without (1) and with (2) 100 /xg/ml 
cycloheximide. Iodination was carried out in 
Ca 2+ -free medium at 5°C (a) and at 20°C (b). The 
difference in the pattern of iodinated bands be- 
tween (a) and (b) is due to the different tempera- 
ture used for the iodination. Bars indicate the 
position of 140-150K molecular weight region. 
Top is cathode. 



directly correlated with the appearance 
of the 140-150 MW protein at the cell 
surface of TE cells (Fig. 41b). 

Evidence was also obtained suggest- 
ing that the 140-150 MW cell-surface 
protein can interact with Ca 2+ . This is 
supported by the finding that the 
radiolabeling of this protein was sensi- 
tive to the presence of Ca in the iodina- 
tion medium. Thus, the 140-150 MW 
protein in TC cells could be iodinated 
only in the absence of Ca 2+ (Fig. 42a vs. 
42c), while the labeling of the other 
proteins in TC cells and all the proteins 
in TE cells were unaffected by the pre- 
sence or absence of Ca 2+ (Fig. 42b). 
This effect of Ca 2+ might be equivalent 
to another effect of Ca 2+ to protect the 
140-150 MW protein from trypsin di- 
gestion. These phenomena suggest that 
the 140-150 MW protein undergoes a 




CM MIGRATED 

Fig. 42. Densitometer traces of autoradio- 
graphs of [ ,25 l] -labeled cell-surface proteins elec- 
trophoresed in 7.5% SDS-polyacrylamide gel. (a) 
TC cells iodinated in the absence of Ca 2 + ; (b) TE 
cells iodinated in the absence or presence of Ca 2+ ; 
(c) TC cells iodinated in the presence of Ca 2+ . Bar 
indicates the position of 140-150K molecular 
weight region in each figure. 



conformational change or alters its 
molecular state in the cell membrane 
as a result of an interaction with Ca 2+ . 
In summary, these observations 
suggest a correlation of the appearance 
of the 140-150 MW surface protein 
with the Ca 2+ -dependent mechanism 
for cell adhesion. The ability of this 
protein to interact with Ca 2+ is consis- 
tent with the possible participation of 
this protein in a Ca 2+ -dependent cell 
adhesion mechanism. Future experi- 



66 



CARNEGIE INSTITUTION 



ments will focus on the isolation and 
purification of this protein to deter- 
mine its role in cellular adhesion pro- 
cesses. 

Distribution of Lipophilic 

Fluorescent Probes in Intact 

Cells 

R. E. Pagano, K. Ozato, and J. M. Ruysschaert* 

The use of small lipophilic fluores- 
cent molecules for probing the physical 
state of lipid bilayer model membranes 
has been the subject of investigation by 
"membranologists" for a number of 
years. Such probes partition into the 
hydrophobic phase of the lipid bilayer 
and can be used to detect, by fluores- 
cence changes, subtle changes in the 
molecular packing of the lipid 
molecules in the vicinity of the probe. 
Recently, such probes have been used 
to monitor the membranes of intact liv- 
ing eukaryotic cells; however, no direct 
examination of the location of these 
molecules in whole cells has been 
made. We report here on the in- 
tracellular distribution of three such 
fluorescent probes, l,6-phenyl-l,3,5- 
hexatriene (DPH), perylene, and 2-p- 
toluidinyl-6-naphthalene sulfonate 
(TNS), in murine thymocytes, using 
radiolabeled probes and the technique 
of EM autoradiography. 

DPH, perylene and TNS were 
radiolabeled by tritium gas exposure 
method of Wilzbach and subsequently 
purified by crystallization from the ap- 
propriate organic solvents. Labeling of 
mouse thymocytes was performed with 
a 2 x 10" 6 M solution of each of the 
probe molecules dispersed in 0.15 M 
KC1. One volume of cell suspension 
(0.5-2 x 10 7 cells/ml) was mixed with 
one volume of probe suspension and in- 
cubated for 60 min at 25 °C. The 
labeled cells were then washed twice, 
fixed in glutaraldehyde, and processed 
for electron microscope autoradiog- 
raphy by standard methods. 

*Free University of Brussels. 



Micrographs of thymocytes treated 
with [ 3 H] fluorescent probes were 
taken at 14,000 x , and the outline of in- 
dividual cells and location of grains 
traced on paper. Each traced cell was 
then divided into two compartments 
and the number of silver grains in each 
compartment determined. A cell- 
surface compartment was defined by a 
zone two half distances (HD) wide and 
centered over the cell-surface mem- 
brane contour. The remaining portion 
of the cell defined the internal com- 
partment. The HD determined by 
Salpeter for the electron microscope 
autoradiographic conditions used in 
the present study ( :i H; 100-nm sections; 
Ilford L4 purple interference color; 
Microdol-X development) was 160 nm. 
For each cell, the areas of these com- 
partments were measured. Grain den- 
sities were obtained by dividing the 
number of grains in each compartment 
by the area of that compartment. The 
ratio of surface grain density to inte- 
rior grain density and the average 
±SEM were then calculated. The re- 
sults of these measurements for each of 
the probes tested are summarized in 
Table 9. It is readily seen, both from 
the percentage of silver grains found in 
the surface compartment and from the 
ratio of surface to internal compart- 
ment grain densities, that in DPH and 
perylene the probes are largely inter- 
nalized in the treated cells while most 
TNS is confined to the cell surface. 

The cell population used for these 
studies was contaminated with a small 
fraction ( — 1%) of red blood cells and 
thus permitted a simultaneous 
analysis of the distribution of each of 
the probes in this cell type. The data 
obtained for [ :! H] DPH is given in Fig. 
43 and shows a symmetrical distribu- 
tion of the radioactivity about the cell 
membrane, with approximately 80% of 
the grains found within a distance of ± 
0.3 microns of the membrane surface. 
Similar data were obtained for [ 3 H] 
TNS and [ 3 H] perylene. 

These autoradiographic findings are 
consistent with the known physical 



DEPARTMENT OF EMBRYOLOGY 



67 



TABLE 9. Distribution of Autoradiographic Grains over Mouse Thymocytes 
Treated with [ :i H] Fluorescent Probes 



Probe 



(Surface Grain 

Percent Grains 

Total Grains in Surface (Interior Grain 

Counted Compartment Density) 



± S.E.M. 



! H TNS 
! H DPH 
! H Penylene 



302 
990 
918 



69 
16 

27 



11.4 ± 1.3 
2.0 ± 0.3 
2.4 ± 0.1 



o 10- 



II 



-0.7 -0.6 -0.5 -0.4 -0.3 



-0.2 -0.1 +0.1 +0.2 

Distance (microns) 



1-0.3 +0.4 +0.5 +0.6 +0.7 



Fig. 43. Distribution of autoradiographic silver grains in murine red blood cells, treated with [ 3 H] 
DPH. Number of grains found is plotted vs. their distance in microns from the cell surface ( + , outside; 
-, inside). 



properties of the fluorescent probes 
used, and suggest that except for TNS 
or cells such as the erythrocyte, consid- 
erable ambiguity may arise in inter- 



preting fluorescence data obtained 
with whole cells containing various in- 
tracellular membranes into which 
these probes can partition. 



MEMBRANE TRANSPLANTATION BY FUSION 

L. Huang and A. K. Ritchie 



The goal of this work is to establish 
methods by which one can transfer a 
membranous protein molecule with a 
well-defined function to the plasma 
membrane of cells normally devoid of 
such function ("membrane transplan- 
tation") and to study the physiological 



function of the protein in its altered 
environment. 

Since most hydrophobic proteins re- 
quire a lipid environment for their 
function, the transplantation process 
can best be accomplished by using the 
lipid membrane as a carrier for the pro- 



68 



CARNEGIE INSTITUTION 



tein. Joining the carrier membrane 
with the receiving membrane, a fusion 
process, should then transfer the pro- 
tein molecules to their target mem- 
brane. One approach is to solubilize 
and isolate protein molecules and to re- 
constitute them into lipid vesicles. The 
reconstituted vesicles can then be used 
for fusion with the recipient cell. An 
alternative method is to fuse intact 
membrane vesicles, isolated from a 
source very much enriched with a pro- 
tein, directly with the plasma mem- 
brane of the recipient cell. The 
molecule used in this study is the 
nicotinic acetylcholine receptor 
(AChR), an integral membrane protein 
responsible for the selective ion per- 
meability found in the postsynaptic 
membrane of some nerve and muscle 
cells. We hope to introduce chemosen- 
sitivity toward cholinergic agonists in 
recipient cells. The ACh receptor is a 
convenient protein, since in addition to 
its well-defined functional activity it 
can also be tagged in a highly specific 
and relatively irreversible manner 
with 125 I-a-bungarotoxin. 

We have isolated AChR from the 
electroplax of Torpedo californica. The 
electroplax contains membrane that is 
extremely rich in nicotinic acetyl- 
choline receptors. Tissue from the elec- 
troplax (stored at -70 °C) was 
homogenized in a buffered isotonic salt 
solution, centrifuged (100,00G£ x 30 
min at 4°C), then successively washed 
in a high and low salt concentration be- 
fore solubilization by 2% triton X-100 
(1 hr at room temperature). All solu- 
tions contained 0.02% NaN :i and 5 mM 
EDTA to minimize bacterial contami- 
nation and protein degradation and 
were kept at 4°C where possible. Par- 
tial purification and detergent ex- 
change were achieved by sedimenting 
the AChR into a sucrose density gra- 
dient containing 1% Na cholate. AChR 
sedimented as a 10S peak, occasionally 
with faster sedimenting shoulder. This 
preparation had about 0.2 /umoles of 
a-bungarotoxin binding sites/g protein 
which is about 20-fold less than that 
reported for Torpedo AChR purified by 



affinity chromatography. The partially 
purified AChR was then mixed with 
dioleyl lecithin (DOL) which had been 
predispersed in 2% Na cholate solu- 
tion. The final concentration of lipid 
was 10 mg lipid/ml in 1.5% cholate, 100 
mM NaCl, 5 mM EDTA, 0.02% NaN 3 
and 10 mM Tris, pH 7.4. The clear 
mixture was then exhaustively 
dialyzed against six changes of the buf- 
fered salt solution at 4°C for 72 hours. 
The solution became slightly turbid 
after dialysis. Thin-section transmis- 
sion electron microscopy revealed the 
presence of very uniform unilamellar 
vesicles with an average diameter of 
500 A (Fig. 44a). If the DOL-cholate 
dispersed solution was dialyzed with- 
out protein, similar vesicles of smaller 
size (average diameter 400 A) were 
found (Fig. 44b). The homogeneity and 
efficiency of vesicle formation was also 
demonstrated by the presence of a 
single peak on Sepharose 4B gel 
chromatography which represented 
99% of the radioactivity when using 
tritium labeled DOL. The reconstitu- 
tion of the AChR into vesicles was de- 
termined by velocity sedimentation in 
a detergent free sucrose density gra- 
dient. AChR was always associated 
with the 3 H-DOL vesicles which floated 
at the top of the gradient (Fig. 45a). If 
vesicles were first ruptured by 2% tri- 
ton X-100, dissociation of AChR from 
the lipids occurred (Fig. 45b) with the 
lipids remaining at the top of the gra- 
dient and receptor migrating to its 
usual 10S position. The amount of re- 
ceptor which could be incorporated into 
the vesicle fraction increased linearly 
then saturated as the protein/lipid 
ratio reached 45 ng protein/mg lipid. 
About two a-bungarotoxin binding 
sites per vesicle were found under sat- 
uration conditions. Such a low satura- 
tion level of reconstitution may be due 
to the differential affinity of AChR and 
other contaminating proteins for the 
hydrophobic sites in the lipid vesicles. 
We expect much higher efficiency for 
reconstitution when pure AChR is 
used. The reconstituted vesicles were 
also tested for 125 I-a-bungarotoxin ac- 



DEPARTMENT OF EMBRYOLOGY 



69 



■r "*0,. 










* it* > 



.'S '<•. 



b 



Wr^ . 



• ># 



.¥. 






m 



3 



m 



,i s " pe 



*£t ■ 



Fig. 44. Reconstituted vesicles prepared by dialysis method, (a) Vesicles containing AChR and 
DOL. (b) Vesicles containing DOL alone. Bar is 2,000A. 




16 20 4 8 12 

Fraction number 



Fig. 45. Velocity sedimentation profiles of '-^I-a-bungarotoxin labeled AChR and 'H-DOL in the 
reconstituted vesicles in (a) detergent-free linear sucrose density gradients (5-20%- w/w), and (b) 
similar gradient but containing 1% triton X-100. Fractions of 0.22 ml were collected from the bottom 
of the gradient. These receptors were prelabeled by treatment of the crude triton extract with 0.1 
/xg/ml '- s I-a-bungarotoxin prior to reconstitution. Free ,25 I-a-bungarotoxin was separated from toxin 
receptor complexes on a BioGel P60 column. 



70 



CARNEGIE INSTITUTION 



tivity in the presence or absence of tri- 
ton X-100. About 80% of the total 
AChR in the vesicle population was 
available for binding when vesicles 
were not ruptured by detergent, i.e., 
most of the reconstituted receptors had 
their toxin-binding sites facing the ex- 
terior of the vesicle. 

Simple adsorption of the receptor to 
the exterior of the lipid vesicle seems 
unlikely, since co-migration of lipid 
and receptor on a triton-free sucrose 
gradient failed to occur when dialyzed 
receptor was mixed with preformed 
vesicles. We have also attempted to 
distinguish between incorporated re- 
ceptor and loosely adsorbed receptor by 
testing for sensitivity to chymotrypsin 
digestion. Chymotrypsin digestion (0.1 
mg/ml) of receptor extracted with tri- 
ton from Torpedo or from cultured 
chick myotubes and of dialyzed Tor- 
pedo receptor results in a shift of 10S 
material to a 3S position on the sucrose 
gradient. On the other hand, receptors 
on the surface of cultured chick 
skeletal myotubes or on reconstituted 
vesicles remain as 10S material after 
digestion by chymotrypsin. Thus, re- 
ceptors associated with vesicles occupy 
a protected position (i.e., failure to be 
degraded to 3S material) similar to the 
receptors in the membrane of intact 
chick myotubes. The comparable ex- 
periments on Torpedo membranes are 
still in progress. These results encour- 
age us to believe that the reconstituted 
receptor is an integral part of the vesi- 
cle membrane and not merely adsorbed 
to its surface. 

When the reconstituted AChR vesi- 
cles were incubated with Chinese 
hamster V79 cells for 45 min at 37 °C, 
extensive uptake of AChR by the cells 
was found with no evidence of 
cytotoxicity. The intact cell-associated 
AChR remained available for specific 
125 I-a-bungarotoxin (the binding was 
inhibited by 10~ 4 M curare and formed 
a receptor toxin complex that migrated 
as 10S on sucrose gradients). The 
vesicle-treated cells acquired about 5x 
10 4 a-bungarotoxin binding sites per 
cell. An autoradiograph of a sparsely 



plated culture dish of V79 cells that 
had been treated with reconstituted 
vesicles then labeled with 125 I-oi- 
bungarotoxin revealed that most of the 
grains were directly over cells. This is 
in contrast to the random labeling of 
the dish when cells were treated with 
dialyzed receptors. An EM autoradio- 
graph of vesicle-treated cells that had 
been labeled with 125 I-a-bungarotoxin 
further showed that the 125 I-label was 
randomly distributed over the cell sur- 
face. The persistence of the surface lo- 
cation of AChR in these cells was also 
examined. Vesicle-treated cells that 
were incubated for various times in 
complete growth medium at 37 °C still 
retained over 70% of their original ac- 
tivity when tested for toxin binding 
after 10 hr of incubation. If vesicle- 
treated cells were labeled with 125 I-a- 
bungarotoxin and subsequently incu- 
bated at 37 °C, those cells also retained 
over 70% of their radioactivity after 10 
hr (Fig. 46). About 43% of the radioac- 




Fig. 46. The persistence of labeled receptor on 
the surface of V79 cells containing transplanted 
AChR. Monolayer cultures of V79 cells were in- 
cubated for 45 min at 37°C with reconstituted 
AChR. The cells were washed three times with 
Gey's medium, then labeled with ,25 I-a- 
bungarotoxin. The washed cultures were then 
incubated at 37°C for the indicated amount of 
time. The percentage of '-"'I-a-bungarotoxin re- 
maining on the culture is plotted against the in- 
cubation time. 



DEPARTMENT OF EMBRYOLOGY 



71 



tivity that was lost from the cells into 
the medium after 10 hr of incubation 
was receptor-toxin complex, while 57% 
was low molecular weight material (in- 
cluded on a Bio-Gel P60 column) simi- 
lar to the receptor- 125 I-a-bungarotoxin 
degradation product that results from 
the metabolism of the internalized 
membrane of cultured chick myotubes. 
Thus, the specific ct-bungarotoxin bind- 
ing activity of AChR can be trans- 
planted to the surface of V79 cells pre- 
viously devoid of any toxin-binding 
ability, and such activity remains on 
the cells for relatively long periods of 
time. 

The uptake of receptor by V79 cells 
treated with vesicles was time depen- 
dent (Fig. 47a) but independent of the 
temperature of incubation. This is in 
marked contrast to the uptake of 
labeled lipid by the same cultures. The 
uptake of labeled lipid was dependent 
on both the time and temperature of 
incubation and consistent with the 
kinetics observed using vesicles formed 
by other methods. The amount and na- 
ture of lipid uptake was the same 



whether pure lipid vesicles or vesicles 
containing receptor were used. Only 
0.1% of the receptor was associated 
with the cells after 45 min of incuba- 
tion at 0°C, whereas 0.33% of the 
lipids were associated with the cells 
under the same conditions (Table 10). 
When the study was repeated with ves- 
icles containing prelabeled receptors, 
then the percentage of incorporation 
was higher, since this assay measures 
internalized receptors as well as the 
externally available receptors. In this 
case the percentage Of incorporation of 
the lipid (0.26%) and! that of the recep- 
tor (0.2%) at 0°CJwere nearly the 
same. The discrepancy between the 
amount of lipid uptake and the recep- 
tor uptake becomes very large at 37 ° 
due to the absence of any enhancement 
of receptor uptake at the higher tem- 
perature. The mechanism responsible 
for the temperature-dependent portion 
of lipid uptake (a process involving ves- 
icle fusion) is thus different from the 
process involving receptor uptake. It is 
not clear to us at this time how the re- 
ceptor is being transplanted. 




Fig. 47. The uptake of AChR and :! H-DOL by V79 cells. A confluent 35-mm dish of V79 cells was 

incubated with reconstituted vesicles at 0° (• • ) and 37°C (O O) for the indicated times. Each 

dish was treated with vesicles containing 8 mg/ml of DOL and 0.36 fig/ ml of protein, then washed 
three times with Gey's medium. The plates were then incubated with 0.1 fig/ml of 125 I-a-bungarotoxin 
for 30 min at room temperature. The cells were washed six times with Gey's medium, then extracted 
with 1% triton and counted by liquid scintillation (for :i H; Fig. 47B) and/or a Gamma well counter (for 
,25 I; Fig. 47A). 



72 



CARNEGIE INSTITUTION 



TABLE 10. Uptake of Lipid and ACh Receptor by V79 Cells Treated with Vesicles 
Containing the Reconstituted Receptor 





Temp. (°C) 


'H-DOL incorporation 

into cells as percent 

of the total :i H-DOL 

applied 


Receptor incorporation 
into cells as percent 
of the total applied 


* Receptor and 3 H-DOL 
vesicles 

t 125 I-a-bungarotoxin 
receptor and :! H-DOL 
vesicles 




37 


37 


0.33 
1.1 

0.26 
0.66 


0.1 
0.1 

0.2 
0.2 



* Cells were treated for 45 min at 37 °C with vesicles containing unlabeled receptors. The cultures 
were then washed three times with Gey's medium and incubated for 30 min at room temperature with 
0.1 /ti-g/ml 125 I-a-bungarotoxin. Cultures were washed 6x5 min, then extracted with 19c triton X-100. 

tCells were treated in the same manner as above except that the vesicles contained receptors that 
had been prelabeled with 125 I-a-bungarotoxin prior to reconstitution. 



We have also attempted to introduce 
the AChR into the plasma membrane 
of V79 cells by treating these cells with 
membrane fragments isolated from the 
Torpedo electroplax. A membrane frac- 
tion containing 10 2 moles of 
a-bungarotoxin binding sites/g protein 
was obtained by sedimenting a crude 
membrane preparation on a discon- 
tinuous sucrose density gradient. 
Thin-section electron microscopy of 
this fraction showed closed membrane 
vesicles of very heterogeneous size dis- 
tribution (0.1-2 /u. in diameter). A sus- 
pension of this material was incubated 

with a monolayer of cultured V79 cells 
in the presence of UV-inactivated Sen- 
dai virus or lipid vesicles comprised of 
90% phosphatidyl choline and 10% 
phosphatidyl glycerol or phosphatidyl 
serine. Some toxin binding activity was 
observed, but this method did not ap- 
pear to be as promising as the use of 
the reconstituted vesicles to transplant 
membrane proteins. 

Finally, we have tried to test our re- 
constituted vesicles and V79 cells 
treated with the reconstituted vesicles 
for some indication of functional activ- 
ity by the receptor (i.e., ion permeation 
specifically induced by a cholinergic 
agonist). The most satisfactory method 
of testing the vesicles for 22 Na ion flux 
was a 1-ml P60 Bio-Gel column in 
which vesicle-associated 22 Na could be 



completely separated from free 22 Na in 
1 min. When reconstituted receptor 
vesicles were tested, they demon- 
strated an uptake of 22 Na which was 
saturated within 10 min. This influx, 
however, was not enhanced by the pre- 
sence of carbachol, a cholinergic 
agonist. Attempts to test the V79 cells 
that had been treated with vesicles for 
AChR activity by electrophysiological 
techniques were also negative. These 
cells had resting potentials of about 
-25 mV but did not respond to bath or 
iontophoretic application of ACh. 
There was no change in the ACh sen- 
sitivity of cultured chick myotubes 
similarly treated with reconstituted 
receptor after blockade of endogenous 
receptors with a-bungarotoxin. There 
are a number of possibilities for our 
failure to detect receptor functional ac- 
tivity: (1) The receptor, although re- 
taining its oi-bungarotoxin binding ca- 
pacity, may have lost its ability to 
function as an ionophore due to some 
denaturation or degradation of the pro- 
tein. (2) The isolated receptor may be 
lacking some other necessary compo- 
nent that was discarded or inactivated 
in the isolation process. (3) The recep- 
tor may have been transferred but was 
not situated in the membrane in an in- 
tegral manner that allowed it to func- 
tion as an ionophore. Limited success 
by other investigators in restoring both 
crude and purified AChR into artificial 



DEPARTMENT OF EMBRYOLOGY 



73 



membranes and vesicles, although 
with only intermittent success, leads 
us to believe that the first possibility is 
the most likely reason for our failure. 

Experiments are in progress to de- 
termine the conditions necessary for 
restoring functional activity to the re- 
ceptor. The mechanism by which 



transplantation occurs will also be of 
primary importance, using either the 
ACh receptor as a marker — -provided 
that functional activity can be 
restored — or some other membrane 
protein. Our preliminary data indicate 
that this is a promising method of 
membrane transplantation. 



MECHANISM OF SYNGENEIC RESPONSE INDUCED BY LECTINS 

K. Ozato and J. D. Ebert 



In Year Book 74, pp. 90-92, we dem- 
onstrated that the modification of the 
lymphocyte surface by lectins renders 
the cells responsive to syngeneic lym- 
phocytes, as revealed by the prolifera- 
tion of T cells stimulated by spleen 
cells bearing the same histocompatibil- 
ity antigens. A syngeneic response can 
be obtained when either responding 
cells or stimulating cells are pretreated 
with native or succinyl-Con A (N-Con 
A or S-Con A). 

During the past year we attempted a 
more analytical study of this syngeneic 
response. Two directions of research 
were undertaken. We first undertook 
to determine the characteristics of the 
cellular stimulus produced by 
syngeneic spleen cells. We have ob- 
tained data that suggest that the re- 
sponse is characteristic of T cells. Con- 
genital athymic mice (nu/nu) provide a 
lymphocyte population deficient in T 
cells. The spleen cells from nu/nu mice 
did not respond to syngeneic cells. 
Further, in an allogeneic situation, 
they did not manifest the Con A pre- 
treatment effect that usually augments 
the mixed lymphocyte reaction. Con A 
coating of either responding cells or 
mitomycin-C treated spleen cells failed 
to respond in nu/nu mice. Since we first 
observed the syngeneic response in a 
pure T-cell population, we now believe 
that the response is a pure T-cell spe- 
cific phenomenon. 

The analysis of the stimulating 
population has been carried out by 
treating the cells with antibodies 
against specific surface components of 



T cells or B cells. Goat antimouse Ig or 
AKR anti CBA# were used for elimina- 
tion of B cells and T cells, respectively. 
Antibody treatment was carried out be- 
fore the cells were coated with Con A. 
The effect of antibodies was studied 
when both responding cells and 
stimulating cells were coated with Con 
A. As shown in Table 11 (a and b) when 
stimulating spleen cells were treated 
with anti-Ig, the response was effec- 
tively suppressed, whereas anti-0 anti- 
body treatment did not affect the re- 
sponse. This finding indicates that 
stimulatory function depends on the 
presence of B cells. We have tested the 
possible participation of adherent cells 
(mostly macrophages). Spleen cell 
populations that are deleted of adhe- 
rent cells by passing the cells through 
plastic petri plates still manifest their 
stimulatory function. 

Then how do responding T cells 
interact with stimulating cells (proba- 
bly B cells)? Do B cells release a factor 
into the medium? Do stimulating cells 
somehow enable Con A molecules to 
associate with T cells more effectively? 
Stimulating and responding cells must 
either be in contact or be "very close 
neighbors," for the culture fluid in 
which stimulating cells have been 
grown does not stimulate. Moreover, 
when stimulating cells and responding 
cells are cultured separately, using 
nucleopore filter membranes, the 
syngeneic response is not observed. 
The role of stimulating cells does not 
appear to be to provide physical sup- 
port of Con A molecules on the surface, 



74 



CARNEGIE INSTITUTION 



TABLE 11(a). Effect of Anti-0 or Anti-Ig on Con A-induced Syngeneic Response 
CRT vs. [N-Con A]-Syngeneic Spleen 





3 H-TdR Uptake on Day 


3 (cpm x 10 :i /culture) 


Treatment 


B10.D2 


CBA 


Medium RPMI 


27.2 ± 3.0 


18.3 ± 1.4 


Normal Serum AKR (1/20) 


30.3 ± 2.4 


16.7. ± 1.0 


Normal Serum Goat (1/20) 


32.4 ± 2.6 


16.2 ± 1.3 


Rabbit C (1/15) 


15.6 ± 1.6 


17.6 ± 1.5 


AKR Anti- 0(1/20) 


25.3 ± 3.0 


18.8 ± 1.0 


AKR Anti- 0(1/40) 


38.0 ± 2.5 


16.0 ± 0.7 


AKR Anti- 0(1/40) + C (1/15) 


29.3 ± 3.0 


17.2 ± 2.0 


Goat Anti-Ig (1/20) 


0.8 ± 0.1 


0.8 ± 0.1 


Goat Anti-Ig (1/40) 


0.2 ± 0.0 


1.0 ± 0.0 


Goat Anti-Ig (1/40) + C (1/15) 


0.4 ± 0.0 


0.3 ± 0.0 



Spleen cells pretreated with mitomycin C were then treated with antibodies for 45 min at 37°C in 
the presence or absence of complement (C). The parentheses indicate the dilution of the antiserum. 
After washing the cells to remove killed cells as well as unbound antibodies, cells were further treated 
with N-Con A at 50 /u-g/ml at 37°C for 30 min. They were then mixed with responding cells 
(cortisone-resistant thymocytes) in a 1:1 ratio (2x 10 6 cells) in 0.5 ml. The response was measured by 
:l H-TdR uptake on day 3. 

TABLE 1Kb). [N-Con A-CRT] vs. Syngeneic Spleen 





! H-TdR Uptake on Day 3 


(cpm x 10 3 /culture) 


Treatment 


C57BL/10 


CBA 


Medium RPMI 
Normal Serum AKR (1/20) 
Normal Serum Goat (1/15) 
Rabbit C (1/15) 


12.7 ± 0.5 

10.3 ± 0.8 

14.4 ± 1.1 
13.0 ± 0.6 


12.3 ± 0.7 
11.5 ± 0.8 
10.0 ± 0.8 
10.0 ± 0.8 


AKRAnti-0(l/2O) 
AKR Anti- 0(1/40) 
AKR Anti- 0(1/40) + C (1/15) 


11.4 ± 0.7 
10.9 ± 0.3 
12.0 ± 1.0 


9.0 ± 0.1 

13.5 ± 1.3 

8.9 ± 0.9 


Goat Anti-Ig (1/20) 
Goat Anti-Ig (1/40) 
Goat Anti-Ig ( 1/40) + C ( 1/15) 


0.3 ± 0.0 
0.9 ± 0.1 

0.2 ± 0.0 


4.3 ± 0.7 
4.7 ± 0.3 
3.0 ± 0.1 



Stimulating spleen cells were treated with antibodies in the same manner as (a), after which they 
were mixed with responding cells that had been treated with N-Con A (50 /u.g/ml). 



for when stimulating cells are killed by- 
fixation or by freezing and thawing 
after they were coated with Con A, the 
response is completely abrogated (Ta- 
ble 12). However, treatment with 
metabolic inhibitors or the microfila- 
ment disrupting agent, cytocalasin B, 
did not affect the response. Thus we 
may suggest that the intact cell mem- 
brane of stimulating cells is required. 
It is an interesting possibility that 
membrane components may be ex- 
changed between responding cells and 



stimulating cells and that Con A 
molecules are transferred from 
stimulating cells to responding cells or 
vice versa. We are now studying the 
fate of Con A molecules, using 125 I-Con 
A bound to either stimulating cells or 
responding cells to test this possibility. 
Another goal of our research has 
been to delineate the role of lectins. If 
the response is entirely dependent on 
stimulating cells, the role of Con A or 
other lectins must be to enhance the 
"physical contact" of responding and 



DEPARTMENT OF EMBRYOLOGY 



75 



TABLE 12. No Syngeneic Response from Fixed "Stimulating" Cells 



! H-TdR Uptake on Day 3 (cpm xlO" 3 ") 



Treatment of Stimulating Cells 



I. Responding Cells: 
Coated with N-Con A 



II. Stimulating Cells: 
Coated with N-Con A 



1. No treatment 

2. Glutaraldehyde(2.5%) 

3. Ethanol 

4. Acetone 

5. Freezing and thawing 

6. Cytochalasin B (40 fxg/ml) 

7. Dinitrophenol (2 mM) 

8. Sodium azide (50 mM) 



18.5 

0.2 

0.3 

0.1 

0.2 

23.7 

22.4 

23.1 



15.6 

0.0 

0.3 

0.1 

0.1 

18.4 

19.2 

17.8 



Stimulating cells (mitomycin-C treated) were treated with various reagents for 15 min at room 
temperature (2 to 4) or treated for 45 min at 37°C (5 to 8). Freezing and thawing were repeated twice. 
In column II, Con-A pretreated cells were used. The response of cortisone-resistant thymocytes was 
measured on day 3. 



stimulating cells. If so, any lectin that 
can agglutinate lymphocytes should 
give the same effect. Among lectins we 
have tested, similar syngeneic re- 
sponse was observed with two other 
lectins, namely, leucoagglutinin 
(purified form of phytohemagglutinin) 



(Fig. 48), and lentil lectin (a gift from 
John Cebra at Johns Hopkins Univer- 
sity). Although wheat germ aggluti- 
nins agglutinate lymphocytes, they do 
not induce the syngeneic response. 
Binding to a sugar does not seem to be 
a specific requirement. The ability to 



15 


1 

1 


i 

h- 


i 


1 " " 1 


10 


1 
1 
i 
I j 

1 f 
I |1 
■1 f 
1 


~> 




s 

s 
s 
\ 

\ 
\ 


( 
5 


I 1 
I j 

i J 






\ 


< 


> I 


~ * 


* 


* * 



O 20 



IOO 200 300 500 

Leucoagglutinin Concentration (fiq/m\) 



Fig. 48. CBA cortisone-resistant thymocytes were cultivated with syngeneic (CBA) (• •) or 

allogeneic (B10.D2) (o o) spleen cells pretreated with leucoagglutinin at the concentrations indi- 
cated at 37°C for 30 min. As controls, spleen cells (A A) were cultured alone. 



76 

produce a syngeneic response appears 
to be limited to those lectins that have 
mitogenic properties in T cells. It is 
therefore conceivable that such lectins 



CARNEGIE INSTITUTION 

not only effect a physical association 
between cells but also provide a 
stimulus to the responding cells. 



STUDIES ON SKELETAL MUSCLE AND NEURONAL PLASMA 

MEMBRANES 

D. M. Fambrough, R. E. Pagano, P. N. Devreotes, K. Tepperman, D. J. Card, and S. Carbonetto 
with the technical assistance of A. Mabin, W. Duncan, andH. W. Chen 



The acetylcholine (ACh) receptor is a 
major glycoprotein in the muscle 
plasma membrane of the neuromuscu- 
lar junction and a minor component of 
extrajunctional plasma membrane. 
The ACh receptor has been our point of 
focus in studies on the differentiation 
and organization of skeletal muscle 
plasma membrane, and it has become 
clear that the number and distribution 
of ACh receptors in skeletal muscle 
(which vary with the developmental 
and physiological state of the muscle) 
are regulated largely by regulation of 
ACh receptor metabolism. Our studies 
of the metabolism of extrajunctional 
ACh receptors have led us to propose a 
model for the major events in receptor 
metabolism, illustrated in Fig. 49. 



The main features of the model are 
as follows. The protein subunits of 
which the ACh receptor is composed 
are synthesized by the conventional 
protein synthesizing mechanisms of 
cells and are rapidly assembled into re- 
ceptor units resembling those in the 
plasma membrane. These newly syn- 
thesized receptors occur upon some 
membrane system(s) in the cytoplasm 
of the muscle cells and are collectively 
referred to as the "precursor pool." The 
average residence time of a new recep- 
tor in the precursor pool is 2-3 hr. Re- 
ceptors are transferred from this pool 
to the surface by an energy-requiring 
process and are then able to function. 
The average half-life of an ACh recep- 
tor in the plasma membrane is 22 hr. 



HYPOTHETICAL "LIFE-CYCLE" of ACETYLCHOLINE 
RECEPTORS 



mRNA(s) 




V = PRECURSOR 

V = SURFACE 



M- 



I-TYROSINE 



Fig. 49. Hypothetical "life cycle" of acetylcholine receptors in cultured chick skeletal muscle. The 
figure depicts a cross section through a myotube with receptors symbolized as triangles in membrane 
profiles. Arrows indicate processes consistent with data on receptor metabolism. Symbols for precur- 
sor and surface receptors and for 125 I-a-bungarotoxin and ,25 I-tyrosine are indicated on figure. 



DEPARTMENT OF EMBRYOLOGY 



77 



Receptors are degraded by an energy- 
requiring process that involves the in- 
ternalization of the receptor and trans- 
port to secondary lysosomes and then 
proteolytic destruction. 

This model for ACh receptor biosyn- 
thesis and turnover was developed 
mostly on the basis of data from exper- 
iments involving the use of 
a-bungarotoxin and [ 125 I] labeled 
a-bungarotoxin as probes for ACh re- 
ceptors. The strategies for using these 
probes to measure the accumulation of 
receptors in plasma membranes, the 
incorporation of newly synthesized re- 
ceptors into plasma membranes, and 
the degradation of receptors have been 
described in previous Year Books. 
Some of the details of the models were 
based upon data from studies involving 
inhibitors of various cellular processes. 
In Year Book 74 (pp. 68-73) we de- 
scribed the application of two new 
techniques to the study of ACh receptor 
metabolism: electron microscope au- 
toradiography and the "density shift" 
technique. EM autoradiographic 
studies during the past year have pro- 
vided quantitative supporting data for 
the degradative aspects of the model of 
receptor metabolism. The density shift 
technique has made it possible to label 
ACh receptors directly and thus to ver- 
ify the kinetics of receptor biosynthesis 
and incorporation into plasma mem- 
branes. 

Now that we can confidently meas- 
ure many steps in the metabolism of 
extrajunctional ACh receptors, we 
have begun to investigate the manner 
in which external stimuli influence the 
number and distribution of ACh recep- 
tors in skeletal muscle fibers via their 
influence upon receptor metabolism. 
The two classes of stimuli postulated to 
affect receptor metabolism are (1) 
hypothetical "trophic" factors released 
by motoneurons onto muscle fibers and 
(2) muscle activity (which is normally 
driven by the activity of motoneurons 
but can be artificially driven by electri- 
cal stimulating). So far we have only 
investigated the effect of artificially 
driven or suppressed muscle activity on 



the metabolism of extrajunctional re- 
ceptors. These studies suggest that 
muscle activity leads to a reduction in 
the number of receptors because of a 
decrease in the rate of receptor biosyn- 
thesis or incorporation into plasma 
membranes. 

The organization of ACh receptors 
into dense clusters at postsynaptic sites 
may involve active "gathering" of ex- 
trajunctional receptors or merely 
stabilization of receptors implanted in 
the plasma membrane at the site of 
nerve-muscle interactions. From an 
experimental point of view, one can 
pose the question: Can preexisting ex- ' 
trajunctional receptors be accumulated 
into postsynaptic receptor arrays? To 
investigate this question and also to 
gain some insight into the number and 
distribution of receptors on embryonic 
skeletal muscle during differentiation 
and synapse formation, we have begun 
a study of the chick anterior latissimus 
dorsi muscles. We are also developing 
techniques to determine whether or not 
ACh receptors on muscle are free to 
move in the plasma membrane. 

We consider the ACh receptor both a 
functional entity of great interest in its 
own right and also an example of a 
membrane glycoprotein. In this latter 
frame of reference, we are interested to 
know whether the kinetics of turnover 
of ACh receptors are typical of or iden- 
tical to the kinetics of turnover of other 
membrane proteins. To investigate this 
matter we have been attempting to 
label a set of membrane proteins by the 
lactoperoxidase iodination technique 
and then to study the turnover of this 
class of proteins. Some progress and 
the definition of some difficulties are 
summarized in the report. 

Our interest in excitable cells has ex- 
tended to neurons, and as a first ap- 
proach to studies of membrane 
metabolism and control of membrane 
properties in neurons we have selected 
chick sympathetic neurons. This choice 
was based largely upon the work of 
Greene and collaborators, who have 
demonstrated the specific binding of 
a-bungarotoxin to such cells. We are 



78 



CARNEGIE INSTITUTION 



currently testing whether or not 
tv-bungarotoxin binds to ACh receptors 
in cultures of such neurons and 
whether the binding is strong enough 
to permit us to develop strategies simi- 
lar to those used to study the 
metabolism of skeletal muscle ACh re- 
ceptors. 

ACh Receptor Biosynthesis 



[ 125 I] mono-iodo-a-bungarotoxin. In 
addition, a-bungarotoxin receptor 
complexes can be sedimented through 
dense sucrose-deuterium oxide gra- 
dients. As will be demonstrated below, 
receptors synthesized from [ 2 H, 13 C, 
15 N] amino acids and complexed with 
ct-bungarotoxin can be separated from 
normal density cc-bungarotoxin- 
receptor complexes on such gradients. 



P. N. Devreotes 

Purification of the plasma mem- 
brane and its proteins poses a major 
technical difficulty to membrane 
biologists. The total plasma membrane 
protein usually represents only about 
1% of the cell protein, and specific 
membrane components may represent 
only a small fraction of this total. In 
their investigations of membrane pro- 
teins, therefore, membrane biologists 
have turned to specific markers or spe- 
cific chemical modification techniques 
to tag these proteins. Marking methods 
generally label membrane components 
exposed on the outer surface of the 
membrane after synthesis and after in- 
sertion into the plasma membrane. Ac- 
cordingly, attempts to study biosyn- 
thesis and turnover of membrane com- 
ponents have been indirect, including 
our studies of ACh receptor metab- 
olism with the use of ct-bungarotoxin. 
Criticism is usually raised that the de- 
gradation or turnover of the bound 
marker rather than the native protein 
itself is being studied. Attempts have 
been made to study biosynthesis and 
turnover directly by labeling the mem- 
brane components with radioactive 
amino acids. With this approach, how- 
ever, the advantages gained through 
the specificity of the marker must be 
sacrificed. Marker specificity can be re- 
tained in direct labeling experiments if 
receptors are directly labeled not with 
radioactive amino acids but with 
amino acids that change a physical 
property of the receptors — their densi- 
ty. 

A specific and nearly irreversible 
marker for acetylcholine receptors is 



['h, I2 c, ,4 n] 

RECEPTORS 




30 
FRACTION 

Fig. 50. Density-shifted receptors from dif- 
ferentiating cells. Twenty large-muscle cultures 
were grown for 24 hr in normal medium. At that 
time, there were very few myotubes in culture, 
and the myoblasts were fed with medium con- 
taining [ 2 H, 13 C, 15 N] -amino acids. After the cells 
had fused in the density-labeled medium, the 
surface receptors were complexed with [ l25 l] 
mono-iodo-a-bungarotoxin. The complexes were 
then extracted into 259c cholate, 10-mM Tris and 
mixed with a marker of [ i: "l] mono-iodo-a- 
bungarotoxin-receptor complexes prepared from 
myotubes grown in normal medium. A 0.4-ml 
aliquot of the mixed extract was carefully 
layered over a 25-40% sucrose-deuterium oxide 
gradient containing Y7c triton, ImM EDTA- 
PMSF, 10 mM Tris,pH 7.8. Centrifugation was 
for 2 days in a Beckman SW41 rotor at 36K at 
4°C. Gradients were pumped into scintillation 
vials and the activity of each isotope determined. 



DEPARTMENT OF EMBRYOLOGY 



79 



So far, the most reliable method of 
preparing a sample of pure density 
labeled [ 125 I] mono-iodo-a-bungaro- 
toxin-receptor complexes is to allow 
cells to differentiate in medium con- 
taining [ 2 H, 13 C, 15 N] amino acids, 
with [ 125 I] mono-iodo-a-bungarotoxin 
and extracted in nonionic detergents. 
Nearly complete separation of these re- 
ceptors from [ X H, 12 C, 14 N]-receptors 
has been obtained, as shown in Fig. 50. 
Over 90% of the acetylcholine recep- 
tors extracted from cells that have dif- 
ferentiated in pH, 13 C, 15 N] amino acid 
medium are of altered buoyant density. 

An estimate of the apparent density 
shift between PH, 12 C, 14 N]-receptors 
and [ 2 H, 13 C, 15 N]-receptors can be ob- 
tained by measuring sedimentation 
rates of the respective complexes with 
a-bungarotoxin in shallow gradients of 
different average densities. The aver- 
age density of the gradients was varied 
by substitution of D 2 for H 2 0. It can 



be shown that for shallow gradients the 
logarithm of the distance traveled by a 
given protein versus time is a straight 
line. For a given protein, the slope of 
this line is a function of the average 
density of the gradient medium. In fact 
if X is the slope, then 

A = k (l-p lp) 

where k is a constant dependent on 
molecular weight, rotor speed, and 
temperature; p is the density of the 
protein; and p is the average density 
of the medium. Plots of A. versus p , 
therefore, intersect the p axis at p. 
This kind of analysis has been carried 
out for PH, 12 C, 14 N]- and [ 2 H, 13 C, 
15 N]-receptors. As shown in Fig. 51, the 
apparent density of PH, 12 C, 14 N]- 
receptors is 1.26 and that of [ 2 H, 13 C, 
15 N]-receptors is 1.33. When correc- 
tions are made for detergent binding to 
the a-bungarotoxin receptor complexes 
(detergent binding causes, in part, the 




4 6 8 10 

TIME (HOURS) 



1.2 1.3 

p (g/CM 3 ) 



Fig. 51. Estimate of density shift. Identical aliquots of the same mixed extract described in the 
legend to Fig. 49 were layered onto identical gradients and centrifuged for different times. Gradients 
for each series were 5-20% sucrose in H 2 0, 50% D 2 or D 2 containing 1% triton, 10 mM Tris. 
Gradients were collected into scintillation vials, and the position of the two bands determined at each 
time for the three different series. X„ and X refer to the distance from the center of the rotor to the 
meniscus and the position of the band, respectively. Lambda is the slope of the straight lines shown on 
the left. Experiment was repeated twice. 



80 



CARNEGIE INSTITUTION 



anomalous low density of the receptor), 
a density shift of 0.08 g/cm 3 is calcu- 
lated. This corresponds to a shift of ap- 
proximately 6%. The maximum at- 
tainable shift (calculated from the 
amino acid composition of purified re- 
ceptors from Electrophorus electricus) 
is about 8.5%. Since the calculated 
shift is a minimum estimate, a sub- 
stantial fraction of the amino acids of 
the receptor must be substituted by 
PH, 13 C, 15 N]-amino acids. 

As has been described in the previ- 
ous section, the model for acetylcholine 
receptor biosynthesis suggests that re- 
ceptors undergo a 3-hr transit time be- 
fore being inserted into the plasma 
membrane. This feature of the model 
has been derived from observations 
using puromycin and cycloheximide to 
block protein synthesis. When protein 
synthesis is inhibited, new receptors 
are incorporated into the surface for 
about 3 hr and then incorporation 
stops, suggesting that the pool of pre- 
synthesized receptor has been depleted. 
Direct evidence of a 3-hr transit time 
for receptors can be obtained by the 
density shift technique. Density- 
shifted [ 2 H, 13 C, 15 N]-receptors should 
begin to appear in the cell surface 
about 3 hr after their synthesis. 

The experiment was carried out as 
follows. The existing surface receptors 
were first blocked by an overnight ex- 
posure to unlabeled a-bungarotoxin. 
After rinsing away the unbound 
a-bungarotoxin, myotubes were incu- 
bated in medium containing [ 2 H, 13 C, 
15 N]-amino acids. New receptors then 
appear on the surface of the myotubes 
as a linear function of time (in these 
experiments, the heavy medium may 
have had a slightly deleterious effect 
on the cultures at late times) and can 
be labeled at subsequent times with 
[ 125 I] mono-iodo-a-bungarotoxin. After 
extraction into nonionic detergents, 
these [ 125 I] mono-iodo-a-bungaro- 
toxin-receptor complexes were mixed 
with a marker of [ ,31 I] mono-iodo-a- 
bungarotoxin-receptor complexes 
extracted from myotubes grown in 
normal medium. The mixed extract 



was then centrifuged through 
sucrose-deuterium oxide gradients to 
determine the fraction of [ 2 H, 13 C, 
15 N]-receptors. These density-shifted 
receptors represent the fraction of 
newly appearing receptors that were 
synthesized after addition of dense 
amino acids to the medium. As shown 
in Fig. 52 new receptors begin to ap- 
pear on the surface without a lag; how- 
ever, [ 2 H, 13 C, 15 N]-labeled receptors 
appear at the maximum rate only after 
a lag of about 3.5 hr. Small amounts of 
[ 2 H, 13 C, 15 N]-receptors appear at 2 and 
3 hr; after 3.5 hr most of the newly ap- 
pearing receptors are density shifted. 

Since most receptors only appear on 
the surface at the maximum rate 3.5 hr 
after their biosynthesis, it is suggested 
that newly synthesized receptors 
undergo a 3-hr transit time within the 
cell. Since small amounts of receptors 
appear at 2 and 3 hr, however, it is 
suggested that the intracellular trans- 
port of receptors is not a strictly linear, 
assembly-line process. Newly synthe- 
sized receptors and receptors already in 
transport can become, to a certain ex- 
tent, intermixed and inserted at ran- 
dom into the plasma membrane. Alter- 
natively, the small amounts of receptor 
appearing at 2 and 3 hr after synthesis 
might be in a subpopulation of 
myotubes in which the transit time is 
shorter than average. 

It has also been learned from these 
experiments that acetylcholine recep- 
tors are not "unmasked" during the de- 
velopment of the cultures. Most of the 
newly appearing receptors are synthe- 
sized a few hours before incorporation 
into plasma membranes. Furthermore, 
as evident from the experiment de- 
scribed in the legend to Fig. 50, the 
acetylcholine receptors expressed dur- 
ing differentiation of myoblasts into 
multinucleated myotubes are newly 
synthesized. It is unlikely, therefore, 
that presynthesized receptors are 
stored within myoblasts and expressed 
during differentiation. 

The density shift technique for 
acetylcholine receptors has enabled us 
to confirm several aspects of the model 



DEPARTMENT OF EMBRYOLOGY 



81 



for acetylcholine receptor biosynthesis 
without purification of the receptor or 
the plasma membrane. We are using 
density labeling technique to measure 
the kinetics of incorporation of isotop- 



ically substituted amino acids into the 
receptors undergoing intracellular 
transport and to measure directly the 
half-life of receptors once they become 
members of the surface population. 




4 6 

TIME (HOURS) 

Fig. 52. Appearance of density-labeled receptors on surface membrane. Three-, four-, or five-day-old 
cultures of myotubes were saturated with unlabeled a-bungarotoxin (0.2 /u,g/ml for about 15 hr). After 
rinsing away unbound a-bungarotoxin, the medium was replaced with medium containing [ 2 H, I:i C, 
15 N] amino acids. After incubation for the indicated times at 37°C, sets of cultures were cooled at 4°C 
and saturated with [ 125 l] mono-iodo-a-bungarotoxin. The [ 125 l] mono-iodo-a-bungarotoxin-receptor 
complexes were then extracted in 2.5% cholate, 10 mM Tris, mixed with a marker of 131 I mono-iodo- 
a-bungarotoxin-receptor complexes prepared from myotubes grown in normal medium. Samples were 
layered in thin bands on 10-20% sucrose-deuterium oxide gradients containing 1% triton, lOmM Tris. 
After centrifugation for 1 day in a SW41 rotor at 36K at 4°C, gradients were collected into scintilla- 
tion vials. Total receptors (o). Amount of material running to the position of [ 2 H, 13 C, 15 N]- 

receptors (• ••) was determined by curve fitting. Different ratios of "heavy" and "light" peaks were 

added together by a computer program which used, for the shape of each peak, the shape of the 
marker on each gradient. These curves were matched to the data and could be fit within 5% . Similar 
results could be obtained by counting the area outside the marker peak and expressing it as a fraction 
of the total area. 



Autoradiographic Analysis of 
Receptor Degradation 

P. N. Devreotes and D. M. Fambrough 

The rate of appearance of radioactiv- 
ity in the medium after [ 125 I]-mono- 
iodo-a-bungarotoxin is bound to ACh 
receptors is a measure of receptor de- 
gradation. Our data on the mechanism 
of degradation of receptors are consis- 
tent with a model (Fig. 49) in which 
receptors are internalized and then de- 



graded. For example, homogenates of 
myotubes labeled with [ 125 I] mono- 
iodo-a-bungarotoxin have little or no 
capacity for degradation; and deg- 
radation is inhibited by 2,4- 
dinitrophenol and is very temperature 
sensitive, having a Q 1() of 8. After a 
brief "pulse" incubation of myotubes 
with [ 125 I] mono-iodo-a-bungarotoxin, 
the rate of release of [ 125 I] mono-iodo- 
tyrosine into the medium (due to deg- 
radation of the [ 125 I] labeled 
a-bungarotoxin initially bound to sur- 
face ACh receptors) requires about 90 



82 



CARNEGIE INSTITUTION 



min to reach a maximum (Fig. 53). 
During this time some of the labeled 
toxin-receptor complexes must move 
into a small compartment of receptors 
earmarked for degradation. Since the 
lag time is equivalent to about one 
hour of degradation and since receptors 
are normally degraded at about 3% per 
hour, the compartment should contain 
about 3% as many receptors as there 
are surface receptors. We have looked 
for the subcellular location of this 
hypothetical compartment, using elec- 
tron microscopy autoradiography. In 
these studies the location of radioactiv- 



ity on and within cultured muscle cells 
was determined as a function of time 
after brief exposure of cells to [ 125 I] 
a-bungarotoxin. The large secondary 
lysosomes of the muscle cells were 
found to accumulate radioactivity up to 
a plateau level of slightly over 2% of 
total cellular radioactivity (Fig. 53). 
This accumulation occurred with kine- 
tics consistent with a model of degrada- 
tion in which the transport of [ 125 I]-a- 
bungarotoxin-receptor complexes to 
lysosomes and their hydrolysis in lyso- 
somes results in the production of 
[ 125 I]-tyrosine. 



.08 



.06 



.04 



.02 



EARLY KINETICS OF 
RECEPTOR DEGRADATION 




[ l25 l] TYROSINE 
IN MEDIUM 



GRAINS IN 
LYSOSOMES 



< 
a 

Id 

< 

UJ 

_l 

UJ 
K 

UJ 

> z 



60 120 

MINUTES AFTER CONTACT 
0F[ l25 l] a-BUNGAROTOXIN 



> C 

O > 

< h- 

Q i 

a , 

2 - 

in 

u. w 

0<— 



_ o 

I- 
o 
< 



Fig. 53. Kinetics of transport of radioactivity to secondary lysosomes (° o) and kinetics of 

liberation of iodo-tyrosine from chick muscle cultures (• •) after brief exposure of muscle to 

'"I-a-bungarotoxin. Data obtained from electron microscope autoradiographs and from chromato- 
graphic analysis of culture medium. 



DEPARTMENT OF EMBRYOLOGY 



83 



Effect of Muscle Activity on 
Receptor Turnover 

D. J. Card and D. M. Fambrough 

Recent experiments have shown that 
muscle activity is important in regulat- 
ing extrajunctional chemosensitivity of 
skeletal muscles. Acetylcholine sen- 
sitivity of muscle increases following 
denervation and decreases upon rein- 
nervation. Electrical stimulation of 
denervated or cultured embryonic 
muscle can lead to a decline in ex- 
trajunctional ACh sensitivity; on the 
other hand, an increased sensitivity to 
ACh can be produced after blockade of 
spontaneous contractions in cultured 
cells by tetrodotoxin (a selective in- 
hibitor of voltage-sensitive sodium cur- 
rent). These changes in ACh sensitiv- 
ity reflect changes in the number of ex- 
trajunctional ACh receptors. Thus, the 
changes should be understandable in 
terms of the incorporation of receptors 
into the muscle membrane and their 
subsequent degradation. 

Experimental strategies for measur- 
ing incorporation of new ACh receptors 
into unstimulated cultured chick 
skeletal muscle fibers and the degrada- 
tion of receptors thereafter have al- 
ready been reported by this laboratory. 
We were able to reproduce the observa- 
tion of Shainberg and Nelson that tet- 
rodotoxin induced a twofold increase in 
ACh receptor number within 24 hr of 
application to active cell cultures. 
However, a quantitative assessment of 
the synthesis and degradation of ACh 
receptors in active vs. quiescent cul- 
tured cells is difficult because it is 
nearly impossible to control or to quan- 
titate the amount of activity of the cul- 
tured cells. After 4 to 5 days in culture, 
muscle cells usually begin to develop 
spontaneous contractions, but not all 
the muscle cells contract. Also, the 
time of onset of spontaneous contrac- 
tions differs greatly between cultures, 
so not all 5-day or even all 11-day cul- 
tures are spontaneously active. It has 
not been possible to electrically stimu- 



late and drive every cell or even most 
cells in a culture dish. 

This laboratory has previously re- 
ported measurements of degradation 
rates of extrajunctional receptors from 
denervated mouse extensor digitorum 
longus (EDL) muscle. Because EDL 
muscle is easily activated by electrical 
stimulation, it was used to study the 
effects of muscular activity on degrada- 
tion of receptors. After mid-thigh re- 
section of the sciatic nerve, the lower 
leg (including the EDL muscle) is de- 
nervated. The time course of develop- 
ment of extrajunctional ACh receptors 
after denervation is tabulated in Table 
13. In our studies on receptor degrada- 
tion, muscles denervated for at least 5 
days were used. 

TABLE 13. Accumulation of ACh Receptors 
Following Denervation of Mouse EDL 





ACh Receptor Sites* 


Days After 


(molecules x 


Denervation 


10 9 /mg muscle) 





2.0 


1 


2.0 


2 


2.1 


3 


3.5 


4 


8.8 


5 


12.5 


7 


15.6 


9 


17.6 


14 


14.1 


21 


13.6 



*Measured as a-bungarotoxin binding sites. 

The degradation rate of extrajunc- 
tional receptors was inferred from 
measurements of the rate at which 
[ 125 I]-labeled a-bungarotoxin bound to 
these receptors was degraded with con- 
comittant release of radiolabeled de- 
gradation products from the muscle fi- 
bers. We have not yet characterized re- 
leased radioactive species, but this has 
been done with denervated rat dia- 
phragm. Metabolic inhibitors such as 
cycloheximide and dinitrophenol and 
reduced temperature decreased the 
degradation rate, as expected. The per- 
fusion chamber used in the degrada- 
tion measurements was similar to that 
reported last year, but instead of 



84 



CARNEGIE INSTITUTION 



mounting the muscle on a screen it was 
tied and stretched between two 
platinum electrodes (used for electrical 
stimulation) mounted on a Plexiglas 
strip. Oxygenated Trowell T-8 medium 
was dripped over the muscle and was 
continuously aspirated out of the mus- 
cle chamber into test tubes; the 
radioactivity of the perfusate was then 
measured in a gamma counter. The 
half-life of ACh receptors in the plasma 
membrane averaged 27 hr under these 
conditions. After several hours, the 
muscle was stimulated at 50 stimuli 
per sec for 1 sec every 10 sec for 8 hr. 
The degradation rate of receptors de- 
creased rapidly to 78% of the value be- 
fore stimulation. Thus stimulation of 
the denervated EDL muscle increased 
the half-life of the receptors in the 
plasma membrane to approximately 34 
hr. 

Degradation rate for extrajunctional 
ACh receptors does not appear to 
change during the first two weeks of 
denervation (Table 14). Also the 
stimulation-induced reduction in rate 
is independent of the length of the de- 
nervation period. Since chemosensitiv- 
ity declines during electrical stimula- 
tion without any acceleration of degra- 
dation, it follows that electrical stimu- 
lation must repress the biosynthesis or 
incorporation of new receptors into 
plasma membranes. 

TABLE 14. Degradation Rate of Receptors from 
Muscle Denervated 5-13 Days 

Denervation Time Receptors Degraded/Hour 
(days) (%) 

5 2.75 

6 2.38 

7 2.15 

12 2.34 

13 2.01 

The effects of muscle activity on ACh 
receptor biosynthesis and incorpora- 
tion into plasma membranes are not 
known. We are attempting to perfect a 
strategy for the high time-resolution 
measurement of incorporation rate of 
new receptors into extrajunctional 
membranes of denervated adult mus- 
cles. 



Organization of Muscle Plasma 
Membrane in vivo 

D. J. Card 

Acetylcholine receptors are found 
over the entire surface of embryonic 
skeletal muscle fibers in vivo. These 
sites gradually disappear after inner- 
vation, as clusters of acetylcholine re- 
ceptors form in the postsynaptic mem- 
brane. The junctional receptors may 
result from the organization of some of 
the extrajunctional receptors into a 
much more compact spatial arrange- 
ment. In assessing the role of ex- 
trajunctional acetylcholine receptors 
as components of newly forming 
synapses, it is necessary to know 
whether junctional receptors appear 
before, during, or only as a result of, 
formation of functional neuromuscular 
synapses. We are studying muscle 
morphology and acetylcholine receptor 
number and distribution at different 
developmental stages of the chick an- 
terior latissimus dorsi (ALD) muscles 
to learn the time of nerve outgrowth, 
synapse formation, appearance of junc- 
tional receptors, and disappearance of 
extrajunctional receptors. 

Muscles were removed from the em- 
bryos, fixed in 2% glutaraldehyde, 100 
xnM cacodylate buffer, sliced into 
1-micron sections and then stained 
with toluidine blue. In order to study 
receptor binding sites the muscles were 
incubated for 1 hr in a solution of 
[ 125 I]-a-bungarotoxin (0.5 /xg/ml), 
washed several hours and then fixed in 
2% glutaraldehyde, 100 mM cacody- 
late buffer. Following this treatment, 
the total number of acetylcholine re- 
ceptors was measured, and single mus- 
cle fibers were dissected for au- 
toradiography. 

Some preliminary observations 
based upon light microscopy and au- 
toradiography and on the binding of 
[ 123 I]-mono-iodo-o;-bungarotoxin follow. 
Initially (5-6 days incubation) the pre- 
sumptive anterior latissimus dorsi 
(ALD) muscle was composed of loosely 
organized, undifferentiated cells. How- 
ever, after 9 days, muscle cells and 



DEPARTMENT OF EMBRYOLOGY 



85 



nerve bundles were evident. Au- 
toradiography of single ALD muscle fi- 
bers (6-9 days of incubation) show 
radioactive label (ACh receptors) dis- 
persed uniformly over the muscle sur- 
face. No receptor aggregations were 
present. After 13-15 days incubation 
the ALD muscle fibers looked mor- 
phologically similar to post-hatch mus- 
cles, except that fiber size was smaller 
and ACh receptors were scattered over 
the cell surface. Some areas of each 
muscle fiber from 13 to 15 days devel- 
opment appeared to have a high recep- 
tor density, but these observations 
have not been quantified. 

At 16 days the ALD had well-defined 
clusters of ACh receptors spaced every 
120-170 microns along each muscle fi- 
ber, with many ACh receptors still 
scattered outside the clusters. The 
spacing of the dense clusters of ACh re- 
ceptors, presumably junctional recep- 
tors, agrees favorably with morpholog- 
ical, electrophysiological, and cholines- 
terase data obtained by others (Gordon, 
Perry, Ruffery and Vrbova, Cell Tiss. 
Res. 155: 13-25, 1974; Bennett and 
Pettigrew, J. Physiol. 241: 515-545, 
1974). 

Total number [ 125 I]-a-bungarotoxin 
binding sites was measured in muscles 
from embryonic day 6 to 2 days post- 
hatch (Table 15). Most of the [ 125 I] is 
present as receptor-[ 125 I]-o;-bungaro- 
toxin complexes as judged by sucrose 
gradient velocity' sedimentation of de- 
tergent extracts. It is not clear yet 
when functional synapses are formed. 
However, Gordon and her collaborators 
and Bennett and Pettigrew have re- 
ported morphological nerve contacts 

TABLE 15. [ 125 I]-a-Bungarotoxin Binding to 

Embryonic Chick ALD Muscles at Different 

Developmental Ages 



Developmental Age 


Muscle Weight (mg) 


(days) 


cpm/muscle 


6 


7,000 


11 


15,300 


13 


9,800 


16 


7,700 


19 


3,200 


20 


3,000 


+ 2 


1,300 



and acetylcholinesterase staining at 
days 9 to 15. The number of ACh recep- 
tors per mg muscle is also highest at 
that time. As extrajunctional binding 
sites disappear, the number of total 
ACh receptors/per mg muscle also de- 
creases. 

Studies on the Mobility of 

Extrajunctional ACh Receptors 

In Plasma Membrane 

R. E. Pagano and D. M. Fambrough 

The principal objective of this study, 
which is aided by a generous grant 
from the Whitehall Foundation, is to 
examine the general role of membrane 
fluidity in membrane organization and 
function. This problem will be tackled 
by measuring the lateral (transla- 
tional) mobility of acetylcholine recep- 
tors in skeletal muscle fibers, and by 
studying the effects which a variety of 
membrane perturbants (local anesthe- 
tics; exogenous phospholipid and 
cholesterol treatments) have on this 
mobility. Quantitative fluorescence 
microscopy will form the basis for de- 
termining protein (ACh receptor) mo- 
bility. The measurement will be made 
by complexing fluorescent-labeled 
a-bungarotoxin with ACh receptors on 
muscle fibers. A small area of fluores- 
cent cell surface will then be bleached 
with a short pulse of bright light, and 
the rate at which unbleached 
molecules from adjacent areas of the 
cell surface migrate into the bleached 
area will be quantified by measuring 
the fluorescence intensity from that 
area as a function of time. 

During the first year of this study, 
some of the necessary background 
work for carrying out our experiments 
was completed. Fluorescent-labeled 
a-bungarotoxin was prepared using 
fluorescein isothiocyanate (FITC), and 
the preparation was well charac- 
terized. It was shown that incubation of 
mono-FITC-bungarotoxin, with skele- 
tal muscle at 1 /xgm/ml results in 
highly fluorescent neuromuscular 
junctions as judged by fluorescence 
microscopy. The rate of FITC-a- 
bungarotoxin binding was also com- 



86 



CARNEGIE INSTITUTION 



pared with that of iodinated 
a-bungarotoxin, and found to be about 
50-fold slower than the radioactive 
species — inconsequential for our exper- 
iments. Preliminary experiments 
further demonstrated that FITC-a- 
bungarotoxin receptor complexes on 
skeletal muscle are readily bleachable 
and that reversal of this process is a 
very slow process. 

Preliminary experiments were car- 
ried out to test the feasibility of man- 
ipulation of cultured muscle cell lipids, 
using artificial phospholipid vesicles as 
agents for introducing foreign phos- 
pholipids into the cell surface. Cells 
treated at 2°C with dimyristoyl phos- 
phatidyl choline (DMPC) vesicles 
showed a significant accumulation of 
CMPC in their surface as evidenced by 
high resolution electron microscopy 
autoradiography. Furthermore, lipid 
analyses showed little degradation of 
the exogenously supplied lipid. Thus, it 
is concluded that significant amounts 
of perturbing foreign phospholipids can 
be introduced into the cell surface. 
Their effect on ACh receptor mobility 
remains to be tested. 

Preparation of a Fluorescent Derivative 
of a-Bungarotoxin 

Native a-bungarotoxin binds ex- 
tremely tightly to acetylcholine recep- 
tors on cultured skeletal muscle cells 
and is routinely used as a potent, very 
selective, and virtually irreversible in- 
hibitor of receptor function. The 
radioactive derivative, mono-iodo-a- 
bungarotoxin, retains the potency of 
the native toxin. 

During the past year we have de- 
veloped methods for the production of 
fluorescent derivatives of a-bungaro- 
toxin and have determined the toxicity 
of the most useful fluorescent deriva- 
tive. In essence the method involves 
the interaction of purified a-bungaro- 
toxin with fluorescein isothiocyanate 
in the cold at pH 9.0 in sodium 
carbonate-bicarbonate buffer. An 
eight-fold molar excess of FITC is used 
and the reaction proceeds slowly, first 



forming a mono-FITC-a-bungarotoxin 
and later, further derivatized forms. 
The unreacted FITC is then separated 
from the FITC-a-bungarotoxin by 
chromatography on Bio-Gel P-4 and 
any unreacted a-bungarotoxin is re- 
moved from the FITC-a-bungarotoxin 
by ion-exchange chromatography on 
CM-Sephadex C-50. It has been found 
that the mono- and di-FITC bungaro- 
toxin are also separable from each 
other in Bio-Gel P-4, the di-FITC 
a-bungarotoxin being retarded by the 
column. When mono-FITC a-bungaro- 
toxin is fractionated on Sephadex C-50, 
using a gradient of NaCl in 3.3 raM 
phosphate bufferpH 7.2, it is eluted far 
ahead of unreacted a-bungarotoxin and 
thus quantitatively freed of this con- 
tamination. 



Properties of Fluorescent 
a-Bungarotoxin 

The purified mono-FITC a-bungaro- 
toxin binds to acetylcholine receptors 
with a very high affinity. After incuba- 
tion of muscles in a solution containing 
about 1 fJLg/m\ FITC a-bungarotoxin, 
followed by rinsing to remove unbound 
toxin, the acetylcholine receptors at 
the neuromuscular junctions are com- 
plexed with FITC-cc-bungarotoxin and 
appear as vividly fluorescent struc- 
tures when examined by fluorescence 
microscopy. This interaction is essen- 
tially irreversible, and these fluores- 
cent structures maintain their fluores- 
cence for more than 6 months at 10°C 
in a dilute buffer solution. 

The relative rates of interaction of 
FITC-a-bungarotoxin and of radioac- 
tive iodinated a-bungarotoxin have 
been compared by mixing these two 
toxin derivatives in different concen- 
tration ratios and then measuring the 
amount of iodinated a-bungarotoxin 
that bound to cultured skeletal muscle. 
Such competition curves have been 
carried out on three preparations of 
FITC-a-bungarotoxin, and all suggest 
that the speed of binding is reduced 
50-fold for the fluorescent derivative. 
Thus, higher concentrations are re- 



DEPARTMENT OF EMBRYOLOGY 



87 



quired to saturate the acetylcholine re- 
ceptors in a short incubation period. 
Even so, 10" 7 M FITC-a-bungarotoxin 
will saturate the receptors in cultures 
of skeletal muscle in about 20 minutes. 

Use of FITC-a-Bungarotoxin as 

a Probe in Studying Membrane 

Structure 

Following our original proposal we 
have developed the FITC-a-bungaro- 
toxin as an easily bleachable fluores- 
cent tag for cholinergic receptors. Pre- 
liminary experiments using micro- 
scopic equipment at the Leitz and Zeiss 
showrooms in New Jersey and New 
York and qualitative experiments done 
with our yet incomplete fluorescence 
microscopy set-up at Carnegie demon- 
strate that fluorescent FITC-a-bun- 
garotoxin-receptor complexes on 
skeletal muscle fibers are readily 
bleached and that the bleaching is 
mainly irreversible or only very slowly 
reversible. Also, stable fluorescence 
measurements can be made when ap- 
propriate filters and light chopping are 
employed. Thus the main prerequisite 
for a fluorescent probe to use in execut- 
ing our proposed experiments on the 
mobility of receptors in plasma mem- 
branes has been met. 

Turnover of Plasma Membrane 
Proteins 

K. Tepperman 

The model for the turnover of the 
ACh receptor of muscle cells raises the 
question of the relationship of the re- 
ceptor to other plasma membrane pro- 
teins. The study of ACh receptor turn- 
over is aided by the availability of the 
specific probe a-bungarotoxin. Since 
other membrane proteins are not so 
easily identified, new approaches to 
studying turnover of populations of 
proteins are being devised. One ap- 
proach is to covalently label surface 
proteins by enzymatic iodination and 
to follow loss of radioactivity from spe- 
cific proteins. Another approach, de- 
veloped by Robert Schimke, is to allow 



the cells to incorporate different 
radioactive isotopes of leucine on two 
successive days and to look at the ratio 
of the different isotopes for each of the 
membrane proteins. We are using both 
of these approaches to study the turn- 
over of plasma membrane proteins of 
cultured muscle cells. 

In Year Book 74, p. 78, we reported 
preliminary observations on enzymatic 
iodination of muscle cells. At that time 
the total material on iodinated tissue 
culture dishes was extracted in SDS 
and run on polyacrylamide gels, result- 
ing in approximately 15 radioactive 
bands. It is difficult to determine which 
of these proteins are actually part of 
the membrane and which proteins may 
be less intimately associated with the 
surface. In the past year, several 
methods for dissociating loosely bound 
proteins from membranes were ex- 
plored. One currently in use is treat- 
ment of cells with low ionic strength 
medium, followed by homogenization 
and centrifugation to obtain a mem- 
brane pellet. When this pellet is used 
for SDS gels, the resulting pattern of 
iodinated polypeptide chains is much 
simpler than when total material is 
used. The number of radioactive bands 
is reduced to approximately seven, and 
one of the major bands in the original 
preparation, one with a molecular 
weight of about 230,000 daltons, is 
among those which are lost, suggesting 
that this protein is not an integral 
membrane protein. 

Preliminary results using surface 
iodination for turnover studies have 
suggested that most of the proteins 
that are accessible to iodination are 
degraded at the same rate. In these ex- 
periments, sets of cultures are iodi- 
nated, then incubated at 37 °C for dif- 
ferent periods of time. After extraction, 
equivalent amounts of radioactivity 
from different chase periods are loaded 
onto gels. Densitometer traces of au- 
toradiographs of the gels show that dif- 
ferent bands have essentially the same 
relative amounts of radioactivity. Ex- 
periments are now in progress in which 
[ 13, I]-bungarotoxin is bound to cells 



88 



CARNEGIE INSTITUTION 



that have been treated with lac- 
toperoxidase and [ 125 I]. This is done to 
provide an internal measure of the rate 
of ACh receptor turnover with which to 
compare the loss of [ 125 I]-label from 
other membrane proteins of the cells. 
Initial results are compatible with the 
idea that turnover of the iodine-labeled 
proteins occurs at the same rate as 
turnover of the ACh receptor. 

In the iodination experiment, several 
points have been raised which require 
further clarification. Although pub- 
lished reports on lactoperoxidase iodi- 
nation of cells generally indicate little 
labeling of lipids, when muscle cells 
are iodinated, approximately 35% of 
the radioactivity in a membrane frac- 
tion is chloroform methanol extracta- 
ble, indicating that lipids have been 
labeled. Thin-layer chromatograms 
have been done which show that the 
label is present in a variety of lipid 
fractions. Another result of the iodina- 
tion experiments is that during the 
course of incubation following iodina- 
tion, the radioactivity lost from the 
cells can be fractionated into three 
components by column chromatog- 
raphy on Bio-Gel P-2. One fraction 
probably represents nonspecifically as- 
sociated iodide, which comes off the 
cells and the tissue culture dishes dur- 
ing the incubation. Another fraction is 
iodotyrosine, which probably results 
from internalization and degradation 
of labeled proteins. A third fraction, 
excluded by the P-2 columns, may rep- 
resent proteins or other material that 
is lost from the cells. 

Another approach to studying turn- 
over of populations of proteins is the 
double isotope method of Schimke. In 
his reports using this method, he gen- 
erally finds a correlation between 
turnover rates of proteins and their 
molecular weights. Preliminary results 
using this method with cultured mus- 
cle cells show that the correlation for 
the major polypeptides of a crude 
membrane preparation holds. How- 
ever, experiments are in progress 
which will study this question on 
plasma membranes specifically. Also, 



the effects of various treatments de- 
signed to remove all but integral mem- 
brane proteins will be studied to de- 
termine whether the molecular weight 
correlation with turnover rate still 
holds. Such a correlation is contraindi- 
cated by the iodination results. 

Studies on Neuronal Plasma 
Membranes 

S. Carbonetto 

Much of our work on the plasma 
membrane during differentiation of 
skeletal muscle has relied upon the use 
of a-bungarotoxin and its radioactive 
derivatives that bind selectively to the 
acetylcholine receptor molecule in 
muscle cells. Similar information 
about formation of neuronal plasma 
membranes has been unobtainable due 
to lack of a suitable ligand. This report 
deals with our initial studies on a 
neuronal preparation that apparently 
binds a-bungarotoxin with high affin- 
ity. 

Sympathetic ganglionic chains have 
been dissected out of 11 to 12 day old 
chick embryos, dissociated in 0.25% 
trypsin, washed and plated onto 
collagen-coated culture dishes in 
Eagles MEM plus Nerve Growth Fac- 
tor (2 units/ml; Burroughs Wellcome). 
Within 24 hr neurons become attached 
to the surface of the plate and begin to 
send out processes. Intracellular elec- 
trophysiological recordings from these 
cells after three days indicate that 
neurons have resting membrane poten- 
tials of 30 mV, and can be stimulated to 
generate action potentials (Fig. 54). 
Furthermore, when cultured chick 
sympathetic neurons are incubated 
with [ 125 I ]-a-bungarotoxin and 
analyzed by light microscope au- 
toradiography, grains appear over the 
neuron soma and down to the tip of the 
processes but are absent over non- 
neuronal cells in the culture (Fig. 54). 
Washing the culture for 28 hr at 14 °C 
after [ 125 I]-a:-bungarotoxin binding 
only slightly reduced the density of 
grains over neurons. In one experiment 



DEPARTMENT OF EMBRYOLOGY 



89 



B 




100 y 



B 



I 



+i& 




"***&, 



Fig. 54. (A) Intracellular recording from a chick sympathetic neuron in culture. The voltage record, 
V, shows an action potential evoked by a stimulating pulse, S, applied to the surface of the neuron by 
an extracellular microelectrode. The uppermost trace is at mV. The calibration is 50 mV; 20 msec. 
(B) Autoradiograph of cultured chick sympathetic neurons incubated with 125 I-a-bungarotoxin. When 
viewed with phase optics the neuron and several nonneuronal cells are visible in the field (B,). With 
bright field illumination (B 2 ) only exposed emulsion grains are visible and these cover the neuronal 
cell body and its neurites. There is no localization of grains over any of the nonneuronal cells. 



10~ 3 M d-tubocurarine prevented the 
association of radioactivity with the 
cells. 

These preliminary data suggest that 
chick sympathetic neurons in culture 



exhibit properties of well-differen- 
tiated neurons and also may specif- 
ically bind a-bungarotoxin. Experi- 
ments are in progress to determine the 
association and dissociation kinetics of 



90 



CARNEGIE INSTITUTION 



bungarotoxin binding to neurons and 
also to localize the appearance of the 
grains by EM autoradiography. We are 
testing whether these neurons contain 
functional acetylcholine receptors by 



electrophysiologically monitoring their 
response to iontophoretically applied 
acetylcholine and testing if such re- 
sponse can be blocked by application of 
bungarotoxin. 



SIGNALING IN THE NERVOUS SYSTEM: SYNAPSES OF SPECIFIC 
NEURONS IN THE LEECH 

K. J. Muller and S. T. Carbonetto 
with the technical assistance of B. Thomas 



The role that a neuron assumes in 
the nervous system is dependent upon 
the connections it makes with other 
neurons or with peripheral targets. 
Therefore, an essential step both in de- 
termining the forces that act initially 
to organize the nervous system and in 
characterizing the mechanisms by 
which a neuron handles, or integrates, 
information is to ascertain the rela- 
tionship between a neuron's function 
and the arrangement of its synapses. A 
related problem in neurobiology has 
been to determine whether a neuron's 
shape and other properties are intrin- 
sic or whether its contacts or associa- 
tions with other neurons are essential 
for certain aspects of its growth and 
maintenance. 

The nervous system of the leech 
Hirudo medicinalis has been particu- 
larly favorable for study of the physio- 
logical role and functioning of specific 
neurons, largely because its organiza- 
tion is relatively simple. Neurons in 
this segmented worm are grouped 350 
together to form essentially identical 
ganglia that are linked in a chain by 
axon bundles; each ganglion mediates 
the reflexes and controls the behavior 
of its segment. It has been possible to 
monitor electrical activity from within 
single sensory and motor neurons with 
microelectrodes and to correlate the 
physiological activity of functionally 
identified neurons with their charac- 
teristic shape and position within each 
ganglion. By recording from pairs of 
neurons one can trace synaptic connec- 
tions, which have been found to be sur- 
prisingly uniform and in that sense 



predictable between specific neurons. 
Last year we described studies under 
way that provided a morphological de- 
scription of sensory and motor neurons 
and of the distribution of their 
synapses within the ganglion (Year 
Book 74, p. 85). These investigations, 
using the enzyme horseradish 
peroxidase (HRP) as an intracellular 
marker for both light and electron mic- 
roscopy, were to lay the groundwork for 
our determining any structural basis 
for physiological changes that we had 
previously produced with selective le- 
sions of the leech nervous system (J. 
Physiol. (Lond) 242, 289, 1974), as well 
as ft)*- charting the course of regenerat- 
ing neurons. An additional feature of 
HRP as an intracellular marker is that 
although it diffuses extensively within 
an injected neuron, it does not pass be- 
tween electrically coupled neurons, as 
do smaller markers used by others. 
Neuron terminals that make electrical 
contacts are therefore sharply demar- 
cated even within an intricate 
meshwork of neuron processes. 

Chemical Synapses in the Leech 

K. J. Muller and B. Thomas 
Structural Details of the Synapse 

A good picture of the normal, fully 
developed synapse is requisite for the 
study of the distribution and formation 
of synapses. Because the chemical 
synaptic transmission that one can 
record in the leech central nervous sys- 



DEPARTMENT OF EMBRYOLOGY 



91 



tern resembles that in other inverte- 
brates and vertebrates, it is not sur- 
prising that in the electron microscope 
customary synaptic features — the pre- 
sence of presynaptic vesicles, mem- 
brane densities and a widened, uniform 
intercellular cleft — serve to delineate 
the synapse. In the leech, synapses are 
made almost exclusively in the center 
of the ganglion in the region called the 
neuropil rather than on cell bodies that 
form layer or cortex around the gan- 
glion. Sections through the neuropil 
manifest a variety of synaptic profiles, 
but careful analysis reveals that most 
result from different planes of section 
through the same structural configura- 
tion (Fig. 55). Typically, agranular ves- 
icles (—50 nm diameter) are clustered 
near a ridge of presynaptic density that 
apposes a pair of conjoint postsynaptic 
profiles across a cleft about 30 nm 
wide. Larger (—100 nm) dense-core 
vesicles and mitochondria are also 
found in the presynaptic terminal. 
Identified mechanosensory neurons 
that have been injected with the HRP 
for an intracellular marker have 
branched processes that exhibit similar 
features (Fig. 56). This indicates, in ac- 
cord with their physiology, that these 
neurons are presynaptic. The same 
processes are also postsynaptic to other 
neurons. In contrast with the sensory 
neurons, the large longitudinal (L) and 
annulus erector (AE) motoneurons that 
act to shorten the animal and to erect 
its annular rings into ridges, respec- 
tively, are seen in the electron micro- 
scope to be solely postsynaptic within 
the neuropil. Physiologically these are 
neurons upon which the mechanosen- 
sory neurons synapse. One class of ap- 
parently presynaptic terminals (Fig. 
59, white arrow) has been found that 
does not readily fit the pattern of ter- 
minals that contain agranular vesicles: 
These terminals, which contain small 
granular vesicles (—50 nm diameter) 
and extremely dense, eccentric cored 
large vesicles (— 100 nm), are generally 
found to be cells that stain with the dye 
Neutral Red and are widely regarded 



to contain serotonin as a transmitter. 
What are in fact the membrane spe- 
cializations at synapses made by such 
neurons remains to be determined. 

The Arrangement of Synapses on 
Sensory Neurons 

One remarkable pattern in the or- 
ganization of the leech nervous system 
is that synapses of mechanosensory 
neurons are clustered upon fingerlike 
swellings or enlargements. Their line- 
aments are sufficiently similar within 
a modality, e.g., touch, pressure or 
stimulus, but different between them 
to permit a reliable distinction to be 
made between sensory modalities 
solely on the basis of the shapes and 
distribution of synaptic terminals 
viewed in the light microscope. A pos- 
sibility raised by the differences in the 
shapes of synapse clusters is that, as 
with certain vertebrate motor neurons, 
the morphologies of terminals are in- 
trinsic to the presynaptic neurons and 
are not dependent upon the type of 
postsynaptic target. This will be dis- 
cussed in the next section. 

From a functional standpoint, the 
separation of clusters from one another 
and the difference in synaptic inputs to 
them suggest that clusters may some- 
times act autonomously. But this 
means a single neuron that synapses 
onto two neurons in two spatially sepa- 
rate regions could produce differential 
postsynaptic effects. We have reported 
such differential actions {J. Physiol. 
(Lond) 238, 357, 1974), and studies are 
under way to determine if indepen- 
dence of clusters from one another can 
account for these and other integrative 
phenomena. 

Sprouting of Sensory Neuron 
Terminals 

The paired connectives that link ad- 
jacent ganglia to form the leech nerve 
cord are axonal pathways that are vir- 
tually free of synapses. Certainly the 
mechanosensory neurons, which syn- 
apse in their own neuropil and in adja- 
cent ganglia, run directly between 



92 



CARNEGIE INSTITUTION 











Fig. 55. Top: General scheme of synaptic specializations in the leech, based on serial sections. The 
presynaptic terminal contains numerous 50-nm agranular vesicles (only those clustered next to the 
plasma membrane are shown) and is separated from postsynaptic processes by extracellular space 
widened to about 30 nm. Lateral to a dense band along the inner surface of the presynaptic mem- 
brane, an occasional vesicle fuses with the membrane. Bottom: A band in such a terminal, cut in cross 
section, is indicated by an arrow. 



ganglia without branching and synaps- 
ing until they pass the connective. It 
was recently reported that crushing a 
leech connective can cause synapses to 



form within the connective at the 
crush. This presents us with a system 
that promises to make it possible to 
examine the formation de novo of 



DEPARTMENT OF EMBRYOLOGY 



93 




Fig. 56. Sites of synapses of the pressure, P, touch, T, and nociceptive, N, sensory cells culled from 
serial electron micrographs of regions like those indicated by arrowhead in Fig. 57 a and b. Three- 
dimensional reconstructions of injected neurons (upper left) were projected (upper right) and stippled 
to indicate approximate distributions of synaptic vesicles within the irregular fingers that arise from 
secondary processes. Points of contact onto pairs of postsynaptic processes are indicated by large dots; 
at the triangles the sensory neuron is postsynaptic. 



synapses made by functionally iden- 
tified neurons whose developed syn- 
apses have now been well described. 

We have found that in animals 
whose connectives have been severed 
or crushed between a pair of ganglia 
several months previously and not 
permitted to regenerate, the crushed 
portion of sensory axons has in some 
cases sprouted a profusion of branches 
at the site of the cut or crush (Fig. 57). 
The branching unfortunately occurs 
within a mass of scar tissue that be- 
comes dense after staining for specific 
neuron processes, so comparisons were 



made with a cleaner, in vitro system 
developed by others. In culture, 
neurons also sprout at the crush, but 
scar tissue is not present to obscure the 
details of branching (Fig. 57). We have 
seen no qualitative differences in vivo 
and in vitro with the light microscope. 
This system provides an ideal chance to 
examine the growth of new processes 
and the requirements for synapse for- 
mation. In the light microscope, we can 
now say that sensory neurons form 
swellings that look like presynaptic 
terminals along and at the ends of new 
branches, and that such branches form 



94 



CARNEGIE INSTITUTION 



not only at a crush or cut between gan- 
glia, but even at free cut end. In oper- 
ated animals a comparison has not yet 
been made between modalities, but the 
shapes of enlarged fingers along new 
branches of touch-sensitive (T) cells do 
resemble normal T-cell presynaptic 
terminals. In culture a clear distinction 
cannot be made between the new T-cell 
arbors and the new branches of 
pressure-sensitive (P) neurons. The 
complex and distinctive cluster of en- 
larged fingers of the P cell within the 
neuropil (see Fig. 57) has not developed 



at the crush for periods up to 3 
weeks — a time sufficient for regenera- 
tion across the cut. It is now necessary 
to examine the sequence of growth and 
to determine if some of the regenera- 
tion we have seen might involve a fu- 
sion of a growing axon with the old, 
distal stump. Whether it is a loss of its 
target, direct damage to its axon, or the 
presence of new targets that fosters a 
neuron's sprouting is testable with a 
variety of lesions, particularly in cul- 
ture. 

It remains unclear whether some en- 




Fig. 57. Experimentally induced sprouting of the sensory neuron axon in the connective. A crush or 
cut was made between two ganglia as shown at left. Sensory neurons were injected at their cell bodies 
with HRP, stained with diaminobenzidine, and their processes traced from whole mount using camera 
lucida. (a) and (b): Normal secondary branches of pressure, P, and touch, T, neurons within the 
ganglion neuropil. Arrowheads indicate where synapses are typically clustered, for comparison with 
swellings along new branches in the connective (c) to (f). (c): Sprouting where the connective was cut 
100 days previously, (d) and (e): Crush between ganglia maintained in culture for 24 days,P, and 13 
days, T. (f): Crush at end of connective rather than between ganglia (24 days in culture), (g): Regener- 
ation across cut at 24 days in culture. Note the fewer sprouts. T andP placed at ends nearest cell body; 
dashed line, edge of connective and lesions. 



DEPARTMENT OF EMBRYOLOGY 



95 



largements at early stages might rep- 
resent growth cones. One likely pros- 
pect is that such enlargements will be- 
come synaptic terminals in a new 
neuropil formed at the site of the crush 
as reported in the connective of another 
species of leech. We have not yet ob- 
served structures that can be clearly 
identified as synapses, but the presence 
of large dense-core and small agranu- 
lar vesicles at ends of branches of iden- 
tified sensory neurons suggests that 
synapse formation may be in its early 
stages. In this case, the structures that 
make synapses take shape before 
synapses are made and would appear to 
dictate the association of synapses that 
are peculiar to sensory cells. 



An Electrical Synapse in the 
Leech 

K. J. Muller and B. Thomas 

Both types of neuronal synapses — 
chemical and electrical — are readily 
detected physiologically with mic- 
roelectrodes in the leech. Yet in leech 
ganglia electron microscopy has failed 
to provide us with a clear example be- 
tween neurons of gap junctions, the 
putative sites of electrical transmis- 
sion. The reason for our failure re- 
mains obscure, but it is likely that the 
actual extent of synaptic membrane is 
limited and that the separation be- 
tween membranes is, as at other inver- 
tebrate gap junctions, wider than at 





tl 



25p 



D 





30 \l 



Fig. 58. Rohde fibers extend to meet in the connective. (A) Rohde neuron injected in the cell body in 
one ganglion reaches halfway to the next. The region of overlap with the uninjected Rohde cell in the 
lower ganglion is included in (B). When both are injected and a cross section taken through such a 
region, axons of the two cells (A, and A 2 ) can be resolved. (C) light micrograph; and (D) corresponding 
electron micrograph of boxed region in (C). The axons run in the small, unpaired nerve (Faivre's 
nerve). 



96 



CARNEGIE INSTITUTION 



gap junctions in vertebrates. It was 
therefore unexpected when an electri- 
cal junction was discovered between 
unpaired neurons in neighboring gan- 
glia, neurons that were reportedly syn- 
cytial. The junction across which dye 
molecules as large as Procion yellow 
(MW about 500) can readily pass but 
not horseradish peroxidase (MW about 
40,000) occurs between "giant" Rohde 
fibers in the middle of the unpaired 
connective (Fig. 58). The electrical 
coupling between adjacent Rohde 
neurons is so strong that no junction 
was thought to exist between them, yet 
it is only at this mid-connective region 
that the two neurons meet and 
synapse. Visible in the electron micro- 
scope is the extensive arborization of 
the two cells into fine, connecting pro- 
cesses, rather than an extensive con- 
tact along a septum (Fig. 59). The de- 
tails of the contact between the 
neurons is still under intensive exami- 
nation, but the region of overlap along 
the connective is nearly 200)U, and the 
extent of the many contacts is nearly as 
great. Also peculiar to this region of 
apposition are numerous terminals 



(Fig. 59, white arrow) containing small 
and large granular vesicles of the sort 
that have been shown by others to be 
inhibitory upon leech muscle. Such an 
inhibitory action upon the fine Rohde 
fiber collaterals could have pronounced 
and perhaps directional effects upon 
electrical transmission between Rohde 
fibers. This possibility is currently 
under physiological and anatomical 
investigation using selective stimula- 
tion and marking techniques. 

Interruption of the Rohde Neuron 

The discovery that Rohde neurons 
contact each other across an electrical 
junction that is physically separate 
from the neuropil and is permeable 
enough for some large molecules to 
pass has presented us with an unusual 
opportunity to examine the interde- 
pendence of the two neurons. 

For example, by crushing or severing 
a connective near one ganglion, the 
distal portion of one Rohde axon is iso- 
lated from the soma. Can it then be 
maintained through its synapse? Or 
does the interruption stimulate the 




Fig. 59. The electrical synapses between Rohde filters in the connective. The principle axons (1 and 
2) branch profusely as they terminate (left). The cells contact each other principally along their 
branches (right) at areas of narrowed extracellular space (black arrow) 3-4 nm wide as at gap 
junctions in some other invertebrates. Terminals containing small granular vesicles (white arrow) 
are seen in close association with the branches. Only neuron 1 has been injected with HRP, but axon 2 
is recognizable by its size and location. 



DEPARTMENT OF EMBRYOLOGY 



97 



other intact Rohde fiber to grow beyond 
the synapse until it can reestablish 
functional contact with the adjacent 
Rohde neuron? Our results are pre- 
liminary, but where we have been able 
to examine this problem, we have 
found in an animal operated upon 79 
days previously that the undamaged 
neuron does grow beyond the synapse. 
The neighboring Rohde neuron, its 
axon severed and unable to reestablish 
contact, has undergone a massive and 
unusual arborization within the gan- 
glion while eliminating much of its 
large axon (Fig. 60). Whether the new 
processes are making functional con- 



tacts is not clear, but it is noteworthy 
that the level of spontaneous synaptic 
and impulse activity that can be re- 
corded from such cells is elevated, while 
the basic features of the nerve impulse 
are preserved. 

A study of the role that this synapse 
can play in the structure and 
metabolism of the interacting cells is in 
progress using ganglia in culture. 
Here, by selectively destroying one 
neuron and thereby eliminating prin- 
cipal target tissue, the relative impor- 
tance of intrinsic and extrinsic stimuli 
to growth and synapse maintenance in 
the nervous system may be weighed. 



Normal 




Crushed 
Connective 

79 days 




crush 



Fig. 60. Plasticity of the Rohde interneuron. The connective between ganglia was crushed in an 
animal 79 days prior to injection of the neuron, identified by the location of its soma and the size and 
shape of its action potential. The principal axon has disappeared beyond the open triangles, even 
within the anterior connective that was untouched, and new sprouts have appeared (black triangles) 
that reach toward the connective but only slightly beyond the ganglion. 



THE COLLECTION OF HUMAN EMBRYOS 

Ronan O'Rahilly and Ernest Gardner 



The Carnegie Embryological Collec- 
tion was opened formally at the Uni- 



versity of California in Davis on 21 
November 1975. Drs. Ronan O'Rahilly 



98 



CARNEGIE INSTITUTION 



and Ernest Gardner were in atten- 
dance, and speakers included Drs. J. D. 
Ebert, Elizabeth M. Ramsey and Ar- 
thur T. Hertig. Dr. Ramsey presented a 
book on the Carnegie monkey colony, 
and Dr. Hertig later donated the 
binocular dissecting microscope he had 
used in his searches for early human 
embryos. Dr. George W. Corner was 
unable to be present. The University 
was represented by Deans Allen G. 
Marr, C. John Tupper, and William R. 
Pritchard. Carnegie President Philip 
H. Abelson visited the laboratories on 
the following day. 

All inquiries concerning the collec- 
tion, as well as requests for permission 
for publication, should be addressed to 
Professor R. O'Rahilly, Carnegie Labo- 
ratories of Embryology, University of 
California, Davis, California 95616. 



Dandy-Walker and Arnold-Chiari mal- 
formations," "The prenatal develop- 
ment of the human eye," and "The tim- 
ing and sequence of events in the de- 
velopment of the limbs in the human 
embryo" were published. An account of 
"The nerve supply and conducting sys- 
tem of the human heart at the end of 
the embryonic period proper" is due to 
appear in the Journal of Anatomy. 
Studies of the developing anterior fal- 
cate artery are being undertaken by 
Dr. F. Muller. 



The Bluntschli Collection 



Developmental Stages in Human 
Embryos 



Work continues on the revision of 
stages 10 to 23 (R. O'Rahilly), on the 
tabulation of the timing and sequence 
of developmental events in staged 
human embryos (R. O'Rahilly), and on 
the establishment of a computer 
catalogue (Alexander Barry). 



Development of the Nervous System 



Work continues on the develop- 
mental neurobiology of primates (E. 
Gardner and associates). A chapter on 
"The developmental anatomy and his- 
tology of the human central nervous 
system" is awaiting publication in the 
Handbook of Clinical Neurology, edited 
by Vinken and Bruyn. Articles on "The 



Renewed interest has been shown in 
the comparative collection of the late 
Professor Hans Bluntschli of Bern. It 
was established on expeditions to 
Amazonia in 1912 and to Madagascar 
in 1930, and was acquired by the Car- 
negie Institution in 1950. The 
Bluntschli collection comprises insec- 
tivores (tenrecs), prosimians (lemurs), 
and platyrrhines (New World mon- 
keys), as shown in Table 16. During the 
past year, the Bluntschli microscope 
slides were studied by Dr. F. Muller 
(Basel) and by Prof. F. Strauss (Bern). 

Professor Strauss had worked with 
Professor Bluntschli in Switzerland 
and is donating further material rele- 
vant to the collection. Professor David 
B. Meyer has been studying the com- 
parative development of the eye. 

Theses (for the M.Sc. degree) on "Em- 
bryonic limb development in the 
rhesus monkey (Macaca mulattaV by 
G. Stone and on "Staging of the early 
embryonic brain in the baboon (Papio 
sp.) and rhesus monkey (Macaca 
mulatto)" by R. Davignon have been 
approved. 



DEPARTMENT OF EMBRYOLOGY 

TABLE 16. Chief Insectivores and Primates in the Carnegie Collection 



99 



BLUNTSCHLI COLLECTION 
Insectivora 

Centetes (Common tenrec) 
Hemicentetes (Banded tenrec) 
Ericulus (Hedgehog tenrec) 



Prosimii 

Microcebus (Mouse lemur) 
Avahi laniger (Woolly lemur) 

Platyrrhinae 

Saguinus fuscicollis (Hapale; Mystax 

bluntschlii) (Tamarin) 
Aotes trivirgatus (Nyctipithecus vociferans) 

(Owl-faced or night monkey) 
Alouatta senculus (Mycetes auratus) 

(Red howler-monkey) 
Saimiri sciurea (Chrysothrix nigrivittata) 

(Squirrel monkey) 
Cebus apella (C. macrocephalus) (Hooded 

or brown capuchin) 
Cebus albifrons (C. gracilis) (White-fronted 

capuchin) 
Lagothrix lagotricha (L. infumatus) (Brown 

woolly monkey) 

HARTMANN COLLECTION 
Catarrhinae 
Macaca mulatta (Macacus rhesus) 
(Rhesus macaque) 

MALL COLLECTION 
Catarrhinae 
Homo sapiens 



Chiarelli's (1972) Decimal Key 



Suborder 


Family 


Genus 


Species 


1. 


4. 


1. 


1 


1. 


5. 


2. 


1 


2. 


1. 


4. 


3 


2. 


3. 


1. 


1 


2. 


3. 


6. 


3 


2. 


3. 


7. 


1 


2. 


3. 


8. 


1 


2. 


3. 


8. 


3 


2. 


3. 


11. 


1 



STAFF ACTIVITIES 



International conferences and sym- 
posia in which members of the De- 
partment participated during the year 
included three in Scotland ("The 
Synapse," in St. Andrews, sponsored by 
the Scottish Electrophysiological Soci- 
ety; "Genetic Engineering," in Glas- 
gow, the Second Tenovus Symposium 
of the Nucleotide Group, Chemical and 
Biochemical Societies; and the annual 
symposium of the British Society of 
Developmental Biologists, also in 
Glasgow). The annual symposium of 
the Japanese Society of Developmental 
Biologists at Osaka also proved to be 
attractive. Others attended symposia 



held before and during the meeting of 
the Federation of European Biochemi- 
cal Societies in Paris, one on "Or- 
ganelle Biogenesis" and another on 
"Structure of the Eukaryotic Genome." 
Similar topics, "Genetic Function of 
Mitochondrial DNA" and "The Organi- 
zation and Expression of Chromo- 
somes," were treated in Bari, Italy, and 
in Berlin, respectively. Also deserving 
mention was the Institut de la Vie's 
Conference on "Teratogenesis" in Mar- 
tinique, and the Symposium on 
"Biochemistry of Cell Surfaces" held at 
the Fogarty Center of the National In- 
stitutes of Health, Bethesda. 



100 



CARNEGIE INSTITUTION 



Among the other conferences engag- 
ing the interest of members of the 
group, the following should be men- 
tioned: the Cold Spring Harbor Sym- 
posium on "The Synapse"; the annual 
Symposium of the Society of General 
Physiologists, on the subject 
"Biogenesis and Turnover of Mem- 
branes"; a related symposium on 
"Membrane Receptors" at Duke Uni- 
versity; a meeting sponsored by the 
Neurosciences Research Program (Bos- 
ton) on "Depolarization-Release Cou- 
pling Systems in Neurons"; Gordon Re- 
search Conferences on "Developmental 
Biology" and "Molecular Pharmacol- 
ogy"; and the ICN-UCLA Symposium 
on "Molecular Mechanisms in the Con- 
trol of Gene Expression." 

Lectures were presented on a 
number of campuses, including Albert 
Einstein College of Medicine, Califor- 
nia Institute of Technology, Case 
Western Reserve University, Columbia 
University, Cornell Medical College, 
DePaul University, Harvard Univer- 
sity, Hershey Medical Center, Middle- 
bury College, Rockefeller University, 
University College, London; and the 
Universities of Alberta, Arizona, and 
Bern; California at Davis, Irvine, Los 
Angeles, San Diego, and San Fran- 
cisco; Chicago, Cincinnati, Connec- 
ticut, Iowa, Kyoto, Maryland, Michi- 
gan, Pennsylvania, Rochester, Tokyo, 
Utah, and Virginia; Vanderbilt Uni- 
versity, Washington University, and 
Yale University. 

Special presentations included The 
John M. Prather Lectures in Biology at 
Harvard, the Mitsubishi Lecture in 
Tokyo, and a Lowell Lecture in Boston. 

Members of the group also spoke at 
hospitals and research centers, among 
them the Radiation Effects Research 
Foundation in Hiroshima, Japan; the 
Beatson Institute for Cancer Research, 
Glasgow; the Roche Institute for 
Molecular Biology; Oak Ridge As- 
sociated Universities; Sloan-Kettering 
Institute; National Institutes of 
Health; and the Muscular Dystrophy 
Research Laboratories at Newcastle- 
upon-Tyne. 



Members of the Department took 
part in meetings of learned societies, 
including, in addition to those already 
mentioned, the American Association 
for the Advancement of Science, 
American Association of Anatomists, 
American Society of Biological 
Chemists, American Society for Cell 
Biology, American Philosophical Soci- 
ety, Biophysical Society, Federation of 
American Societies for Experimental 
Biology, National Academy of 
Sciences, Society for Developmental 
Biology, Society of Neurochemistry, 
Society for Neuroscience, and Tissue 
Culture Association. 

Advisory and consultative services 
included Editor-in-Chief of Develop- 
mental Biology and membership on the 
editorial boards of Anales del Desar- 
rollo, Cell, Excerpta Medica (section on 
Human Developmental Biology), Cur- 
rent Topics in Developmental Biology, 
Oceanus, and Quarterly Review of Biol- 
ogy. 

Members of the staff also acted in 
these capacities: Chairman, Assembly 
of Life Sciences, National Academy of 
Sciences-National Research Council; 
Member of the Board of Governing 
Trustees and the Board of Scientific 
Overseers, Jackson Laboratory; Trus- 
tee, President, and Director, Marine 
Biological Laboratory; Member of Cor- 
poration, Woods Hole Oceanographic 
Institution; Trustee of International 
Medical Congress, Ltd., and member of 
its program committee planning the 
Fifth International Congress on Birth 
Defects. 

Other posts occupied by members of 
the Department include the following: 
in the American Institute of Biological 
Sciences, member, Council of Past 
Presidents and member, Finance 
Committee; in the Institut de la Vie, 
Chairman, World Committee on the 
Formative Weeks of Human Life; in 
the National Academy of Sciences, 
member, Committee on Energy; in the 
National Institutes of Health, member, 
Board of Scientific Counselors, Na- 
tional Institute of Child Health and 



DEPARTMENT OF EMBRYOLOGY 



101 



Human Development, member, Geron- 
tology Research Center Resources Ad- 
visory Committee, member, Physiolog- 
ical Chemistry Study Section, and 
member, Cell Biology Study Section; 
and in the Society for Developmental 
Biology, Past-President. 

Members of the Department served 
on several visiting committees, includ- 
ing those for the Departments of Biol- 
ogy at Harvard University, Mas- 
sachusetts Institute of Technology, and 
Princeton University; the Department 
of Molecular Biology and Biochemistry 
at Harvard; and the External Advisory 
Committee of the University of 
Chicago's Cancer Research Center. 

Special service was rendered as co- 
Chairman of the "Cluster on Develop- 
mental Biology" of the President's 
Panel on Biomedical Research. 

Staff members taught courses at the 
Marine Biological Laboratory as well 
as at Johns Hopkins (in the Depart- 
ments of Biology, Biophysics, Pediat- 
rics and Physiological Chemistry). 



Seminars 

The roster of speakers at the semi- 
nars organized by the Department to 
serve all those working in develop- 
mental biology in the region included: 
Aftab Ahmed, National Naval Medical 
Center; S. T. Carbonetto, University of 
North Carolina; James Darnell, Rocke- 
feller University; Sarah Elgin, Har- 
vard University; Walter Gilbert, Har- 
vard University; Robert Grainger, 
Yale University; Alan F. Horwitz, 
University of Pennsylvania; Karl 
Illmensee, Institute for Cancer Re- 
search, Philadelphia; Lionel Jaffe, 
Purdue University; Stuart Kauffman, 
University of Pennsylvania; M. S. 
Letinsky, University of California at 
Los Angeles; D. K. Morest, Harvard 
Medical School; Josh Sanes, Harvard 
Medical School; Susumu Tonegawa, 
Basel Institute for Immunology; Her- 
man Vandenburgh, University of 
Pennsylvania; David Ward, Yale Uni- 
versity; and K-W Yau, Stanford Uni- 
versity. 



102 



CARNEGIE INSTITUTION 



BIBLIOGRAPHY 



Adler, W. H.,see Ozato, K. 

Albright, J. F., J. W. Deitchman, S. A. Has- 
sel, and K. Ozato, Differential antibody 
production by adherent and nonadherent 
spleen cells transferred to irradiated and 
cyclophosphamide- treated recipient 
mice. J. Reticuloendothelial Soc., 17, 
195-209, 1975. 

Biroc, S. L., and R. H. Reeder, Iodination of 
Xenopus laevis histone F2al in chroma- 
tin. Biochemistry, 15, 1440-1448, 1976. 

Bokhon'ko, A., and R. H. Reeder, The sub- 
unit structure of mouse satellite chroma- 
tin. Biochem. Biophys. Res. Coram., 70, 
146-152, 1976. 

Brown, D. D., Genome organization in 
higher organisms, Federation Proceed- 
ings, 35, 11-12, 1976. 

Brown, D. D., see also Brown, R. D., Carroll, 
D., and Thomas, C. 

Brown, R. D., and D. D. Brown, The nuc- 
leotide sequence adjoining the 3' end of 
the genes coding for oocyte-type 5S 
ribosomal RNA in Xenopus. J. Mol. Biol., 
102, 1-14, 1976. 

Carroll, D., and D. D. Brown, Adjacent re- 
peating units of Xenopus laevis 5S DNA 
can be heterogeneous in length, Cell, 7, 
477-486, 1976. 

Carroll, D., and D. D. Brown, Repeating 
units of Xenopus laevis oocyte-type 5S 
DNA are heterogeneous in length, Cell, 
7, 467-475, 1976. 

Dawid, I. B., see Leister, D. E., and Wel- 
lauer, P. K. 

Deitchman, J. W., see Albright, J. F. 

Devreotes, P. N., and D. M. Fambrough, 
Synthesis of acetylcholine receptors by 
cultured chick myotubes and denervated 
mouse extensor digitorum longus mus- 
cles. Proc. Nat. Acad. Sci., 73, 161-164, 
1976. 

Devreotes, P. N., and D. M. Fambrough, 
Turnover of acetylcholine receptors in 
skeletal muscle, Cold Spring Harbor 
Symp. Quant. Biol, 40, 237-251, 1976. 

Ebert, J. D., "Katsuma and Dan and the 
Quiet Revolution," in Development, 
Growth and Differentiation, Vol. 18, pp. 
i-ii, 1976. 

Ebert, J. D., see also Ozato, K. 

Fambrough, D. M., see Devreotes, P. N., 
and Ritchie, A. K. 

Gardner, E., and R. O'Rahilly, Neural crest, 
limb development, and thalidomide em- 
bryopathy, Lancet, 1, 635-637, 1976. 



Gardner, E., R. O'Rahilly, and D. Prolo, The 
Dandy-Walker and Arnold-Chiari mal- 
formations, Arch. Neurol., 32, 393-407, 
1975. 

Gardner, E., see also O'Rahilly, R. 

Hassel, S. A., see Albright, J. F. 

Huang, L., and R. E. Pagano, Interaction of 
phospholipid vesicles with cultured 
mammalian cells. I. Characteristics of 
uptake. J. Cell Bio/., 67, 38-48, 1975. 

Huang, L., see also Pagano, R. E. 

Leister, D. E., and I. B. Dawid, Mitochond- 
rial ribosomal proteins in Xenopus 
laevislX. mulleri interspecific hybrids, J. 
Mol. Biol, 96, 119-123, 1975. 

O'Rahilly, R., The prenatal development of 
the human eye, Exp. Eye Res., 21, 93- 
112, 1975. 

O'Rahilly, R., and E. Gardner, The timing 
and sequence of events in the develop- 
ment of the limbs in the human embryo, 
Anat. Embryo!.., 148, 1-23, 1975. 

O'Rahilly, R., see also Gardner, E. 

Overton, C. G., see Weinberg, E. S. 

Ozato, K., Review of J. Klein, "Biology of 
the mouse histocompatibility-2 complex. 
Principles of immunogenetics applied to 
a single system." Science, 190, 978, 1975. 

Ozato, K., W. H. Adler, and J. D. Ebert, 
Synergism of bacterial lipopolysac- 
charides and concanavalin A in the acti- 
vation of thymic lymphocytes, Cellular 
Immunology, 17, 532-541, 1975. 

Ozato, K., J. D. Ebert, and W. H. Adler, 
Pretreatment of thymocytes by 
phytohemagglutinin inhibits the binding 
of :3 H-Concanavalin A, J. Immunol., 115, 
334-339, 1975. 

Ozato, K., and J. D. Ebert, Concanavalin A 
potentiates syngeneic response in 
murine lymphocytes, J. Exp. Med., 143, 
1-14, 1976. 

Ozato, K., J. D. Ebert, and W. H. Adler, The 
differentiation of suppressor cell popula- 
tions as revealed by studies of the effect 
of mitogens on the mixed lymphocyte 
reaction and on the generation of 
cytotoxic lymphocytes, Cell Immunol., 
22, 32.3-333, 1976. 

Ozato, K.,see also Albright, J. F. 

Pagano, R. E., and L. Huang, Interaction of 
phospholipid vesicles with cultured 
mammalian cells. II. Studies of mecha- 
nism, J. Cell Biol., 67, 49-60,1975. 

Pagano, R. E., see also Huang, L. 

Prolo, D., see Gardner E. 



DEPARTMENT OF EMBRYOLOGY 



103 



Reeder, R. H., see Biroc, S. L., Bokhon'ko, 
A., and Weinberg, E. S. 

Ritchie, A. K., and D. M. Fambrough, Ionic 
properties of the acetylcholine receptor 
in cultured rat myotubes. J. Gen. 
Physiol., 65, 751-767, 1975. 

Ritchie, A. K., and D. M. Fambrough, Elec- 
trophysiological properties of the mem- 
brane and acetylcholine receptor in de- 
veloping rat and chick myotubes, J. Gen. 
Physiol., 66, 327-355, 1975. 

Shutt, R. H., see Weinberg, E. S. 

Suzuki, Y., Fibroin messenger RNA and its 
genes, Adv. Biophys., 8, 83-114, 1975. 



Thomas, C. and D. D. Brown, Localization 
of the genes for silk fibroin in silk gland 
cells of Bombyx mori, Developmental 
Biology, 49, 89-100, 1976. 

Weinberg, E. S., C. G. Overton, R. H. Shutt, 
and R. H. Reeder, Histone gene ar- 
rangement in the sea urchin, Stron- 
gylocentrotus purpuratus, Proc. Nat. 
Acad. Sci., U.S.A. 72, 4815-4819, 1975. 

Wellauer, P. K., and I. B. Dawid, Structure 
and processing of ribosomal RNA: A 
comparative electron microscopic study 
in three animals, Brookhaven Symposia 
in Biology, 26, 214-223, 1974. 



PERSONNEL 

Year Ended June 30, 1976 

(including those whose services began or ended during the year) 



Research Staff 

Donald D. Brown, Biochemistry, 

Director-designate 
Igor B. Dawid, Biochemistry 
James D. Ebert, Director 
Douglas M. Fambrough, Biochemistry 
Kenneth J. Muller, Neurobiology 
Richard E. Pagano, Biophysics 
Ronald H. Reeder, Biochemistry 
Yoshiaki Suzuki, Biochemistry 

Research Associates (extramural) 

Bent G. Boving, Detroit, Michigan 
Robert L. DeHaan, Atlanta, Georgia 
Ernest Gardner, Davis, California 
Arthur T. Hertig, Boston, Massachusetts 
Irwin R. Konigsberg, Charlottesville, 

Virginia 
Ronan O'Rahilly, Davis, California 
Elizabeth M. Ramsey, Washington, D.C. 

Postdoctoral Fellows and 
Grant-Supported Associates 

Peter Botchan, U.S. Public Health Service 
Grant (Reeder) 

Diana Card, Fellow of the U.S. Public 
Health Service; Fellow of Muscular 
Dystrophy Associations of America, Inc. 



S. T. Carbonetto, Fellow of Carnegie 

Institution of Washington 
Jeffrey Doering, Fellow of the U.S. Public 

Health Service 
Scott Emmons, Fellow of the National 

Cystic Fibrosis Research Foundation 
Nina Fedoroff, Fellow of the U.S. Public 

Health Service and Carnegie Institution 

of Washington 
Paul Geshelin, Fellow of the U.S. Public 

Health Service 
Elizabeth Godwin, U.S. Public Health 

Service Grant (Dawid) 
Leaf Huang, Fellow of Carnegie Institution 

of Washington and the U.S. Public 

Health Service 
Yasumi Ohshima, Fellow of Carnegie 

Institution of Washington 
Keiko Ozato, Joint Fellow of Carnegie 

Institution of Washington and Marine 

Biological Laboratory 
Aileen K. Ritchie, Fellow of Muscular 

Dystrophy Associations of America, Inc. 
Alex Sandra, U.S. Public Health Service 

Grant (Pagano) 
Masatoshi Takeichi, Fellow of Carnegie 

Institution of Washington 
Katherine Tepperman, Fellow of the U.S. 

Public Health Service 
Harvey Wahn, U.S. Public Health Service 

Grant (Reeder) 



104 



CARNEGIE INSTITUTION 



Peter K. Wellauer, Fellow of the National 
Cystic Fibrosis Research Foundation 

Students 

Peter Devreotes, Graduate, Johns Hopkins 

University 
John Gardner, Graduate, Johns Hopkins 

University 
Carol Kaushagen, Graduate, Johns 

Hopkins University 
Jose Ramirez, Graduate, Johns Hopkins 

University 

Visiting Investigators and 
Extramural Collaborators 

W. H. Adler, Baltimore, Maryland 
George Brownlee, Cambridge, England 
John W. Chase, Boston, Massachusetts 
Hayden G. Coon, Bethesda, Maryland 
Michael Edidin, Baltimore, Maryland 
J. W. S. Harris, London, England 
Uel J. McMahan, Boston, Massachusetts 
R. M. Ruysschaert, Brussels, Belgium 
Christian Thomas, Brussels, Belgium 
Witold Wozniak, Poznan, Poland 

Clerical and Technical Staff 

Elaine S. Asch, Senior Technician 
Judith M. Bernstein, Secretary 
James H. Blackwell, Custodian 



Leo Blackwell, Recorder 

Paul Blackwell, Custodian (part-time) 

Julian L. Certain, Librarian (part-time) 

Huei-Wen Chen, Technician 

Jeffrey Ciemny, Recorder 

William J. Cleary, Recorder 

William H. Duncan, Senior Technician 

Ernestine V. Flemmings, Laboratory 

Helper 
Kate Francis, Secretary 
Paul Giza, Technician 
Richard D. Grill, Photographer 
Virginia S. Hicks, Laboratory Helper 
Mary E. Hogan, Technician (part-time) 
John E. Jones, Custodian 
Eddie D. Jordan, Senior Technician 
Carrie Kahn, Technician (part-time) 
Catherine R. Lane, Librarian (part-time) 
Alice H. Mabin, Laboratory Helper 
Thomas F. Malooly, Business Manager 
Rudolph Matthews, Recorder 
Judy Miller, Recorder 
Thomas F. Miller, Custodian 
Ann N. Murphy, Secretary 
Joyce I. Patterson, Laboratory Helper 
John Pazdernik, Jr., Building Engineer 
Betty Lou Phebus, Bookkeeper-Technician 
Martha L. Rebbert, Senior Technician 
Adrienne Robinette, Secretary 
Bessie H. Smith, Laboratory Helper 
Delores V. Somerville, Senior Technician 
John L. Wiser, Machinist 



Department 
of Terrestrial Magnetism 

Washington, District of Columbia 



George W. Wetherill 
Director 



Carnegie Institution of Washington Year Book 75, 1975-1976 



Contents 



Introduction 109 

Galaxies and Stars 118 

Evidence for motion of the Galaxy 118 

A search for extragalactic extinction 119 

The statistics of galaxy alignments 121 

Kinematic studies of NGC 3351, 3115, and 1275 122 

An illustration of galaxy redshifts 127 

The irregular galaxy WLM and its surroundings 130 

The nucleus of M31 133 

Star counts in globular clusters 134 

Neutral hydrogen supernova remnant study 136 

Type I supernovae: Interpretation of 

optical spectra 139 

The 21-cm receiver for the Instituto Argentino 

de Radioastronomia 143 

820-MHz survey of the Southern sky 144 

Image tube systems 145 

Activities of the Instituto Argentino de 

Radioastronomia 147 

The Earth and the Solar System 149 

Gas-solid interactions in the solar nebula 149 

Interplanetary bodies and solar system history 151 

The role of large impacting bodies in the 

formation of the earth and moon 152 

Late (4.0 b.y.) bombardment of the moon 

and terrestrial planets 157 

The earth-crossing Apollo objects 164 

Particle tracks and micro-impact features 

in grains from gas-rich meteorites 170 

Cosmic-ray research 176 

Rb-Sr mantle isochrons 176 

Part I: Continental regions 177 

Part II: Oceanic regions 191 

Geochemistry of late Cenozoic volcanic rocks from 

the Nevado de Toluca area, Mexico 207 

Preliminary Rb/Sr data on the minimum age of the 

central Andean Precambrian basement complex 213 

Volcanoes, Q , and seismicity in western 

South America 216 

Determination of the lithosphere-asthenosphere 

boundary using converted waves 223 

Shallow seismicity in subduction zones 229 

A model for laterally heterogeneous velocity 

structure at the base of the mantle 233 

Single site phase velocity determinations 240 

Maximum entropy spectral analysis 243 

Atomic Physics and Properties of Materials 249 

Diffusion in silicate melts and glasses 249 

Diffusion measurements using fast deuterons for 

in situ production of radioactive tracers 259 

Observation of x radiation characteristic of 

a united atom 263 

Bibliography 268 

Personnel 270 



INTRODUCTION 



It has become a rather tiresome 
cliche to contrast the universal genius 
or "Renaissance man" of former years 
with today's specialist. Concentration 
of one's work into limited areas of 
knowledge is often blamed for the fre- 
quent, inability of scientists to grasp 
adequately the relationships between 
their work and that of others, and to 
achieve the unifying syntheses which 
are the goal of intellectual endeavor. 
Worse yet, in the political and enter- 
tainment world simple mention of the 
titles of specialized research projects 
results in serious scholars becoming 
sources of public amusement or objects 
of public ridicule. 

In this context it is sometimes forgot- 
ten that there is a unifying countercur- 
rent upon which even the most spe- 
cialized scientists are borne. The main- 
stream of twentieth century physics 
and chemistry has been the develop- 
ment of a universal theory of matter, 
applicable to all times and all places. In 
addition to blurring the boundary be- 
tween physics and chemistry almost to 
the point where the difference is unrec- 
ognizable, this development has been 
of profound importance to all of natural 
science. Astronomy, geology, and biol- 
ogy have lost their taxonomic and de- 
scriptive flavor. 

For a time those in the vanguard of 
this development felt it necessary to 
identify their activity (often in a 
somewhat superior tone) as geophysics 
or geochemistry rather than geology, 
or as astrophysics rather than as- 
tronomy. (Even between these so- 
called interdisciplinary fields there is a 
certain hierarchy. I once annoyed a 
famous solar spectroscopist by intro- 
ducing him to a class of chemistry stu- 
dents as an astrochemist; upon gaining 
the floor he firmly reestablished his 
identity as an astrophysicist.) Al- 
though these distinctions are still 
made, they are less useful, and possibly 



as part of the current vogue for nostal- 
gia, there are now many scientists 
primarily concerned with ther- 
modynamics of multiphase systems 
who proudly describe themselves as 
geologists, and experts in nuclear reac- 
tions who would object to being called 
anything but astronomers. In many 
universities, if you really want to learn 
thermodynamics you will attend lec- 
tures in the geology department, and if 
you really want to learn fluid mechan- 
ics you will go to meteorology or 
oceanography; and the genuine physics 
and chemistry departments find them- 
selves only one among many and not 
always the best. Thus the battle is 
over; physics and chemistry are now 
the cornerstone of all science. The 
earth is now treated as a chemical and 
mechanical system; stars and galaxies 
are to be understood only through 
knowledge of plasmas, nuclear reac- 
tions, and molecular chemistry. 

However, even though physicists and 
chemists have reproduced themselves 
within geology and astronomy depart- 
ments to the point where their progeny 
are now dominant, it is also apparent 
that the unity in the basic approach to 
science which has now been achieved 
has not been as fully exploited as it 
could be, and that many of the old bar- 
riers remain, even though they are 
more often surmounted today than in 
the past. 

The reasons for the continued exis- 
tence of these obstacles are manifold. 
To a significant extent they are a con- 
sequence of practical problems bas- 
ically of a nonscientific nature. In a 
university, lecture courses have to be 
taught, and students have to be accred- 
ited as bona fide geologists, astrono- 
mers, or meteorologists if they wish to 
find employment. The department 
needs to be sure it gets at least its fair 
share of university positions, funds, 
and space. The need to deal with these 



109 



110 



CARNEGIE INSTITUTION 



problems can unite the faculty of a de- 
partment more effectively than their 
mutual scientific interests, and it de- 
flects their attention from what might 
become mutually stimulating scientific 
interaction with those in other de- 
partments. In another area, the sheer 
size of the federal funding structure re- 
quires agencies such as the National 
Science Foundation to be com- 
partmented into "disciplines," even 
though this word has lost most of the 
significance it formerly possessed. Pro- 
gram directors and the associated peer 
reviewers can impose severe penalties 
on those scientists who stray too far 
from what may be regarded as the most 
fundamental and important subjects 
for research even though they may 
simply be the most fashionable at the 
moment within a limited group of sci- 
entists. 

We have been given the opportunity 
to be remarkably free of these essen- 
tially nonscientific pressures to iden- 
tify with a particular professional 
group. Our more informal mode of in- 
struction avoids the problem of re- 
peatedly "covering" a prescribed body 
of knowledge. Our endowed research 
support and our freedom from govern- 
ment influence is much greater than 
that available to our colleagues 
elsewhere, and historically there has 
been within the Department a tradi- 
tion of extending our research beyond 
the point which a reviewer might well 
regard as the limits of our competence. 
Freedom from duties associated with a 
professorial position and the obligation 
to serve on institutional and federal 
committees also can provide us with 
the time and tranquility necessary to 
look inward to the Department, to un- 
derstand and value the work of one 
another, and to contemplate in greater 
depth the meaning of our work and its 
relationship to that of others. With this 
unusual opportunity comes the respon- 
sibility to break away from artificial 
boundaries associated with our profes- 
sional origins and to build a research 
community dedicated to viewing the 
earth, solar system, stars, and galaxies 



as a continuous and unified field of sci- 
entific investigation. Although at 
greater risk, workers elsewhere have 
taken steps to achieve this. The princi- 
pal barrier to our success in this effort 
should be only our finite abilities. 

The reader of this report can judge 
the extent to which we have accepted 
this responsibility. Some changes from 
previous reports can be seen. There are 
no longer organized reports of "sec- 
tions" of astrophysics, geophysics, and 
atomic physics. The report has been 
subdivided into units only in order to 
make it easier to read. The boundaries 
between these units are intentionally 
arbitrary. New work has begun in an 
area of science transitional between 
our previous work in astronomy and 
earth science. This is the study of the 
solar system and its history, as re- 
vealed by the most primitive material 
available, the meteorites, and by 
theoretical investigations ultimately 
based upon observations of the earth, 
the moon, and the planets. Of course, 
all of this could be viewed as a facade; a 
perceptive reader will note that to a 
large extent the same people are work- 
ing on the same problems as before. A 
cynical reader could conclude that all 
of the foregoing is a pretense and that 
things will always be the same. That is 
not our intention. Only time will tell. 

Current Research in the 
Department 

There is a natural hierarchy in the 
organization of the universe. On the 
largest scale there are the clusters of 
galaxies, galaxies, and their con- 
stituent stars and interstellar mate- 
rial. Within the stars are formed the 
elements that are the matter from 
which new stars and planetary systems 
are in turn formed. The formation of 
new stars and planetary systems is 
controlled by the physical and chemical 
structure of the interstellar medium. It 
is the structure of the interstellar 
medium that defines the initial and 
boundary conditions for the formation 
of planetary bodies such as the earth 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



111 



and moon. These conditions largely de- 
termine the nature and magnitude of 
the processes which have occurred dur- 
ing subsequent earth history, and 
which have resulted in the present 
chemical and physical state of our 
planet. Underlying and unifying all 
are the fundamental properties of mat- 

tter as learned from atomic and nuclear 
physics. 
At all levels in this hierarchy we are 
confronted with mysteries. Although 
the earth has been extensively studied, 
one need go only a few tens of kilome- 
ters beneath its surface to arrive at the 
frontier area of current research. The 
formational solar system has tended to 
be treated as a magical wonderland 
where one qualitative speculation is as 
good as another. Although much prog- 
ress has been made in understanding 
stellar evolution during the past sev- 
eral decades, there does not exist at the 
present time any usable quantitative 
theory for understanding either the 
beginning or the end of a star's life. In 
the area of galactic evolution we are at 
best emerging from a descriptive stage 
where masses, velocity fields, and 
chemical abundances are measured for 
various types of objects including 
"peculiar" ones in the expectation that 
before too long these data will form the 
basis for a more general understanding 
of galaxies. The strong coupling be- 
tween the nature and history of 
galaxies, stars, and planets results in 
this imperfection of understanding in 
one area affecting our ability to under- 
stand another. Some sort of simultane- 
ous attack on the unknown is needed. 
We are obviously too small to under- 
take this mighty task. However, in 
working simultaneously at several of 
these frontiers along with other scien- 
tists throughout the world we are in an 
excellent position to contribute to the 

I solving of these problems and to the 
understanding of their relationship to 



Galaxies and Stars 



During the past year investigations 
of the universe at the galactic scale 



have made use of the all-sky sample of 
Scl galaxies which was established by 
Ford, Rubin, and Thonnard in their 
study of the anisotropy of the Hubble 
expansion. This work has now been 
completed, with the conclusion that the 
observed anisotropy has its origin in 
motion of both our Galaxy and the 
Local Group of galaxies, relative to the 
more distant sample galaxies, at a ve- 
locity of about 450 km s" 1 . The conflict 
between this result and the reported 
isotropy of the 3°K relic black body 
radiation remains as a problem to be 
resolved theoretically or observation- 
ally. 

Now that optical spectra and radio 
observations exist for such an all-sky 
sample, they can be used for the inves- 
tigation of other fundamental prob- 
lems. Rubin and Thonnard have in this 
way explored the possibility that be- 
tween the members of the group of 
nearby galaxies there is an excess of 
extragalactic matter, analogous to the 
interstellar material within a galaxy. 
Such material would be expected to lie 
within the plane defined by these 
nearby galaxies, and should lead to ex- 
cess extinction in the direction of this 
plane, compared with the perpendicu- 
lar direction. No significant excess ex- 
tinction is found. It is concluded that 
the density of this intergalactic mate- 
rial is similar to the average density of 
the universe, and that no significant 
portion of the missing matter required 
to cause the universe to be gravitation- 
ally closed was found in the form of this 
"intergalactic medium." 

In past exploration of the nature of 
the various types of galaxies, principal 
attention has been directed toward ob- 
servation of spiral galaxies, their veloc- 
ity fields and masses being determined 
by observation of bright emission lines 
of the gas that is abundant in these 
galaxies. The velocity fields of the un- 
derlying stars that form the bulk of the 
mass of galaxies have been relatively 
little studied in the spirals and in the 
equally important gas-free elliptical 
galaxies. Ford, Peterson, and Rubin 
report on the use of absorption lines 



112 



CARNEGIE INSTITUTION 



originating in the stars. Velocity fields 
and mass and angular momentum 
models have been obtained for two ob- 
jects, a barred spiral galaxy and an 
early-type galaxy. It is found that the 
rotation over the radial extent of the 
bar is that of a solid body, but whether 
the stars are permanently associated 
with the bar or the bar is a density- 
wave phenomenon passing through the 
stellar distribution cannot be deter- 
mined from the data. Overall the 
dynamics of the barred galaxy are very 
similar to those of a normal galaxy. For 
the first time, high resolution spectra 
of an elliptical galaxy are available to 
provide a sound observational basis for 
constructing a realistic dynamical 
model. In addition, investigations of a 
peculiar colliding pair of galaxies dis- 
covered by Minkowski 20 years ago 
show that the galaxies are still ap- 
proaching one another, and the evi- 
dence for an earlier explosive event in 
one of the galaxies cannot be explained 
by the presence of the other galaxy. 

The all-sky sample of galaxies has 
also been used by the same workers to 
illustrate the good general agreement 
of both optical and 21-cm meas- 
urements of redshifts when measured 
on the same galaxies. 

Ford, ^Peterson, and Rubin also re- 
port on the more detailed structure of 
the nearby Andromeda galaxy, M31. 
They find evidence that the angular 
momentum vector of the nucleus may 
be tilted with respect to the plane of the 
disk of the galaxy. If this is the case, 
this will be an important fact to be in- 
cluded in attempts to understand the 
general principles of the dynamics of 
galaxies. Other interpretations are 
possible. However, future observations 
should permit these alternatives to be 
tested. 

Thonnard reports 21-cm observa- 
tions of neutral hydrogen in the irregu- 
lar galaxy WLM, which lies in proxim- 
ity to the Magellanic Stream. He finds 
that this galaxy, although containing 
relatively few stars, contains almost as 
much neutral hydrogen as the Large 
Magellanic Cloud, and that this hydro- 



gen is located within WLM rather than 
being a portion of the adjacent Magel- 
lanic Stream. 

Turner has carried out a Monte 
Carlo study of the statistics of galaxy 
alignments in order to understand the 
significance of reported observations of 
such alignments. He finds that the 
probability of such alignment by 
chance is surprisingly high, and that a 
fair number of such alignments should 
occur even in the absence of a genetic 
or dynamic association between the 
aligned galaxies. 

Peterson has continued his work on 
globular clusters, and reports new star 
counts and comparisons with dynami- 
cal models for 27 clusters. Globular 
clusters are perhaps the oldest type of 
object available for investigation and 
may predate the galaxy with which 
they are associated. Their structure 
may therefore still bear a record of the 
earliest events of galactic history. 
These data provide constraints which 
must be satisfied by theories of galactic 
evolution. 

Going to more recent events in the 
history of the Galaxy, new results on 
supernovae are described. An under- 
standing of these explosion events is 
essential to understanding the termi- 
nal stages of stellar evolution, element 
formation, x-ray sources, pulsars, 
neutron stars, cosmic ray sources, and 
possibly the initiation of the formation 
of new stars. Consequently there has 
been considerable recent interest in the 
development of supernova models. 
Such models require observational 
data against which they may be tested. 
Assousa reports calculations which 
may explain why in his previous work 
neutral hydrogen was found in only a 
few of the supernova remnants studied. 
Use of remnant expansion velocities 
from other workers permits calculation 
of the mass of the objects studied and 
shows good agreement between the 
predicted and the observed neutral hy- 
drogen mass. 

Assousa, Ford, Peterson, and Rubin 
describe new spectra of extragalactic 
type I supernovae and their compari- 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



113 






son with generated synthetic spectra. 
They conclude that these spectra ap- 
pear to be dominated by transitions in 
ionized iron, which could possibly be 
produced by radiative excitation dur- 
ing the explosive event. 

In the area of instrumentation, much 
effort of both the scientific and techni- 
cal staff (Ecklund, Little, Thonnard, 
and Turner) is going into the construc- 
tion of a modernized 21-cm receiver for 
use on the 30-meter radio telescope at 
the Instituto Argentino de Radioas- 
tronomia. It is expected that this will 
permit much higher quality observa- 
tions of Southern sky high velocity 
clouds and spiral galaxies than has 
hitherto been possible. Digital equip- 
ment for an 820-MHz survey of the 
Southern sky is also under construc- 
tion. 

Ford continues his work with image 
tube systems, in addition to his col- 
laborative use of the present "Carnegie 
Image Tubes" in the current investiga- 
tions described earlier in this report. 
Two new spectrographic image tube 
systems have been constructed in our 
shop. In addition, facilities are under 
development for analysis and evalua- 
tion of the new generation of digital 
imaging devices as they become avail- 
able. 

A brief report is also given of the ac- 
tivities of the Instituto Argentino de 
Radioastronomia, with which we at 
DTM have been closely associated 
since its beginning. In spite of severe 
economic difficulties, a significant pro- 
gram of instrument development and 
observation has continued. Of particu- 
lar significance has been the near com- 
pletion of a low velocity neutral hydro- 
gen survey which reveals the large- 
scale structure of this material over 
most of the Southern sky. 

The Earth and the Solar System 

At the interface between astronomi- 
cal science and earth science lies inves- 
tigation of the formation of the solar 
system. The processes are adjuncts of 
star formation. Many of the facts 



against which theories can be tested 
are based on measurements of rocks, 
particularly meteorites. 

In this area of science there is a se- 
vere need for the development of 
theories sufficiently quantitative to be 
testable and communicable. There 
needs to be developed a community of 
scientists who have enough respect for 
the work of one another to seriously at- 
tempt to understand its details, 
criticize what they believe to be wrong, 
and build upon what is of value. Unfor- 
tunately such a community does not 
exist at present; rather too frequently, 
investigations are pursued in splendid 
isolation from one another, and while 
there is much disagreement, there is 
too little effort to understand the origin 
of these differences. 

Weidenschilling has made signifi- 
cant progress in development of a 
quantitative model of the dynamics of 
particles of various sizes moving in a 
gaseous disk of the type generally be- 
lieved to have been present during the 
early stages of planet formation. This 
builds upon and extends earlier work of 
Whipple. He shows that there is a 
strong dependence on particle size 
which may influence the rate at which 
the initial ^ 1-km bodies are formed 
and possibly also the chemical compo- 
sition of these bodies. 

Somewhat later in the formation of 
the solar system these earlier conden- 
sations must somehow or other have 
coagulated to form planets. There has 
developed in the past five years or so a 
common belief that the time scale for 
this accumulation must have been very 
short, on the order of 10 3 to 10 4 years. 
There exists at present no quantitative 
dynamical theory which predicts such 
a rapid time scale. Perhaps the princi- 
pal reason for widespread acceptance of 
this short time scale has been the evi- 
dence that at least much of the moon 
was heated above its melting point es- 
sentially at the time of its formation, 
leaving insufficient time for the slow 
buildup of temperature by the decay of 
long-lived radioactive elements and 
therefore requiring the trapping of 



114 



CARNEGIE INSTITUTION 



gravitational energy associated with 
rapid accretion. In 1969, V. S. Saf- 
ronov, working at the Institute of the 
Physics of the Earth in Moscow, pub- 
lished a theory of early terrestrial 
heating in which a calculated distribu- 
tion of large impacting bodies during 
accretion "buried" a sufficient quantity 
of their interplanetary kinetic energy 
beneath the accreting surface of the 
earth to heat much of the interior to 
near its melting temperature. We were 
fortunate in having the opportunity of 
hearing Safronov explain this work 
further at a recent DTM seminar. 

When applied to the moon, however, 
this theory still fails to provide the 
necessary initial temperature, primar- 
ily because in the case of the earth the 
interplanetary kinetic energy is aug- 
mented by acceleration during the fall 
of the body into the earth's gravita- 
tional field. This effect will be much 
less for the smaller moon. 

During the past year a variant of 
Safronov's theory has been developed 
(Wetherill) using techniques previ- 
ously employed by the author for the 
study of the evolution of meteorite or- 
bits. This leads to a considerably 
greater fraction of the accreting bodies 
being large (>200 km in radius) and of 
high velocity (~8 km/s). Both of these 
result in much greater burial of kinetic 
energy during the formation of the 
moon, and thereby provide a mecha- 
nism for obtaining initial high lunar 
temperature even if the time scale is as 
large as that given by quantitative 
dynamical theories (~10 8 yr). 

Application of the same and similar 
techniques to two other solar system 
problems are reported. These are the 
causes of the late heavy bombardment 
(4.0 b.y. ago) which produced the 
crater-saturated surface of the moon 
(and probably other planets as well) of 
that age and the role played by present 
earth-crossing interplanetary bodies 
(Apollo objects) as sources of chondritic 
meteorites. 

Extension of realistic theoretical in- 
vestigations into early solar system 
history, both here and elsewhere, may 



be expected to clarify considerably our 
thinking with regard to these prob- 
lems, but taken alone, are not likely to 
be able to distinguish between alterna- 
tive hypotheses. Fortunately they do 
not have to be taken alone, as there are 
available samples of meteoritic rocks 
which in various senses have preserved 
relic information concerning the ear- 
liest history of the solar system and 
quite likely of some presolar events as 
well. In addition, at least some meteor- 
ites have been affected by more recent 
events prior to their impacting the 
earth, and these must be disentangled 
somehow from the primordial effects in 
order to be well understood. A promis- 
ing opening into this area was found by 
Brownlee and Rajan several years ago 
in the discovery of submicron impact 
craters on the surfaces of meteorite 
grains. When combined with the oc- 
currence of solar flare tracks in the 
same grains, this leads to an analogy 
between the surface of the meteorite 
parent body and the lunar regolith 
(soil), for which such phenomena have 
been extensively studied. Rajan and 
his collaborator Poupeau of C.N.R.S., 
France, report a continuation of this 
work, directed toward an understand- 
ing of similarities and differences be- 
tween the lunar regolith and the in- 
ferred regolith surfaces of the parent 
bodies of meteorites of various kinds. 

Lunar studies show that the proba- 
bility of a grain found on the surface 
being cratered increases as the regolith 
matures, as a consequence of being 
continually impacted, turned over, and 
mixed. These new results show the 
meteoritic data lie on the extrapolation 
of this lunar trend and correspond to 
very immature regoliths. This correla- 
tion suggests a common dynamical 
mechanism that is operative on all re- 
goliths. On the other hand, glassy 
splashes are absent from meteorite 
grains, although they are quite abun- 
dant even on immature lunar soils, an 
effect which is not yet understood. 

Other extraterrestrial research re- 
ported here concerns measurement of 
reversals of the sun's poloidal magnetic 






DEPARTMENT OF TERRESTRIAL MAGNETISM 



115 



field as inferred from ionization cham- 
bers operated continuously by the De- 
partment since 1937 (Forbush). This 
time scale of almost three decades is 
long compared with that of changing 
emphases in the field of high energy 
physics with which cosmic-ray re- 
search was associated in 1937, but it 
represents less than two cycles in the 
variations of the sun's poloidal field. 
The success of these measurements is a 
tribute to the value of making observa- 
tions on the time scale peculiar to the 
phenomena being studied rather than 
on the basis of changing fashion. 

Since the beginning much of the re- 
search of this Department has been 
strongly oriented toward global prob- 
lems of the earth and its history. This 
continues to be the case. Currently 
major emphasis is being placed upon 
using geochemical and geophysical 
techniques to learn as much as possible 
concerning the nature of the earth's 
mantle. 

During the past year, much of the 
time of those working in isotope 
geochemistry (Brooks, James, and 
Hofmann, in collaboration with Hart, 
now at M.I.T.) has been devoted to tak- 
ing stock of the isotopic data developed 
here and elsewhere in recent years, 
and attempting to understand its im- 
plications regarding the earth's mantle 
beneath the oceans and the continents. 

A compilation of all the data of suffi- 
cient quality shows that in both these 
regions the initial isotopic composition 
of the strontium mantle-derived rocks 
is not uniform. The initial 87 Sr/ 8H Sr 
ratios of rocks from both oceanic is- 
lands and continental areas are usu- 
ally distinctly higher than the much 
more uniform values found in mid- 
ocean ridge basalt. Such elevated Sr 
isotope ratios have commonly been in- 
terpreted as indicating either a crustal 
origin of the rocks or at least crustal 
contamination. This interpretation is 
particularly untenable for oceanic 
rocks; arguments are presented 
against this being the case for most of 
these continental rocks as well. 



A key point in these arguments is 
that in a given region the data indicate 
not only higher initial Sr isotope ratios 
but a correlation between the 87 Sr/ 86 Sr 
ratio and the Rb/Sr ratio. Thus a 
"pseudoisochron" is obtained which re- 
sembles the more familiar whole-rock 
isochrons which are used for age de- 
terminations and are based on the pre- 
sent 87 Sr/ 86 Sr ratio of the rocks rather 
than their initial ratio. This correla- 
tion is interpreted as indicating that 
the present rocks possess not only a 
"memory" of some mean Rb/Sr ratio of 
their source as indicated by their ini- 
tial Sr isotope ratio but that some in- 
formation concerning the time scale for 
the variation of this Rb/Sr ratio in the 
mantle source has also been preserved. 
Thus "pseudoisochrons" are in fact 
mantle isochrons and may possess real 
time significance, even though 
obscured by scatter in the data result- 
ing from the superposition of later 
events. More specific models are pro- 
posed for the chemical development of 
these mantle source regions. The seis- 
mic evidence for the great thickness of 
the continental lithosphere and the 
possible complete absence of continen- 
tal asthenosphere is incorporated into 
these models, and it is hypothesized 
that the continental mantle isochrons 
are the result of more or less closed sys- 
tem evolution of an ancient heteroge- 
neous continental lithosphere, with a 
typically higher Rb/Sr ratio than the 
more depleted asthenosphere sources of 
ocean ridge basalts. In the oceanic is- 
lands, there is not thick lithosphere of 
this kind, and it is proposed that the 
deeper mesosphere plays an analogous 
role in the formation of these rocks. 

In addition to this synthesis of 
isotopic data obtained during the past 
several years, some new results are 
also reported. Whitford and his col- 
laborator Bloomfield in the Institute of 
Geological Sciences in London are 
working on orogenic volcanic rocks 
from Mexico which are situated on old, 
typically continental crust adjacent to 
a convergent plate boundary. The rocks 
are compared with typical island arc 



116 



CARNEGIE INSTITUTION 



volcanics from the Sunda arc of In- 
donesia. The Mexican rocks appear to 
be geochemically distinct from those of 
the island arc. They are richer in Si0 2 
and Na 2 and are characterized by 
higher atomic Mg/Mg+Fe 2+ ratios. Al- 
though this work is still in progress, 
the isotopic data now obtained together 
with the high Mg/Mg + Fe 2+ ratios 
suggest a mantle origin for the Mexi- 
can rocks. 

James has obtained a minimum 
Rb/Sr whole-rock age of 1012 ± 52 m.y. 
for rocks from the central Andean 
basement complex near Arequipa, 
Peru. This is the oldest age measure- 
ment reported from this basement 
complex and suggests a similar or older 
age for the mantle lithosphere underly- 
ing this region, which is believed to be 
the source of those rocks exhibiting the 
mantle isochrons discussed earlier in 
this introduction. 

These geochemical results are re- 
lated to geophysical data in the same 
area. Sacks, Snoke, and Linde sum- 
marize their work on the distribution 
of volcanism, mantle anelasticity, and 
seismic activity in South America. 
They find that in southern Peru and 
Chile there is an anelastic zone (low Q) 
between the subducting oceanic plate 
and the >300 km thick continental 
lithosphere. This is correlated with 
seismic activity in the downgoing plate 
and presence of volcanoes, indicating 
partial melting of the mantle in this 
region. Farther to the north in Peru, 
the low-Q zone is absent, the subduct- 
ing slab has low seismicity, and vol- 
canoes are absent. It appears that the 
development of a well-defined seismic 
plane in the subducting slab of oceanic 
lithosphere near its interface with the 
rocks above requires these overlying 
rocks to be hot and partially molten. 

Both geophysical and geochemical 
data indicate more and more that old 
continental areas are underlain by 
lithospheres hundreds of kilometers 
thick, and thus despite the occurrence 
of continental drift on moving plates, 
continents are not an extremely super- 



ficial feature of the earth. This gives 
rise to the question of whether a lower 
boundary to the continental litho- 
sphere exists at all, and if so, if it can 
be detected. Conventional techniques, 
using surface waves, have no resolu- 
tion below about 200 km depth. Sacks 
and Snoke describe seismic arrivals in 
Peru, northern Chile, and Argentina 
which they have been able to model 
only as converted phases arising from a 
nearby horizontal transition zone at a 
depth of 400 km. Furthermore, these 
data indicate the seismic shear velocity 
is lower below the interface, which 
leads to the interpretation that a boun- 
dary between the thick continental 
lithosphere and an underlying as- 
thenosphere has been found. 

Sacks, Rodriguez, Snoke, and Linde 
report results from the long-term col- 
laboration between DTM and the Uni- 
versity of San Agustin in Arequipa, 
Peru. These data show that there is an 
earthquake-free shallow (<50 km 
thick) region on the continental side of 
the trench extending for about 80 km. 
This same feature is found elsewhere, 
e.g., in the Aleutians. The explanation 
offered for this phenomenon is that al- 
though the rocks in these regions are of 
normal strength they are subjected to a 
superposition of stresses (a compres- 
sion resulting from the relative plate 
motion and a tension caused by the 
curvature of the plates at the trench) 
which leads to the region being nearly 
stress free. 

These results in observational seis- 
mology have been supported by several 
interesting theoretical studies. In 1974 
Sacks reported anomalies in the 
amplitudes of seismic phases that have 
traversed the earth's core, and he 
showed that these are correlated with 
certain regions on the core-mantle 
boundary. Thus in some unclear way 
these results were maps of "geogra- 
phic" regions on the core-mantle inter- 
face. Snoke and Sacks now propose a 
velocity model that fits these data. In 
this model the anomalous regions con- 
sist of cells ~ 150 km in dimension 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



117 



which multiply scatter the seismic sig- 
nals traversing them along near- 
grazing ray paths. The size and veloc- 
ity variations required are of a mag- 
nitude compatible with their being 
small convection cells. These cells are 
believed to occur in clusters in those 
areas of the lowermost mantle corre- 
sponding to the anomalous regions 
found earlier. 

Sacks, Snoke, Evans, and their col- 
laborators King and Beavan at Cam- 
bridge University report a technique 
for the measurement of seismic phase 
velocity at a single site. Conventional- 
ly, determination of the phase velocity 
requires a widely spaced network that 
measures travel times through the 
network as a function of frequency. The 
large spacing required limits the seis- 
mic wavelength that can be used and 
hence limits determination of the earth 
structure to relatively shallow depths 
(<200 km). It is now found that a verti- 
cal seismometer and a volume 
strainmeter operating at a single site 
can, at least in principle, be used to de- 
termine phase velocity without any in- 
trinsic upper wavelength limit and 
without uncertainties introduced by 
local heterogeneities and tilts. 

Seber, James, Snoke, and Linde have 
investigated the application of 
maximum entropy spectral analysis to 
seismic data. This has the potential 
advantage that in contrast to the more 
usual Fourier analysis this technique 
is well suited to extract the spectral 
content from noisy or incompletely re- 
corded signals. Calculations using 
simulated signals bear out this expec- 
tation to a considerable extent, but 
some puzzling results were found 
which will require further investiga- 
tion. 

Atomic Physics and Properties of 
Materials 

Hofmann and Magaritz have consid- 
erably extended earlier work on cation 
diffusion in natural melts. In addition 



to increased understanding of the role 
played by diffusion in natural systems, 
these new results display effects that 
must be relevant to the fundamental 
atomic and molecular structure of sili- 
cate liquids. 

Measurements of diffusion rates in 
basaltic melts show that diffusive mix- 
ing distances in a partially molten 
mantle may be expected to be on the 
order of hundreds of meters in 10 9 yr. 
These results are used to test a model 
for the development of "hot spot" 
sources of oceanic islands which has 
been proposed by others and for which 
mixing by processes other than diffu- 
sion is not expected. The limited diffu- 
sion lengths appear to preclude the de- 
velopment of enriched regions suffi- 
ciently large to be of geochemical im- 
portance. 

Measurements of diffusion in Sr and 
Ba obsidian glass also indicate the ex- 
tent of diffusive transport as a 
geochemical mechanism. These results 
show that at metamorphic tempera- 
tures of 700°C, transport distances may 
be limited to 20 cm in a million years. 
This shows that large widely spaced 
whole-rock samples may not indicate a 
reliable age of metamorphism when 
used for Rb-Sr dating, and that small- 
scale (~cm) sampling may be required 
to ensure the isotopic equilibration re- 
quired to establish an age. 

These workers give examples of the 
wide applicability of these diffusion 
data to problems of trace element 
geochemistry, igneous petrology, and 
to the microscopic structure of liquids. 

Hofmann and Brown have developed 
a new technique for measurement of 
cation diffusion in basaltic melts 
wherein the radioactive tracer is ini- 
tially located in a sharply defined zone 
within the sample rather than being 
located on the surface. This is accom- 
plished by bombardment of basaltic 
glass with 40-Mev deuterons for the 
formation of radioactive species such as 
22 Na, 4,i Sc, and 5,i Co within the sample. 
This approach minimizes convective 
mixing during the subsequent melting 



118 



CARNEGIE INSTITUTION 



of the sample and eliminates evapora- 
tion of volatile tracers such as sodium 
from the surface. This technique is 
found to be highly successful and has 
been used to confirm and extend the 
measurements found using the earlier 
technique. 

Brown and Pepper continue their in- 
vestigations of fundamental problems 
in atomic structure. These are ap- 
proached by study of the x rays pro- 
duced when heavy ions collide at ener- 
gies of a few Mev. From the theoretical 
point of view it is not clear how the 
transient "molecule" formed by the two 
ions at the moment of their collision 
should be treated, and there are a 
number of experimental results that 
still require explanation. 

These workers report the most clear 
example of the formation of a "united 
atom" in which both nuclei temporarily 



combine as a single center of charge. 
The l and m x-ray bands characteristic 
of such an atom are clearly observed, 
and the limits of these bands change 
exactly as predicted when the atomic 
number of the target is changed. 

In another experiment it is found 
that when praseodymium is bom- 
barded with potassium atoms, the 
praseodymium l shell x-ray lines are 
found as expected, as a consequence of 
transfer of electronic vacancies from 
the k shell of the potassium ion. How- 
ever, the remarkable thing is that only 
two of the several possible l lines are 
seen. The exchange is selective. This is 
believed to represent a form of charge 
exchange not observed before, and it is 
suggested that it is a consequence of 
rotational coupling that occurs while 
the two colliding nuclei are relatively 
distant from one another. 



GALAXIES AND STARS 

G. E. Assousa, E. T. Ecklund, W. K. Ford, Jr., C.A. Little, C. J. Peterson, V. C. Rubin, N. Thonnard, 

K. C. Turner, and M. A. Tuve 

Collaborators 

B. Balick, B. F. Burke, J. W. Erkes, J. Graham, C. K. Kumar, J. Oort, M. S. Roberts, and R. E. White 



Evidence for Motion of the 
Galaxy 

W. K. Ford, Jr., V. C. Rubin, and N. Thonnard 

During the past year we completed 
our study, made in collaboration with 
Dr. J. A. Graham from Cerro Tololo 
Inter-American Observatory and Dr. 
M. S. Roberts from the National Radio 
Astronomy Observatory*, of a sample 
of Sc I galaxies, which was undertaken 
to investigate the isotropy of the Hub- 
ble expansion (Rubin et at., 1976a,b). 
As discussed in detail in last year's re- 
port {Year Book 74, p. 113), we have 
detected an anisotropy, in the sense 

* Operated by Associated Universities, Inc., 
under contract with the National Science Found- 
ation. 



that galaxies on one half of the sky 
have velocities that are small for their 
magnitudes; galaxies on the remaining 
half of the sky have velocities that are 
large for their magnitudes. From the 
extensive observational and statistical 
study making use of optical mag- 
nitudes, optical velocities, 21-cm mag- 
nitudes, 21-cm velocities, and diamet- 
ers and maximum velocities of the ro- 
tation curves, we conclude that a mo- 
tion of the Galaxy and of the Local 
Group of galaxies relative to the sam- 
ple is the only interpretation consistent 
with all of the observations. The 
Galaxy is moving approximately edge 
on, with the leading edge in the anti- 
center direction. The velocity of the 
center of the Galaxy is V GM ~ 450 ± 
125 km s _1 ; as observed at the sun (and 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



119 



therefore including the rotation of the 
sun about the center of the Galaxy) the 
velocity is V ~ 600 ± 125 km s '. 

Alternative explanations of the ob- 
served effect which we have considered 
but rejected include the following: (1) 
Galaxies on one half of the sky are in- 
trinsically fainter than those on the 
other half. (2) Galaxies observed be- 
yond the central region of our Galaxy 
are less obscured than those observed 
in the anticenter direction. (3) The 
Hubble expansion increases by 20% 
across the sky for galaxies in the dis- 
tance range 3500 < V c < 6500 km s "'. 

After correction for the motion of the 
observer, the random motions of gal- 
axies are small [o-(Au) < 200 km s -1 ], 
and the Hubble flow is very uniform 
[a(AH/H) < 0.04]. This result thus sup- 
ports the similar earlier conclusions of 
deVaucouleurs (1958) and Sandage 
and Tammann (1975). The conflict 
with the observed isotropy of the 3°K 
relic radiation, which implies V < 300 
km s _1 , remains unresolved. 

References 

deVaucouleurs, G., Further evidence for a 
local supercluster of galaxies: rotation 
and expansion, Astron. J., 63, 253, 1958. 

Rubin, V. C, W. K. Ford, Jr., N. Thonnard, 
M. S. Roberts, and J. A. Graham, The 
motion of the Galaxy and the Local 
Group determined from the velocity 
anisotropy of distant Sc I galaxies. I. The 
data, Astron. J., 81, in press, 1976a. 

Rubin, V. C., N. Thonnard, W. K. Ford, Jr., 
and M. S. Roberts, The motion of the 
Galaxy and the Local Group determined 
from the velocity anisotropy of distant Sc 
I galaxies. II. The analysis for the mo- 
tion, Astron. J., 81, in press, 1976b. 

Sandage, A. R., and G. A. Tammann, Steps 
toward the Hubble constant. V. The 
Hubble constant from nearby galaxies 
and the regularity of the local velocity 
field, Astrophys. J., 196, 313, 1975. 

A Search for Extragalactic 
Extinction 

V. C. Rubin and N. Thonnard 

Knowledge of the type and quantity 
of intergalactic material is of interest 



because it can help determine the total 
amount of matter in the universe and 
place limits on the amount of material 
outside the galaxies. Attempts to de- 
termine this have involved searching 
in the direction of galaxy clusters for 
excess reddening of galaxies, changes 
in the number density of distant 
galaxies, and excess luminosity in the 
space between galaxies. We have ex- 
tended the approach used by Rubin et 
al. (1976) in determining the extinction 
within our Galaxy to search for excess 
extinction between the galaxies in the 
direction of the supergalactic plane, 
the flattened disk of nearby galaxies 
(radial velocities less than 2000 km s _1 ) 
of which we are an outlying member. 

Because we are located in the super- 
galactic plane, if extragalactic matter 
is distributed in this plane, our line of 
sight to more distant galaxies passes 
through this layer of obscuring matter. 
Taking a uniform sample of distant 
galaxies that have a small spread in 
intrinsic luminosity (Sc Fs with 3500 < 
V < 6500 km s _1 ) and correcting the 
observed magnitudes and velocities for 
all known systematic effects, one can 
form a parameter called the Hubble 
Modulus. When we examine the Hub- 
ble Modulus as a function of super- 
galactic latitude, any systematic 
change can be interpreted as a varia- 
tion due to the change in path length 
through the obscuring material for 
galaxies at different latitudes. We find 
no variation in extinction as a function 
of supergalactic latitude, which implies 
that we have detected no intergalactic 
material. 

An analysis by Takase (1972) of the 
corrected colors for 650 galaxies in the 
Reference Catalogue of Bright Galaxies 
marginally showed that elliptical and 
SO galaxies within 10° from the super- 
galactic plane are reddened with re- 
spect to galaxies at supergalactic 
latitudes greater than 30° by approxi- 
mately 0.04 magnitude at blue 
wavelengths. G. deVaucouleurs et al. 
(1972) repeated this analysis and in- 
cluded colors for 262 additional 
galaxies observed at the 91-cm 



120 



CARNEGIE INSTITUTION 





1 1 1 1 ! 




i 




i i i i_ 

Sc galaxies: 












3500 < V < 6500 - 


I.I 


- 








0<SGL<I80° 

• 


1.0 


• 


• 










•• • 

• 

• 




• 




• - 


0.9 


• 


' * «" 




• 




i 




l ill" 



--0.8 



-0.4 



90 45 40 35 30 25 20 15 

I Supergalactic Latitude I (degrees) 



E 

<d 



+0.4 



10 



Fig. 1. Hubble Modulus for 49 Sc I galaxies at supergalactic longitude less than 180° as a function 
of the absolute value of supergalactic latitude. Solid line is least-squares fit to data giving total 
change from SG pole to SG equator of -0.07±0.10 magnitudes. Dashed line is from de Vaucouleurs et 
al., (1972), using galaxy colors, and gives a total change from SG pole to SG equator of +0.18±0.05 
magnitudes. 



McDonald reflector. Their results also 
marginally indicate excess reddening 
in the supergalactic plane for both el- 
liptical and spiral galaxies and fur- 
thermore indicated a greater effect in 
the direction of the Virgo cluster. 

If one assumes that the excess red- 
dening is due to wavelength-dependent 
intergalactic absorption and that the 
reddening mechanism for galaxies in 
the supergalactic plane is the same as 
in the galaxy, then the excess redden- 
ing seen by Takase implies an excess 
extinction of +0.16 magnitude between 
the supergalactic plane and the pole. 
However, we see the not significant dif- 
ference of -0.12 ±0.08 magnitude. In 
Fig. 1 we plot the Hubble Modulus 
against the argument of the empirical 
reddening law derived by deVau- 
couleurs. The solid line is a least- 
squares solution to the data, while the 
dotted line is the effect found by 
de Vaucouleurs et al., converted from 
colors to blue magnitudes. Their re- 
sults indicated a total extinction of 
+ 0.18 + 0.05 magnitude toward the 
supergalactic plane, whereas we see 



-0.07+0.10 magnitude. Our results 
imply an upper limit to the density of 
intergalactic material in the Local 
Supercluster of 10~ :!I g cm ! , which is 
approximately the same as the 
minimum average density of the uni- 
verse and therefore would at most only 
double the density. Therefore, the 
"missing matter" needed to close the 
universe is not in intergalactic space in 
a form that can cause significant ex- 
tinction. 



References 

de Vaucouleurs, G., A. deVaucouleurs, and 
H. G. Corwin, Jr., Intergalactic extinc- 
tion in the Local Supercluster. I. Selec- 
tive extinction, Astron. J., 77, 285, 1972. 

Rubin, V. C., W. K. Ford, Jr., N. Thonnard, 
M. S. Roberts, and J. A. Graham, The 
motion of the Galaxy and the Local 
Group determined from the velocity 
anisotropy of distant Sc I galaxies. I. The 
data, Astron. J., 81, in press, 1976. 

Takase, B., Intergalactic reddening of 
galaxies, Publ. Astron. Soc. Japan, 24, 
295, 1972. 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



121 



The Statistics of Galaxy 
Alignments 

K. C. Turner 

From time to time in the astronomi- 
cal literature, processes are inferred on 
the basis of the alignment of various 
objects. For example, a number of 
galaxies are found to lie along a 
straight line. Such alignments have 
been offered as evidence for the ejection 
of small galaxies by large ones. Quan- 
titative estimates of the unlikelihood of 
such arrangements, however, have 
been very rare and not of high quality. 
An attempt to understand the probabil- 
ity structure of such alignments was 
begun some two years ago. 

Extensive Monte Carlo calculations 
to determine the probability of finding 
m objects lying within a given distance 
of a straight line when there are N ob- 
jects in a square field have been com- 
pleted (see Year Book 73, pp. 886-887.) 
This approach has been necessary be- 
cause the problem is quite intractable 
analytically. (A recently published at- 
tempt at such a calculation [Barnothy, 
1974] is incorrect). Figure 2 illustrates 






d/l_\ 
.005 
.01 
.02 

.03 
.04 
.05 

.06 
.07' 

.08 

.09 

.10 



N 



3,4, 5 | 6 | 7 | 8 , 9 , 10 



6 



7 



the general nature of the results. Given 
the number of objects, N, and the ratio 
of the half width of the line to the side 
of the square field, the areas where one 
may find m objects aligned with proba- 
bility greater than 10% are labeled 
with the value of m. Thus, for example, 
in a field of nine objects, the odds are 
greater than 1 in 10 that one can find 
five objects lying within 5% of the field 
size of some straight line. Figure 3 is a 
similar representation at the 1% prob- 
ability level. 



N 



a/i_ \ 


3 


4, 


5 , 


6 , 


7 


8 


9. 


10 


005 








.01 






.02 


3 


4 






.03 










.04 






5 






.05 












.06 










6 




.07 














.08 










( 




.09 














8 


.10 




6 






9 



P > 0.10 

Fig. 2. Contours of TV for P &10% . 



P > .01 

Fig. 3. Contours of N for P ^ 1%. 

Tables produced by these calcula- 
tions have been used to make estimates 
of the likelihood that the alignments of 
galaxies proposed by Arp (1972 and 
references therein) as evidence for the 
expulsion of QSOs and galaxies from 
galactic nuclei are merely coinci- 
dences. In all cases examined, the 
probability of finding such alignments 
by chance has exceeded 10~ 3 . Figure 4 
illustrates one such case, the region of 
NGC 520, a peculiar galaxy repre- 
sented by the square. All quasi-stellar 
objects in the Parkes Catalog in this 
region are plotted as open circles if 



122 



CARNEGIE INSTITUTION 




Fig. 4. Quasi-stellar objects near NGC 520. 



they lie near Arp's suggested line of 
ejection and as filled points if they are 
far from the line. The field contains ten 
objects, six of which are near the line. 
The ratio of line width to the side of the 
area searched (calculated from dis- 
tance of the farthest open circle to the 
line) is 0.03. The probability of finding 
such an arrangement by chance is 
0.013 (If we eliminate the farthest 
point, we can reduce the accuracy ratio 
to 0.02, but then only five of the ten 
points lie on a line, and the probability 
becomes 0.048.) 

References 

Arp, H. C, Ejection of small compact 
galaxies from larger galaxies, IAU 
Symp. No. 44, D, Reidel Publ. Co., New 
York, p. 380, 1972. 

Barnothy, M. F., The NGC 520 chain of 
quasars, Astron. J., 79, 8, 1974. 



Kinematic Studies of NGC 3351, 
3115, and 1275 

W. K. Ford, Jr., C. J. Peterson, and V. C. Rubin 

A fundamental question of cosmol- 
ogy concerns the arrangement of mat- 



ter in the universe. Is most of the mass 
of the universe contained in galaxies, 
or is there a sizable fraction of mass 
outside the galaxies? A second question 
concerns the significance of the divi- 
sion of galaxies into two major types. 
Why do some galaxies have flattened 
disks, contain gas, dust, and young 
stars in significant quantities, and 
usually exhibit spiral structure; while 
other galaxies are spheroidal, dust and 
gas free, generally structureless, and 
contain principally older, evolved 
stars? Answers to these questions can 
be approached by (1) statistical exami- 
nation of large galaxy samples, and (2) 
studies of masses of fairly nearby typi- 
cal galaxies (deduced from a detailed 
study of their velocity fields). It is the 
latter approach which has occupied 
part of our research efforts over the 
past few years. 

During the last decade, significant 
progress has been made in observing 
the velocity fields and hence the mas- 
ses of spiral galaxies, as measured from 
the emission lines in their spectra. 
These emission lines arise in the ex- 
cited gas in each galaxy. Although the 
gas constitutes in general only a small 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



123 



percentage of the mass of the galaxy, it 
is assumed that the gas is in dynamical 
equilibrium in the gravitational field 
of the stellar population, and can thus 
be used as a tracer of the underlying 
velocity pattern. 

For gas-free stellar populations such 
as are found in the bars of barred spiral 
galaxies or in elliptical or SO galaxies, 
velocity data are almost nonexistent 
and mass data are meager. During the 
past year, we have directed our obser- 
vations toward these difficult objects, 
and have obtained spectra of suffi- 
ciently high quality so that we could 
measure the velocities of the stellar 
populations from the absorption lines 
in the integrated galaxy spectra. We 
report here on studies of three such 
galaxies: NGC 3351, a barred spiral 
galaxy in which for the first time it has 
been possible to measure the velocities 
of stars in a bar; NGC 3115, an E7/S0 
galaxy that had been studied earlier 
but with contradictory results; and 
NGC 1275, an exceedingly peculiar 
galaxy whose nature has long been a 
puzzle. 

Motions of the Stars and the Excited- 
Gas in the Barred Spiral Galaxy NGC 
3351 

Although numerous studies of mo- 
tions in barred spiral galaxies have 
been made, such studies have been re- 
stricted to the motions of the excited 
gas in the nuclei and in the outer spiral 
arms. Because of the difficulties in de- 
termining accurate velocities from stel- 
lar absorption features, no velocities 
for the stars in the bars had been ob- 
tained. We have now completed a study 
of the velocities of both the stars and 
the gas in the barred spiral NGC 3351, 
and so for the first time can describe 
the motions of the stars in the bar of a 
galaxy of this type. 

NGC 3351 is one of the few bright 
barred spirals in the Northern sky 
(Fig. 5a). It is a particularly good can- 
didate in which to search for noncircu- 
lar motions of the stellar population in 
the bar because the galaxy is oriented 
on the sky with the bar only a few de- 



grees from the line of sight. A spectrum 
taken at the 4-m telescope with the 
spectrograph slit aligned along the bar 
is shown in Fig. 5a (right). Prominent 
absorption lines arising from the stel- 
lar population in the bar are marked. A 
careful inspection of the spectrum re- 
veals that each galaxy line is red- 
shifted — 800 km s _1 (due to the sys- 
temic velocity of the galaxy) from a 
corresponding but generally weaker 
line at V = km s _1 . The latter lines 
arise from the scattered solar spectrum 
in the night-sky radiation. It is the 
high resolution of the spectrum that 
permits us to resolve the galaxy fea- 
tures from these night-sky features 
and to obtain accurate measures for the 
galaxy lines. Velocities measured 
along the bar indicate that the stars 
are moving in circular orbits around 
the nucleus, with velocities that in- 
crease linearly with distance from the 
nucleus. There is no evidence of large- 
scale stellar streaming motions in the 
bar. 

Measures of both stellar and gas fea- 
tures in the galaxy lead to the follow- 
ing conclusions: 

1. The nuclear region contains a 
ring of HII regions r ~ 340 pc, which is 
rotating with V rot = 126 ± 16 km s" 1 
and contracting to the nucleus with V c 
= 34 ± 11 kms" 1 . 

2. The stars in the bar (r =£ 3 kpc) 
are rotating with constant angular ve- 
locity 80 ± 20 km s" 1 kpc" 1 . The bar is 
thus a quasi-stationary feature in this 
galaxy. There is no evidence for large- 
scale streaming motions of the stars in 
the bar, nor evidence of excited gas in 
the bar. 

3. Beyond the region of the bar, the 
velocities of the HII regions within the 
outer ring reach a maximum of V rol ~ 
220 km/s at R ~ 3-5 kpc and then 
begin to decrease. 

4. A simple mass model for the 
galaxy gives a total mass and angular 
momentum consistent with values 
found for normal spiral galaxies. Thus 
in NGC 3351 there is no indication that 
the presence of the bar, so prominent in 
optical photographs, distorts the 



124 



CARNEGIE INSTITUTION 



NGC 3351 





NGC3115 



X 




NGC 1275 



so 



uth 



4— — i smmmm t m 



• 



north 



Fig. 5. Absorption line spectra from three different types of galaxies; all spectra taken with the 
Kitt Peak 4-m spectrograph, which incorporates a Carnegie image tube, (a) NGC 3351, a barred spiral 
galaxy. Spectrum of the bar shows prominent galaxy absorption lines displaced to red from corre- 
sponding lines in the night sky. (b) NGC 3115, a spheroidal (E7/S0) galaxy and a spectrum along the 
bar. Note the variation in velocity along the disk, indicated by the curvature in the absorption lines. 
Galaxy photograph courtesy of S. Strom, KPNO. (c) The peculiar galaxy NGC 1275 and a spectrum 
crossing the nucleus. To the south, prominent absorption lines are seen which are obscured to the 
north. This is attributed to the presence of an intervening galaxy. Emission lines from both galaxies 
(at different velocities) are seen to the north (bottom of spectrum). 



large-scale gravitational field in the 
galaxy. The surprising result from this 
study is that NGC 3351 has a velocity 
field that appears normal, much like 
that of a typical nonbarred spiral 
galaxy. 

The Rotation Curve of the E7/S0 Galaxy 
NGC 3115 

NGC 3115 is a nearby galaxy origi- 
nally classified as an elliptical. More 
recent photographs have revealed a 
flattened disk embedded in a spheroi- 
dal system (Fig. 5b). Because of its rel- 
atively high surface brightness, its 
large angular extent, and the presence 



of a disk, it is a prime target for a study 
of the dynamics and kinematics of an 
elliptical galaxy, albeit a nontypical 
one. Earlier spectroscopic studies at- 
tempted to determine the rotation of 
these galaxies. An early one by Min- 
kowski (1960) indicated a minimum in 
the circular velocities at large dis- 
tances from the nucleus; a more recent 
study by Williams (1975) failed to re- 
veal the minimum, but his data showed 
a large scatter in the measured veloci- 
ties. 

We have obtained two spectra of very 
high resolution along the major axis of 
the galaxy (Fig. 5b), using the 4-m 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



125 



Mayall telescope with Carnegie image 
tube spectrograph. Measurement of the 
positions of 14 absorption features of 
Call, Fel, H, Mgl, and CrI as a function 
of distance from the nucleus gives rota- 
tion curve data with extremely small 
statistical errors (Fig. 6). The central 
angular velocity is 17.5 ±1.0 km s" 1 
arcsec -1 . The velocity curve rises 
rapidly, but beyond 20 arcsec it re- 
mains level at V ~ 270 km s" 1 . Al- 
though we intend to obtain additional 
observations at greater distances from 
the nucleus before making a final mass 
model, we can draw some preliminary 
conclusions based on an extremely 
simple model. A spheroid of axial ratio 
cla = 0.4 and density distribution given 
by p = 9 x 10 10 [1 + r/0.2] " 2 ' will re- 
produce the flat rotation curve outside 
of the nucleus. An additional nuclear 
mass of some 10 9 9TC©is implied by the 
shape of the rotation curve and the ve- 
locity dispersion: The mass out to r = 5 
kpc is about 7 x 10 10 3TC©. The projected 
surface density of the adopted spheroid 
gives a mean surface brightness as a 



function of distance from the nucleus 
which is too bright near r = 5 kpc com- 
pared with the observed surface 
brightness (Strom et al., 1976). Hence 
the mass-to-light ratio of the inte- 
grated starlight must increase by a fac- 
tor of 3 or 4 from r = 1 to r — 5 kpc in 
the disk. We will attempt to use this 
variation, together with the observed 
radial gradient in color, to model the 
dynamical and chemical history of the 
galaxy. 

The distance of NGC 3115 is uncer- 
tain. It does not appear to be a member 
of any clearly definable group of 
galaxies, and its systematic velocity of 
395 km s" 1 (corrected for the motion of 
the sun and Galaxy in the Local Group 
of Galaxies) is too small to give an ac- 
curate distance estimate by use of the 
Hubble law. Minkowski (quoted by 
Williams 1975) gives a distance of D — 
6.6 Mpc based on the apparent bright- 
ness of the globular clusters associated 
with the galaxy. We have adopted a 
distance of R = 10 Mpc for our calcula- 
tions. 



in 



1000- 



£ 800 

o 
o 

_J 

LU 

> 600 

o 
or 

2 400 

o 
o 

-J 

UJ 



♦ * * V •* 



*♦ . 



NE 



NGC 3115 

SPECTRUM 4M-II26A 
PA. 44° (MAJOR AXIS) 



SW 



-100 



-80 



-60 -40 -20 20 40 60 

RADIAL POSITION (ARC SEC) 



80 



100 



Fig. 6. The observed velocity in the E7/S0 galaxy NGC 3115 as a function of distance from the 
nucleus. Each data point represents the mean velocity over a radial range of 5 " from measurements of 
stellar absorption lines due to Call, Cal, Fel, Mgl, CrI, and H. 



126 



CARNEGIE INSTITUTION 



The Peculiar Galaxy NGC 1275 

NGC 1275 (Perseus A) is one of the 
most peculiar objects in our region of 
the universe. Twenty years ago, Min- 
kowski (Year Book 54, p. 25) discovered 
in its spectrum two sets of emission 
lines, one set with a redshift of V ~ 
5200 km s" 1 and a second with V— 
8200 km s" 1 . Since that time NGC 1275 
has been interpreted as either an 
exploding galaxy or a collision between 
two galaxies. In order to study the un- 
derlying stellar population and the ex- 
cited gas in NGC 1275, an extensive 
body of spectroscopic and photographic 
material has been obtained at the 
2.1-m and 4-m telescopes of Kitt Peak 
National Observatory. Analysis of this 
material in collaboration with Profes- 
sor J. H. Oort leads to the following 
conclusions: 

1. The luminosity profile of the stel- 
lar continuum (see Fig. 7) shows that 






■ 






NGC 1275 






. 


PLATE 4M-I938 

■■-. 














"■.. 
















"■. 








N 








KING MODEL CURVE ■ 








C = 1.5 ''■•■ 








'■ 








■ _ 











1.2 

Log r 1 



2.0 



Fig. 7. The luminosity profile of the con- 
tinuum light from NGC 1275, from an east-west 
trace. The plate is a 144-min exposure at the 
prime focus of the 4-m Mayall telescope, N 2 - 
baked 0.98 emulsion + A.6558 filter; FWHM 
92 A. In the passband of this plate and filter com- 
bination, there is no light from emission in either 
velocity system in the galaxy. The ordinate is the 
logarithm of the surface brightness, f, of the 
galaxy in units of the surface brightness of the 
sky. The abscissa is the logarithm of the radius 
in seconds of arc. The dashed line is a profile from 
the dynamical models of King (1966) with con- 
centration parameter C = 1.5. The surface 
brightness (solid squares) diverges from the 
model curve at small radii (r < 4"( due to an un- 
resolved component in the nucleus of the galaxy. 



the main galaxy is spheroidal (Fig. 5c). 
A strong absorption line spectrum is 
seen at V = 5200 km s ', which agrees 
with the velocity of the extensive fila- 
mentary material surrounding the 
galaxy. South of the nucleus is a strong 
A-type absorption line spectrum; to the 
north the Balmer lines are obscured. 
Emission lines of H, O, and N are also 
seen (Fig. 5c). 

2. Emission patches at V = 8200 km 
s -1 over the northern part of NGC 1275 
are associated with obscuration that 
hides the stellar continuum of the un- 
derlying galaxy. The pattern of veloci- 
ties in the high- velocity system (Fig. 8) 
shows a fairly smooth transition from 
velocities near 8100 km s" 1 in the east 
to velocities of 8400 km s~' in the west. 
This pattern is consistent with that ex- 
pected from a late-type galaxy rotating 
with a maximum velocity of V r max = 150 
km s~Vsin (inclination angle). In addi- 
tion, the emission line ratios seen in 
this high velocity gas are typical of 
large, low-density HII regions observed 
at large distances from the nucleus in 
late-type galaxies. No stellar absorp- 
tion lines have been found associated 
with this system. This high-velocity 
gas is in front of the low- velocity sys- 
tem. 

NGC 1275 is at the center of the Per- 
seus cluster of galaxies. A count of 
galaxies in the central region of the 
cluster suggests that the probability of 
finding a spiral galaxy within 30" of 
the nucleus of NGC 1275 has the rea- 
sonably high value of 1/30. 

3. NGC 1275 is a strong radio source 
as well as a strong infrared and strong 
x-ray source. This plus the optical ap- 
pearance of the low-velocity fila- 
mentary material, reminiscent of the 
Crab Nebula supernova remnant in 
our own Galaxy, implies that the object 
has undergone a violent event in the 
past. The A-type stellar spectrum indi- 
cates that a burst of stellar formation 
was associated with this event. It is 
likely that the time scale required for 
the dispersal of material into the low- 
velocity filament system is longer than 
the time scale over which the two 



DEPARTMENT OF TERRESTRIAL MAGNETISM 

NGC 1275 HIGH VELOCITY SYSTEM 



127 



N 



82808340 840 
8108 "» 8122 8167 8 02 8100 8209 8302 8344 8392 8386 837C 

8360 

' 8236 



8144 



8167 . 



10" 



8279. 



8399 
i 

8390 



8384 



-w 



Fig. 8. Observed velocities in the high velocity system in the galaxy NGC 1275. The coordinate 
system is centered at the nucleus of the early-type galaxy. High velocity gas is seen only in the 
northern section of this peculiar object. The smooth variation in observed velocity from east to west is 
consistent with that expected from an almost edge-on galaxy, rotating with a maximum velocity near 
200 km s-'. 



galaxies may have been in gravita- 
tional interaction (the two objects are 
still approaching each other); thus, the 
presence of the second galaxy cannot 
have triggered the explosion in the un- 
derlying galaxy. Hence our interpreta- 
tion of the NGC 1275 phenomenon re- 
quires both an explosive event in the 
elliptical galaxy and a high-excitation 
late-type intervening galaxy. While it 
is unlikely that the two main galaxy 
masses are interacting, it is still possi- 
ble that an energetic nuclear source in 
the underlying elliptical may be the 
source of excitation for the high- 
excitation spectrum observed in the 
late-type galaxy. 

Moderately peculiar galaxies are 
commonplace in the universe. Severely 
peculiar galaxies are less so, but the 
attempt to understand them has led 
some astronomers to question whether 
yet unknown laws of physics govern 
their behavior. Our observations of 
NGC 1275 show that individual prop- 
erties, i.e., the luminosity profile of the 
underlying galaxy, the observation of 
the northern part, the velocity and 
line-intensity pattern of the high veloc- 
ity system, all can be understood in 
terms of previously known characteris- 
tics of galaxies. Such is the progress of 
astronomy. Only when detailed studies 
reveal characteristics that are outside 
the realm of previously observed prop- 



erties of galaxies can we properly ap- 
peal to an unknown physics. 

References 

King, I. R., The structure of star clusters 
III. Some simple dynamical models, As- 
tron. J., 71, 64, 1966. 

Minkowski, R., Problems of extragalactic 
spectroscopy, Ann. Astrophys. 23, 385, 
1960. 

Strom, S. E., K. M. Strom, J. W. Goad, F. J. 
Vrba, and W. Rice, Color and metallicity 
gradients in E and SO galaxies, As- 
trophys, J., 204, 684, 1976. 

Williams, T. B., The rotation curve of NGC 
3115, Astrophys, J., 199, 586, 1975. 



An Illustration of Galaxy 
Redshifts 

W. K. Ford, Jr., C. J. Peterson, V. C. Rubin, and 
N. Thonnard 

In connection with the observing 
program to study the radial velocities 
of high-luminosity spiral galaxies, we 
have obtained optical and 21-cm radio 
spectra of a homogeneous sample of 
galaxies. Figure 9 has been constructed 
to illustrate the effect of increasing 
redshift with distance, using both the 
optical Ha and the 21-cm hydrogen 
lines. Photographs of the galaxies, all 
at the same scale, are copied from the 
National Geographic Society-Palomar 
Observatory Sky Survey prints. 



128 



CARNEGIE INSTITUTION 






O 

II 

X 



u 



Z 

o 



5 
z 



u 
z 
< 

I— 
</) 



-8 



-? 



U 






z > 



CN IT) 
1= > 



UK 
O ii 
Z > 



o o 2 o o o 

i o i oo i oo 

K rO OO O A. O 

oi oo u-> o- So 

n n cs ii — ii 

O > O > CM > 



u o 
Z > 



■> «- 



/ 



_/ 



.7 

I 






. ?" 



/ «_/- _. 



«= i =? 1 == ~* 






u 





DEPARTMENT OF TERRESTRIAL MAGNETISM 



129 



The optical spectra of the Northern 
Hemisphere galaxies were obtained 
with the 84-inch (2.1-meter) reflector of 
Kitt Peak National Observatory, and 
spectra of the Southern galaxies with 
the 60-inch (1.5-meter) reflector at the 
Cerro Tololo Inter-American Observa- 
tory near La Serena, Chile. A two- 
stage Carnegie image tube was incor- 
porated into each spectrograph. The 
spectra are reproduced here as nega- 
tives; dark features indicate emission. 
Wavelength increases toward the 
right. In each spectrum, the horizontal 
band is the spectrum from the inte- 
grated starlight in the nucleus. The 
superimposed emission knots arise 
from the hot gas in the nucleus and in 
regions of ionized gas surrounding 
young massive stars in the spiral arms. 
The principal emission line is Ha, indi- 
cated by the arrow. 

The Ha emission arises when an 
electron in an excited atom makes a 
transition from the n = 3 to the n = 2 
orbit. The wavelength of the emitted 
light, 6563A, corresponds to 6.563 x 
10 5 cm. In contrast, the neutral hydro- 
gen emission at 21 cm (a wavelength 3 
x 10 5 larger than that of Ha) occurs 
when the electron in the hydrogen 
atom reverses its spin with respect to 
the spin of the nucleus. Because neu- 
tral hydrogen is a significant compo- 
nent of the interstellar material and 
hence of the total mass of an Sc I 
galaxy, the 21-cm line can be observed 
in these galaxies even though the spin 
reversal in an individual atom is a rare 
event. 

The theoretical resolving power of a 
telescope is a measure of the diameter 
of the telescope in units of the 
wavelength of the observed light; the 
size of the 84-inch (2.1-m) is 2.1 
m/6.563 x 10" 5 cm = 3 x 10 ,; Ha 



wavelengths. In practice, however, the 
limiting resolution is set by the turbu- 
lence of the atmosphere, and the 
resolving power is generally about 1", 
or 2 x 10 5 . To observe a galaxy at a 
wavelength of 21 cm with the equiva- 
lent resolving power would require a 
telescope of 21 cm x 2 x 10 5 = 40 km, 
or about 25 miles. Lacking this, we 
have used the largest single telescope 
at the National Radio Astronomy Ob- 
servatory, the 300-foot (91-m), which 
gives a resolving power of 91 m/21 cm 
= 4 x 10 2 , or only l/500th that of the 
84-inch telescope. Hence, instead of re- 
solving 1" in the galaxy, the radio tele- 
scope integrates over about 10' of the 
sky. 

With the 300-ft transit telescope 
equipped with a new low-noise cooled 
receiver, we have obtained spectra of 
program galaxies with velocities up to 

V = 10,800, or AA/A = 0.036. The 
galaxy with the largest redshift, 
1058 + 11, is the most distant one thus 
far for which the 21-cm emission line 
has been detected. The galaxy velocity 

V is adopted as the midpoint of the in- 
tensity profile; the arrow beneath each 
profile marks the optical velocity. In 
general, the optical and the 21-cm ve- 
locities agree well. Where they do not, 
the difference might represent a dif- 
ference in the velocity of the excited 
hydrogen in the nucleus compared with 
the mean velocity of the neutral hydro- 
gen throughout the galaxy. More 
likely, the velocity difference reflects 
uncertainties in the optical and 21-cm 
velocities. The general agreement of 
the two velocities over a wavelength 
range of 3 x 10 5 is a stringent test of 
the wavelength invariance of the Dop- 
pler expression. 

The final adopted velocity V for each 
galaxy is listed in Fig. 9. With a value 



Fig. 9. A new illustration of the Hubble recession of the galaxies, showing both optical and radio 
spectra for 13 spiral galaxies. The galaxy photographs are reproduced from the National Geographic 
Society-Palomar Observatory Sky Survey. The optical spectra were taken with an image tube spec- 
trograph on the 84-in (2.1-m) reflector at Kitt Peak National Observatory or the 60-in (1.5-m) reflec- 
tor of the Cerro Tololo Inter-American Observatory near La Serena, Chile. The 21-cm observations 
were taken with the 300-ft (91-m) transit telescope of the National Radio Astronomy Observatory. 
With increasing distance from the observer, the emission line of Ha in the optical spectra and the 
21-cm line in the radio spectra are shifted farther toward longer wavelengths (right). 



130 



CARNEGIE INSTITUTION 



for the expansion parameter ofH = 50 
km s" 1 Mpcr 1 , the distance of each 
galaxy is found from W50. These dis- 
tances are indicated at the top of Fig. 
10. Because the value of H is still un- 
certain, the indicated distances will 
have to be changed if the value fori/ is 
revised. 

The Irregular Galaxy WLM and 
Its Surroundings 

N. Thonnard 

Irregular galaxies frequently have 
neutral hydrogen envelopes that are 
considerably larger than their optical 
image. Recently, a large band of ex- 
tragalactic neutral hydrogen, a few de- 
grees wide and extending nearly 180° 
across the sky, was discovered by 
Mathewson et al. (1974). They named it 



the Magellanic Stream, as one end of it 
was in the direction of the Magellanic 
Clouds. It was soon realized that many 
irregular galaxies were close to the 
stream, and it was even suggested that 
some of the concentrations in the 
stream might be "proto-galaxies" that 
had not yet formed stars. Observations 
made in 1973 with the 100-foot CIW- 
IAR* radio telescope in Argentina 
(Year Book 73, pp. 918-919) of WLM a 
low surface brightness irregular 
galaxy, similar in many respects to the 
Large Magellanic Cloud but much 
smaller in angular size — indicated that 
the neutral hydrogen extent is consid- 
erably larger than the optical image. In 
addition, a large concentration of neu- 
tral hydrogen at approximately the 

*Carnegie Institution of Washington-Instituto 
Argentino de Radioastronomia. 





■I5°I8 




24 




30 




36 


-z. 
o 

fee 


42 
48 


_l 
o 

LU 
Q 


54 



■16 - 



12 - 




O h l m 24 : 



36 23 59 48 59 5812 

RIGHT ASCENSION 



Fig. 10. Contours of neutral hydrogen integrated flux density in Jy km s ' superimposed on the 
optical image of WLM. 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



131 



systemic velocity was seen in the vicin- 
ity of WLM, which in retrospect was 
realized to be part of the Magellanic 
Stream. To study the neutral hydrogen 
structure of WLM and to attempt to 
understand its relationship with the 
Magellanic Stream, it was decided to 
make higher resolution and higher 
sensitivity observations with the 
facilities of the National Radio As- 
tronomy Observatory (NRAO). 

The WLM galaxy was mapped with 
the 300-foot transit telescope at NRAO 
on a 6 x 6 arcmin grid. Figure 10 is a 
contour map of the integrated neutral 



hydrogen distribution of the galaxy 
superimposed on a high-contrast copy 
of the National Geographic Society- 
Palomar Observatory Sky Survey 
blueprint. Note the large extent of the 
neutral hydrogen contours compared 
with the optical image, especially at 
the southern end of the galaxy. At the 
northern end, there is some suggestion 
that the hydrogen contours are dis- 
torted westward in the same sense as 
the "kink" in the optical image. Pro- 
files from cuts through the major and 
minor axes are shown in Fig. 11a and 
Fig. lib. The rotation of WLM can 




-370 -320 -270 -220 -170 -120 -70 -20 30 



-370 -320 -270 -220 -170 -120 -70 -20 30 80 



VELOCITY (km S" 1 ) 



Fig. 11. (a)Velocity profiles along the major axis of WLM. The declination is indicated at the right 
of each profile, (b) Velocity profiles along the minor axis of WLM. The right ascension is indicated at 
the right of each profile. Note that the intensity scale is twice as sensitive as in Fig. 12a. 



132 



CARNEGIE INSTITUTION 



a 



S =-14° 30 

140' Observations 
(22 arc min beam) 




-360 -320 -280 -240 -200 -160 -120 -80 



S=-I4°30' 

300 ft Observations C 
(10 arc mm beam) 




-360 -320 -280 -240 -200 -160 -120 



HELIOCENTRIC VELOCITY ( km s"') 



Fig. 12. (a) Position-velocity map at 8 = -14°30' in the vicinity of WLM observed with the 140-ft 
telescope, (b) Identical map, but observed with the 300-ft telescope. The intensity scales were nor- 
malized on the broad feature at 23 h 45 m . 



readily be seen in the velocity shift of 
the profiles from approximately —80 
km s _1 to -190 km s _1 along the major 
axis. The weak signals at - — 200 km 
s ' seen at the southern and western 
edges are due to the eastern edge of the 
Magellanic Stream; the peaks at zero 
velocity are from neutral hydrogen in 
our own galaxy. 

The total neutral hydrogen inte- 
grated flux density for WLM is 446 Jy + 
km s ] . Using a distance of 2 Mpc to 
WLM, (Abies, 1974) the total neutral 
hydrogen mass is 4.2 x 10* 9TC Q . From 
the velocity width of the profiles and 
approximate inclination for the galaxy 
derived from the neutral hydrogen con- 
tours, the total mass of WLM is 3 x 10 9 
9TC . Therefore, WLM is about half as 
massive as the Large Magellanic Cloud 
but contains more than three fourths 
as much neutral hydrogen as the LMC, 
indicating it is a rather normal irregu- 
lar galaxy. 

A 5° x 3° region around WLM was 
also mapped with the NRAO 140-foot 



*1 jansky (Jy) = lO" 26 W m"- Hz~ 



telescope on a 15 x 15 arcmin grid to 
see the fine structure of the Magellanic 
Stream and determine its relationship 
with the galaxy. One cut in right as- 
cension was also made with the 300- 
foot telescope to determine the 
minimum size of some of the features. 
The analysis of these observations is 
just beginning, but the remarkable 
similarity between Fig. 12a and Fig. 
12b indicates that most features are 
completely resolved with a 20-arcmin 
beam. Even for the "hot" spot at a = 
23 h 53 m , one can estimate from the dif- 
ference in peak signal between the two 
telescopes that its angular dimensions 
are not much smaller than approxi- 
mately 30 minutes of arc. Therefore, 
the hydrogen in the Magellanic Stream 
in the vicinity of WLM is of much lower 
concentration and of much larger an- 
gular extent than in the galaxy itself. 

References 

Abies, H. D., private communication, 1974. 
Mathewson, D. S., M. N. Cleary, and J. D. 

Murray, The Magellanic Stream, As- 

trophys. J., 190, 291, 1974. 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



133 



The Nucleus of M31 

W. K. Ford, Jr., C. J. Peterson, and V. C. Rubin 

There are very few galaxies suffi- 
ciently close so that their nuclear re- 
gions can be studied in detail. M31, the 
Andromeda Nebula, is a major excep- 
tion. The limiting resolution generally 
available on earth, 1", subtends 3 pc on 
M31. (In our Galaxy, at the position of 
the sun, a sphere of radius 3 pc con- 
tains only about ten known stars; at 
the nucleus of M31, a comparable re- 
gion contains about 10 (i stars.) High- 
resolution photographs of M31 can 
supply details of galactic structure 
with high spatial resolution. Above the 
atmosphere, still higher resolution can 
be obtained; a recent Stratoscope bal- 
loon flight photographed the nucleus of 
M31 with a resolution of 0"2 (Light, 
Danielson, and Schwarzschild 1974). A 
major result of these observations is an 
indication that the position angle of the 
major axis of M31 is not constant with 
distance from the nucleus but appar- 
ently rotates with increasing nuclear 
distance. If this apparent change in the 



major axis arises from the presence of 
absorbing clouds and lanes near the 
nucleus, then it has no important rele- 
vance to the dynamics of M31. How- 
ever, if the observations reflect a true 
tilt of the angular momentum vector of 
the nucleus with respect to the disk, 
this fact may be an important piece in 
the puzzle of what relation the nucleus 
bears to the structure and dynamics of 
the outer disk of a fairly normal 
galaxy. 

Because the earlier observations 
cover only a limited range in r, we have 
obtained a sequence of direct narrow- 
band red photographs of the nucleus of 
M31 with a Carnegie image tube at the 
Ritchey-Chretien focus of the 4-m tele- 
scope with a large spatial scale (6"7/ 
mm). We have fitted ellipses to the 
isophotes to determine the alignment 
of the major axis as a function of 
radius. In Fig. 13, we show our data 
together with previous measures. In 
the region from r — 0"2 to r - 1000", 
the position angle of the major axis de- 
creases uniformly from PA ~ 70° to PA 
~ 40°. 



90 



< 

Cl. 

<jO 
X 

< 

O 

< 



80 - 



70- 



60- 



50- 



40 



30 



M31 



THIS STUDY 

LIGHT etal. (1974) 

JOHNSON (1961) 

DE VAUCOULEURS (1958) 

LINDBLAD (1956) 

HUBBLE (1929) 



A 
A 



AAA 

A A 



10 12 3 

Log f" 

Fig. 13. The alignment of the major axis as a function of radius for the Andromeda Nebula. 



134 



CARNEGIE INSTITUTION 



It is not possible to deduce the cause 
of this variation from our results. How- 
ever, from a similar set of observations 
in a different spectral region it would 
be possible to separate dynamical ef- 
fects from absorption effects because of 
the wavelength dependence of absorp- 
tion. 

References 

Johnson, H. M., The nucleus of M31, As- 

trophys, J., 133, 309, 1961. 
Light, E. S., R. E. Danielson, and M. 

Schwartzchild, The nucleus of M31, As- 

trophys. J., 194, 257, 1974. 

Star Counts in Globular 
Clusters 

C. J. Peterson 

A resurgence of interest in globular 
clusters has occurred in the last few 
years. A primary factor has been the 
totally unexpected discovery (Clark et 
at., 1975; Canizares and Neighbors, 
1975) that several clusters are the 
sources of variable x-ray emission. The 
role of the globular clusters in the 
study of the Galaxy is important: The 
numerous cluster color- magnitude dia- 
grams that have recently become 
available have permitted a reassess- 
ment of the scale of the Galaxy (Harris 
1975), and other spectroscopic and 
photometric studies are being inter- 
preted in terms of the evolution of the 
chemical abundances in the Galaxy. 

As part of a long-term study of the 
globular cluster system {Year Book 74, 
pp. 133-135), star counts have been ob- 
tained in 27 clusters, using available 
photographic material in the plate col- 
lections of David Dunlap Observatory, 
Hale Observatories, and Ray White at 
Steward Observatory. The derived dis- 
tribution of stars (log surface density 
versus log radius) is shown in Fig. 14 
for four representative clusters. The 
observed data has been fitted with 
theoretical curves from self-consistent 
dynamical models (King 1966); these 
model curves depend on three parame- 



ters. The core radius r c is the radius at 
which the observed surface density has 
dropped to one half the central value; it 
is closely related to the binding energy 
of the cluster. The tidal or limiting 
radius r, defines the "edge" of the clus- 
ter; this limit is imposed upon the clus- 
ter by the gravitational tidal field of 
the Galaxy during the closest approach 
of the cluster to the Galactic Center. A 
third parameter indicates the bright- 
ness scale of the cluster and may be 
either the central surface brightness 
m or the total brightness of the cluster 
m,. The lack of correlation between r c , 
r,, and either scale factor m or m, 
shows that three parameters are in- 
deed necessary to define the structure 
of any given cluster. 

The statistical distributions of these 
data quantities, as well as their corre- 
lations with other cluster properties 
and with the spatial position in the 
Galaxy of the clusters, are the result 
not only of the formative process of the 
Galaxy but also of the subsequent 
dynamical evolution of the clusters in- 
dividually and within the framework of 
the Galaxy as a whole. In principle, 
therefore, these data hold clues to dis- 
tinguish between the two models that 
have been proposed for the origin of the 
cluster system. The first, proposed by 
Peebles and Dicke (1968), suggests 
that the clusters formed very early in 
the history of the universe. At a later 
epoch galaxy formation occurred and in 
that process the clusters became spa- 
tially associated with the much more 
massive galaxies. The alternative 
hypothesis suggests that the clusters 
formed coevally with the initial col- 
lapse of the material that was to form 
the Galaxy. Neither theory, however, 
has advanced to the point where either 
general or specific predictions of clus- 
ter properties have been made. 

Any theory of the formation and evo- 
lution of the cluster system must be 
consistent with the following prop- 
erties of the cluster system: 

1. The three structural parameters 
do not correlate with each other or with 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



135 




CF-1030] 

CF-1027 > Star counts 

C- 1910 J 



J I i I I l_ 




-0.4 0.0 0.4 0.8 1.2 



-0.8 -0.4 0.0 0.4 0.8 1.2 




Kron andMayall photoelectric 
CTIO Schmidt 64287 
C -2631 




• Illingworth 

' C-76 

v C-814 \ Star counts 

- □ CTIO Shmidt5l50 



Fig. 14. Surface density profiles for a representative set of four globular clusters. The data repre- 
sents star counts and concentric aperture photoelectric photometry. The ordinate is the logarithm of 
the surface density (stars per unit area in arbitrary units), and the abscissa is the logarithm of the 
radius in minutes of arc. The solid line in each figure is a theoretical surface density profile from the 
dynamical models of King (1966). Core and tidal radii are indicated as well as the concentration class 
(c = log r,lr ( .) for each cluster. 



other cluster properties (e.g., color, 
metallicity, HR diagram morphology) 
which are related to the stellar popula- 
tion of the cluster. 

2. There is no correlation between 
the concentration class (defined as c = 
log r,/r ( .) of a cluster and its position in 
the Galaxy. There is, however, a gen- 
eral tendency for clusters with large 
core radii (i.e., weakly bound clusters) 
to be absent from the center of the 
Galaxy. This is not unexpected, as 
these weakly bound clusters would be 
preferentially disrupted near the cen- 
ter of the Galaxy by the strong galactic 



gravitational tidal field. Because the 
limiting radius r, is set by the tidal 
field of the Galaxy, larger clusters tend 
to be found farther from the Galactic 
Center. 

3. As the radial extent of a cluster is 
set by the tidal field of the Galaxy at 
that point in the cluster orbit where 
the gravitational forces are strongest, 
this datum (r t ) can be used to deter- 
mine the closest approach of the cluster 
to the Galactic Center. This gives a 
fifth orbital parameter which, when 
combined in a statistical analysis with 
the three components for the present 



136 



CARNEGIE INSTITUTION 



position of the cluster and its observed 
line-of-sight velocity, yields the distri- 
bution of orbital eccentricities for the 
cluster system (Fig. 15). This distribu- 
tion is consistent with an isotropic ve- 
locity distribution (i.e., the dependency 
on velocity of the phase space distribu- 
tion functionary) goes only as |l>| 2 ); 
this result has one immediate conse- 
quence of great importance. As the 
clusters range to great distances from 
the Galaxy, they are the best objects to 
use for the study of the effect of the 
total gravitational field of the Galaxy. 
With this conclusion, more stringent 
limitations can be applied to the 
theoretical analysis of the positions 
and velocities of the clusters which will 
yield a more accurate estimate for the 
total mass of the Galaxy. 




0.2 0.4 0.6 0.8 

ECCENTRICITY 



1.0 



Fig. 15. A histogram showing the distribu- 
tion of orbital eccentricities computed from the 
data for 61 globular clusters. The straight line is 
the distribution of eccentricities expected if the 
phase space distribution f(r,v) for the clusters is 
isotropic in velocity. 



References 

Canizares, D. R., and J. E. Neighbors, Ob- 
servation of a bright variable x-ray 
source in a globular cluster, Astrophys. 
J. (letters), 199, L97, 1975. 

Clark, G. W., T. H. Markert, and F. K. Li, 
Observations of variable x-ray sources in 



globular clusters, Astrophys, J. (letters), 
199, L93, 1975. 

Harris, W. E., Distance to the galactic cen- 
ter measured from globular clusters, 
Bull. Amer. Astron. Soc, 7, 436, 1975. 

King, I. R., The structure of star clusters. 
III. Some simple dynamical models, As- 
tron, J., 71, 64, 1966. 

Peebles, P. J. E., and H. R. Dicke, Origin of 
the globular star clusters, Astrophys. J., 
154, 891, 1968. 



Neutral Hydrogen Supernova 
Remnant Study 

G. E. Assousa 

The survey of old galactic supernova 
remnants (SNR) at 21 cm reported ear- 
lier (Assousa and Erkes, 1973; As- 
sousa, Balick, and Erkes, 1974) was 
carried out to study interaction of 
SNRs with the interstellar medium, 
and in particular to test the hypothesis 
that SNRs accrete a shell of interstel- 
lar material during the last phase of 
their evolution. This is the phase of the 
SNR's history during which the rem- 
nant expands while conserving 
momentum but not kinetic energy. The 
onset of this phase is characterized by a 
shock velocity of 200 km/s (Woltjer, 

1972) and ends when the shock velocity 
becomes comparable to the random in- 
terstellar gas motions, i.e. —10 km/s. 

Although in principle, estimates of 
the mass accreted by the SNR can be 
made on the basis of the Sedov-Oort So- 
lution (Woltjer, 1972), it could not be 
calculated without knowing both the 
SNR's radius and its expansion veloc- 
ity. 

Until recently, only two large rem- 
nants, Cygnus Loop and HB21, with 
filament expansion velocities below 
200 km/s., had been measured. To date 
we could only verify the existence of 
expanding fragmentary neutral hydro- 
gen shells in the vicinity of two rem- 
nants: HB21 (Assousa and Erkes, 

1973) and S147 (Assousa, Balick, and 
Erkes, 1974). We have obtained their 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



137 



masses and expansion velocities. We 
found marginal evidence for a shell as- 
sociated with the Cygnus Loop (As- 
sousa, Balick, and Erkes, 1974) and 
W41 (Assousa, 1975). A detailed study 
of 3C396.1, W50, CTB 63, W63, CTB 1, 
HB 3 and HB 9, and PKS 0607 + 17 
shows no evidence for any shell struc- 
ture. The remaining objects CTB 72, 
CTA 1, CTB 13, VRO 4205.01, and OA 
184 have been partially analyzed, but 
even for these objects, with the excep- 
tion of VRO 4205.01, there appears lit- 
tle evidence for HI which we are able to 
detect with present instrumental sen- 
sitivity (peak-to-peak noise 0.3° K). 

The absence of these shells, initially 
puzzling, becomes understandable, at 
least for some remnants, with the 
availability of filamentary velocity 
data (Lozinskaya, 1975). Knowledge of 



these velocities and the radii of the 
SNR allows us to make the mass esti- 
mates. Table 1 lists in columns 1 
through 9 the object name, longitude 
and latitude of object, distance to object 
(R) in kpc, filament or shell velocity (V) 
in km/s; radius (r) in pc; interstellar 
density (p ) in H atoms/cm 3 , accreted 
mass (3TC) in 911 , age (t) in 10 3 yr, and 
initial explosion energy (EJ in 10 50 
ergs. 

Given R and V, the interstellar 
medium density p is uniquely defined 
by the Sedov (adiabatic-energy con- 
serving phase)-Oort (isothermal- 
momentum conserving phase) solution 
(Woltjer, 1972) for an initial explosion 
energy of 1 x 10 50 ergs. The family of 
solutions forp = 0.01, 0.1, 1.0, and 10.0 
H atoms/cm 3 is given in Fig. 16. 
Examination of the mass column shows 



io 3 



10° 




lO* 10- 

TIME (YEARS) 



Fig. 16. The evolution of a 1 x 10 5 " erg SNR. The family of curves represents the expansion 
velocity vs. time (double-slope lines, decreasing), radius vs. time (double-slope lines, increasing), and 
accreted mass vs. time for a range of ISM densities. 






138 



CARNEGIE INSTITUTION 



PS 



CO 



O 



w 



CO CM 


o 


O O 


i— 1 r- 1 CO 




CO ^ 






o 


o o o 


o m 


■* cm in 


CD ■<* 


t-H Tf T? 



T t 



o o o o o 

f"tONON 
(N rH ■<* iH 



O l> 
O CN 



^ Tf S? 






co co 

tH O 

in p oq co © 

O r-i O O r-i 

• m • -co 



. OQ 



O CO i— I I-H T— I 



OKffi >C0 



£> 



in » 



CN ^ 

o I C# 

INOOlOO 


inskaya, 

vey. 

dov-Oort 

ents that 


H(M D- C005 


t> O t-h o o 


OHH^H 


8 5% 6 


CD t> CO CO t~ 

CO Tt OJ Tt T-H 

OHU3NO 


ned from L 
ned from s 
ined from 
11 detected, 
no HI fila 


rfOJOtOO 


t> CO CD <X> 00 





+5 j$ wQ 



fl-S 



^ cosiz; 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



139 



good agreement with measured and es- 
timated mass where available and 
shows a mass too low to be detected for 
the remaining objects, consistent with 
our results. The list represents objects 
observed in the survey for which veloc- 
ities are available. 

Lozinskaya (1975) has suggested 
that a homogeneous class of objects ex- 
panding into a medium of variable 
density may account for the observed 
properties of a sample of remnants she 
has studied. Our observations are con- 
sistent with this suggestion. More pre- 
cisely, we see that 10 50 ergs is a good 
estimate of the initial energy. 

We thus summarize that present ob- 
servational data for old galactic SNRs 
may have arisen from nearly identical 
initial conditions (presupernova mass 
and SN kinetic energy 10 50 ergs) but 
expanded into regions of varying in- 
terstellar medium densities. 

References 

Assousa, G. E., Search for shell structure in 
association with old galactic supernova 
remnants in neutral hydrogen and for- 
maldehyde, Carnegie Inst. Wash. Year 
Book 74, 126, 1975. 

Assousa, G. E., B. Balick, and J. W. Erkes, 
Supernova remnants, Carnegie Inst. 
Wash. Year Book 73, 899, 1974. 

Assousa, G. E., and J. W. Erkes, An ex- 
panding shell of hydrogen surrounding 
the supernova remnant HB 21, Astron. 
J., 78, 885, 1973. 

Lozinskaya, T. A., Supernova remnants as 
optical nebulae, Sou. Astron., 19, no. 1, 
21, 1975. 

Woltjer, L., Supernova remnants, Ann. Rev. 
Astron. Astrophys., 10, 129-158, 1972. 



Type I Supernovae: 

Interpretation of Optical 

Spectra 

G. E. Assousa, W. Kent Ford, Jr., C. J. Peterson, 
and V. C. Rubin 

Supernovae (SN) represent an end- 
stage of stellar evolution. An under- 



standing of this phenomenon is critical 
for the determination of the rates at 
which SNs contribute to the synthesis 
of elements, to cosmic rays, and to the 
energy released into the interstellar 
medium. 

Observationally one is hampered by 
the relatively low rate of SN produc- 
tion. Ilovaisky and Lequex (1972) es- 
timate on the basis of a galactic survey 
of supernova remnants a rate of 
(50 ±25 years/SN). 

For the optical astronomer the 
spectra of SNs and their evolution with 
time, following initial detection, pro- 
vide one observational handle. An un- 
derstanding of the nature and the 
time-behavior of these spectra may not 
provide direct evidence of heavy- 
element (more massive than oxygen) 
synthesis. It may do so indirectly by 
identifying those elements which give 
rise to the spectrum. Simultaneously, 
it provides information regarding the 
physical conditions in the SN which 
give rise to these lines. 

To date, the problem of line identifi- 
cation remains only partially solved. 
This is particularly true for supernovae 
of type I (SNI). This problem is aggra- 
vated by the lack of data in the litera- 
ture. While many spectra of super- 
novae exist and have been used for 
classification of their spectral type, 
most spectra have never been pub- 
lished. Published spectra exist for only 
about 10% of recorded supernovae (Oke 
and Searle, 1974). 

There are, of course, difficulties in 
obtaining supernovae spectra. Few are 
bright at the time of discovery. Their 
rapid decline in brightness severely 
limits the time over which spectra may 
be easily acquired; in addition, nearly 
one half of the possible observation 
time is lost due to the brightness of the 
moon-lit sky. The development of fast 
image tube systems in the past decade 
has made practical the recording of 
spectra of faint supernovae. As part of 
a continuing program on the study of 
extragalactic systems, two of us (VCR 
and WKF) have made a practice of se- 
curing spectra of supernovae whenever 



140 



CARNEGIE INSTITUTION 



possible (Rubin and Ford, 1967; Ford 
and Rubin, 1968). 

Here we present the spectra of three 
supernovae, SN 1970j in NGC 7619, 
SN 1971 1 in NGC 6384, and SN 1971 p 
in MCG 5-26-14. 

Based on the present and other 
available data on the spectra of SNIs, 
we have generated synthetic spectra 
that closely resemble the observed 
spectra. 

The original spectra and microden- 
sitometer tracings are shown in Figs. 
17 through 19. In Fig. 20, the spectrum 
of SN 1971 I (C-47) at 18 days after 
maximum has been replotted. The bot- 
tom plot is a synthetic spectrum com- 
posed of emission time of Fell, Call, 
Nal, H u , H/i, H y , H«, and the Hell R, 
line at 4686 A. There are 48 lines due 
to Fell, 2 lines each for Call and Nal, 4 
lines for HI, and a single line for Hell. 
The wavelength is the rest frame of the 
galaxy. The lines have been Gaus- 
sian-broadened to reflect a velocity 
dispersion of 3000 km/s. For all except 
Hell, the relative strengths of the lines 
were obtained from the relative line 
widths in the Nova DQ Hercules (Mus- 
tel, 1974). The Hell contribution was 
arbitrarily chosen at twice Ha. Al- 



though the C-47 spectrum is in density, 
the marked similarity of the actual and 
synthetic spectra is very striking. In 
the synthetic spectra the Fell is the 
dominant contributor. Kirshner et al. 
(1973) have shown that the peak at 
4600 A gradually shifts with time. A 
reasonable interpretation is that the 
Hell 4686 line becomes more promi- 
nent with time, becoming considerably 
stronger than the Fell. 

Improved spectral fits may be ob- 
tained by including other lines ex- 
pected to be present, such as Sill, but 
the improvement remains small. 

We are thus led to conclude that 
spectra of SNIs are primarily domi- 
nated by Fell transitions. We have not 
attempted to devise a complete model 
to determine the physical conditions in 
the SN which give rise to this result. A 
radioactive excitation mechanism by 
resonance transitions at 2500 A, fol- 
lowed by permitted transitions to lower 
metastable states as suggested by 
Wampler and Oke (1967) for identify- 
ing the Fell lines in the QSO 3C273 is 
physically plausible for SNIs. 

But, regardless of the mechanism, 
the presence of Fe in SNI appears ines- 
capable. 





REST W AVELE NGTH 




(A) 








3800 4200 4600 5000 


5400 5800 


6200 


6600 


6 


SN I970j 




ton 




■ 


5 


1943 

Hg 

1 








: 


4 

>- 


H 9 iii i>VVr\ 








■ 


h- 


1 /VU, ff \k A 


Hg 
I 


NoD 




■ 


2 a 


a J i il*» ' wl r^/vAi a 


1 


1 




■ 


UJ 

1- 


2 1 v V 1 III J 

\ vr 






con 




? 2 






1 








^Vv-vngvvi 


\ Hvni 


A 


1 


iVtJw 1 i i i 1 1 1— 1 1 


i i i i 


\r^ 


V 



4600 

OBSERVED 



5000 5400 

WAVELENGTH 



5800 

(A) 



Fig. 17. Spectrum of supernova 1970j in the galaxy NGC 7619 at 1 day before maximum light. The 
bottom scale gives the observed wavelength; the top scale gives the wavelength in the rest frame of 
the galaxy. No corrections have been made for change of instrumental sensitivity with wavelength. 
Original dispersion is 57 A/mm. 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



141 



REST WAVELENGTH 

4400 5000 5600 



6800 7400 



SN 19711 
C-28 



NIGHT SKY LINES 
[OT] No D [Oil 




4400 
OBSERVED 



5000 5600 

WAVELENGTH 



Fig. 18. Spectra of supernova 1971 in the galaxy NGC 6384. Spectrum C-28 was obtained on 29 
June 1971 at maximum light (Barbone^ ai, 1973). Spectrum C-47 was obtained 18 days later on 17 
July 1971. Because no calibrations exist for these photographic spectra, the data are presented in 
terms of density. The top and bottom scales are galaxy rest-frame and observed wavelengths, respec- 
tively. No corrections have been made for change of instrumental sensitivity with wavelength. Origi- 
nal dispersion is 300 A/mm. 



References 

Barbon, K., F. Ciatti, and L. Rosino, Obser- 
vations of five supernovae 1970-71, 
Memoria della Societa Astronomica 
Italiana, 44, 65, 1973. 

Branch, D., and B. Patchett, Type I super- 
novae, Mon. Notic. Roy. Astron. Soc, 
161, 71, 1973. 

deVaucouleurs, G., and A. deVaucouleurs, 
Reference Catalogue of Bright Galaxies, 
(Austin: University of Texas Press), 
1964. 

Ford, W. K., Jr., and V. C. Rubin, The spec- 
trum of the 1968 supernova in NGC 
2713, Proc. Astron. Soc. Pac, 80, 466, 
1968. 



Ilovaisky, S. A., and J. Lequex, A study of 
galactic supernova remnants, Astron. 
Astrophys., 20, 347, 1972. 

Kirshner, Robert P., J. B. Oke, M. V. 
Penston, and L. Searle, The spectra of 
supernovae, Astrophys. J., 185, 303, 
1973. 

Mustel, E. R., Analysis of the physical con- 
ditions inside the envelope of DQ Her- 
cules 1934, Vistas in Astronomy, vol. 16, 
(A. Beer, ed., Pergamon Press, New 
York), pp. 260-269, 1974. 

Oke, J. B., and L. Searle, The spectra of 
supernovae, Ann. Rev. Astron. As- 
trophys., vol. 12, 315-329, 1974. 

Rubin, V. C, and W. K. Ford, Jr., The spec- 
trum of the 1967 supernova in NGC 



142 



CARNEGIE INSTITUTION 



3400 



REST WAVELENGTH (A) 

3800 4200 4600 5000 5400 



5800 



6200 



SN I97lu. 
2II6B 




SN I97lu. 
2I20A 




3400 



4200 4600 5000 

OBSERVED WAVELENGTH 



5400 

(A) 



Fig. 19. The spectra of supernova 1971u in the anonymous galaxy MCG 5-26-14. Spectra 2116 B 
and 2120 A were obtained on successive nights (16 and 17 December 1971, respectively) and are 
consistent with type I SN spectrum at 15±2 days after maximum light. The contributions from the 
night sky have been removed from both spectra. As in the previous two figures, no corrections have 
been applied for instrumental sensitivity with wavelength. The bottom scale gives the observed 
wavelength; the top scale gives the observed wavelength; the top scale gives the wavelength in the 
rest frame of the Galaxy. Original dispersion is 109A/mm. 



3389 and H-alpha velocities in the 
Galaxy, Proc. Astron. Soc. Pac, 79, 322, 
1967. 
Rubin, V. C, W. K. Ford, Jr., N. Thonnard, 
M. S. Roberts, and J. Graham, The mo- 
tion of the Galaxy and the Local Group 



determined from the velocity anisotropy 
of distant Sc I galaxies, I. The Data, As- 
tron. J., in press, 1976. 
Wampler, E. J., and J. B. Oke, The emission 
line spectrum of 3C273, Astrophys. J., 
148, 695, 1967. 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



143 



i — i — i — i — i — i — I — i — i — i — i — i — r 



~i i i r 




3600 4000 4400 4800 5200 5600 6000 6400 6800 

REST FRAME WAVELENGTH IN A 

Fig. 20. The SN 1971 / spectrum at 18 days after maximum (top) and the synthetic spectrum 
(bottom). The lines used in obtaining the spectrum were broadened with a Gaussian assuming a 
velocity spread of 3000 km/s. Positions of the NAI, Call, and Ha,/3,y,8, lines are indicated. The 
presence of the Balmer lines produces minor changes in the shape of the spectrum. The Hell line 
at 4686 clearly has the effect of redshifting the 4600 A peak. The dashed spectrum is due to Fell 
alone. All wavelengths are in the rest frame of the Galaxy. 



The 21-cm Receiver for the 
Instituto Argentino de 

RADIOASTRONOMiA 



E. T. Ecklund, C. A. Little, N. Thonnard, and 
K. C. Turner 



The joint proposal to modernize the 
21-cm receiver at the IAR and to 
undertake an investigation of high- 
velocity clouds and southern spiral 
galaxies at the neutral hydrogen line 
was funded as of November 1, 1975, by 
the National Science Foundation and 
the Agencia Ejecutiva Argentina. We 
have already had a four-week visit in 
January by Ing. E. Filloy, the chief en- 
gineer of the IAR, during which we 
finalized all of the details in the re- 
ceiver design. 

Most of the components for the 
front-end of the receiver have been re- 
ceived or are on order. A block diagram 
of the receiver front-end package is 
shown in Fig. 21. The parametric 



amplifier has been received, and using 
the noise figure and/or losses in the 
other components, we expect a total 
receiver-noise temperature of 56°K. 
This leads us to expect a system tem- 
perature of 70°K on the sky. The use of 
dual frequency conversion allows a 
large separation between signal and 
image frequencies and simplifies the 
design of band rejection filters. 

Figure 22 is a block diagram of the 
"back end" of the receiver. By utilizing 
an on-line computer for phase detection 
and integration, we hope to avoid many 
nonlinearities and temperature- 
dependent effects which proved 
troublesome in the old system. In addi- 
tion, we will be able to automatically 
calibrate the system gain and baseline 
every few seconds, thus eliminating 
several other possible instability prob- 
lems. All components for the digital 
part of this system are now on hand, 
and their assembly into a working sys- 
tem is under way. 



144 



CARNEGIE INSTITUTION 






lAtten. 



~l 



Vorioble 

Noise I* Variable current source 

Source 



Precision constant 
current generator 




First L.O. 



Mixer 



Parametric 
Low loss Amplifier 

line for 
future switch 



Transistor 
Amplifier 



Temperature controller 
25°C ± 0. I 



L_ 




Second 

H6 L.O. 



J 



v To receiver back end 
Fig. 21. Front end for the new 21-cm receiver at the IAR. 



820-MHz Survey of the Southern 
Sky 

N. Thonnard and K. C. Turner 

A recent development in radio as- 
tronomy, pleasing both aesthetically 
and scientifically, is the possibility of 
constructing maps of the whole sky 
with respectable angular resolution 
(i.e., —1°). Such pictures are revealing 
structures so large in angular size as to 
be heretofore unobservable. 

One such survey of local neutral hy- 
drogen by Heiles and his collaborators 
at the University of California, Berke- 
ley, is now being completed at the IAR 
in cooperation with our Argentine col- 
leagues. 

We are presently designing a system 
to produce such an all-sky picture of 



the continuum radiation at 820 MHz in 
a cooperative program with members 
of the IAR and Professor H. Weaver's 
group at Berkeley. The northern por- 
tion of the survey already exists down 
to declination —7° by Berkhuijsen 
(1972), done at Leiden. The southern 
portion will be done at the IAR, and a 
central region overlapping both halves 
to ensure proper calibration of scales 
will be done at the Berkeley Hat Creek 
Observatory. Both polarizations of the 
incoming radiation will be measured. 

Design and construction of the 
back-end and data-acquisition systems 
will be done at DTM, while the Berke- 
ley group will be responsible for the 
front-end system. All of the digital 
equipment has already been acquired 
and is now being assembled. A KIM-1 
microprocessor based system made by 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



145 



-> 



64 
Channel 



Analog 

to 

Digital 

Converter 



Data 
Acguisition 
Computer 

32 K Bytes 



~fc 



Present 

Filter Spectrometer 

30 channels of 100 kHz BW 

56 channels of 10 kHz BW 



Teletype 
Terminal 



Tape Rec 
Interface 



D to A 
for 

Display 



Auxi liary 
Interface 



D toA 

for 
Chart 



9 -track 
Digital 
Tape Rec 



^Oscilloscope 



-~> 



Strip Chart 
Recorder 



-> to Dicke Switch 



-> to Noise Source 



Sidereal time, hour angle, declination 
and local oscillator frequency inputs 

Fig. 22. Phase detection, integration, and control system for line spectrometer at the IAR. 



MOS-Technology will serve as the cen- 
tral control for the system. The 
availability of such sophisticated data 
handling systems at very low prices 
makes possible much more adequate 
control of the gain and baseline offset 
of the receiver, as these parameters can 
be measured automatically every few 
seconds. 



Reference 

Berkhuijsen, E. M., A survey of the con- 
tinuum radiation at 820 MHz between 
declinations -7° and +85°. I. Observa- 
tions and reductions, Astron. Astrophys, 
Suppl, 5, 263, 1972. 



Image Tube Systems 

W. K. Ford, Jr. 

The combination of the Carnegie 
Image Tube and the Ill-aJ recording 
emulsion continues to be used to pro- 
duce astronomical data of high quality. 



The tubes, which are manufactured by 
RCA (Type C33063EP3), were origi- 
nally developed with the support of 
grants from the National Science 
Foundation. These improved versions 
of the original Carnegie Tube are now 
in operation at a number of obser- 
vatories including Kitt Peak, Cerro To- 
lolo, and Las Campanas. 

During this report year two image 
tube systems for spectroscopic work 
were built in the Department of Ter- 
restrial Magnetism's shop. One of these 
systems was designed in collaboration 
with Drs. Searle and Brucato of the 
Hale Observatories for use with a 
Bowen-type f/1.4 Schmidt camera on 
the Boiler and Chivens spectrograph at 
the new Irenee du Pont Telescope at 
Las Campanas. The second system was 
built for use on the DTM spectrograph 
at Lowell Observatory. It replaces the 
early Carnegie Image Tube System 
(incorporating RCA tube type C33011) 
that has been in regular operation at 
Lowell Observatory, especially by Drs. 
Rubin and Ford, since 1966. 



146 



CARNEGIE INSTITUTION 



We anticipate that much of our as- 
tronomical work in the next few years 
will continue to be done with photo- 
graphic recording of the image inten- 
sifier output. In spite of the problems of 
accurate photometric calibration of the 
plates on one hand and the great prog- 
ress that is being made with direct re- 
cording of data with digital sensors on 
the other, there are some types of ob- 
servations for which the size (or 
number of picture elements), stability, 
and efficiency of the present intensifier 
systems still give an overwhelming ad- 
vantage. Therefore, we are undertak- 
ing a modest renovation and modern- 
ization of the DTM scanning mic- 
rophotometer. The principal mechan- 
ical improvement is the replacement of 
the old traveling stage with a precision 
transverse stage of one micron resolu- 
tion and driven by a stepping motor. 
Also, the card-punch recording of the 
data is being replaced by a digital 
magnetic tape recorder. These changes 
will ease the task of extracting relative 
intensity information from the plates. 

The intensifier-image dissector 
scanners at Lick Observatory and at 
the Kitt Peak National Observatory 
and the Silicon Intensified Target (SIT) 
systems at Hale, Kitt Peak, and Cerro 
Tololo Observatories are operating 
successfully and are yielding photo- 
metric astronomical data with sky 
background subtraction. However, 
these types of systems appear to fall 
short in performance over what might 
be achieved with some silicon detectors 
now in the development stage. In these 
detectors an image is read out serially 
point by point by moving photogener- 
ated charge laterally through the bulk 
silicon of the device to an output gate. 
The charge can be accumulated during 
an exposure or integration period and 
then moved by manipulating in appro- 
priate phase the potential applied to 
metallic electrodes insulated from the 
bulk silicon. These structures are gen- 
erally described by the generic term 
charge coupled devices, or more fre- 
quently simply CCDs. 



The basic advantage of the CCD over 
more conventional camera tubes that 
astronomers seek to exploit is the in- 
herent geometrical stability, compact- 
ness, and long-term stability of sen- 
sitivity. There are two broad ap- 
proaches: the photon-in mode, analo- 
gous to the silicon target vidicon; and 
the electron-bombardment mode, 
analogous to the SIT vidicon. The 
photon-in mode has the advantage of 
high quantum efficiency, a high 
signal-to-noise ratio for high signal 
levels, and probably a higher dynamic 
range than conventional camera tubes. 
The relative lack of problems as- 
sociated with electron beam-landing on 
the target should simplify obtaining 
quantitative information. The elec- 
tron-in, or event-counting, mode may 
eventually offer advantages at very 
low light levels with the quantum ef- 
ficiencies associated with photoemis- 
sive surfaces. The commercial devel- 
opment of prototypes of both these de- 
vices is proceeding along several com- 
petitive lines. 

As a result of the long program of 
development and evaluation of image 
tube systems at DTM we are in a good 
position to analyze and evaluate the 
characteristics of some of the more 
promising of these devices. In particu- 
lar, the data handling system we have 
used with our image dissector scanner 
can be adapted to measuring the per- 
formance of the CCDs as they become 
available. This preliminary evaluation 
can be done without the expensive de- 
velopment of a full data handling sys- 
tem. As a step toward making some in- 
itial tests at DTM and in order to pro- 
vide some on-line data compression for 
spectroscopic applications, Ford and 
Thonnard, assisted by a very capable 
summer student, Mr. Charles Bennett, 
have designed and built a fast arithme- 
tic logic unit to operate with our signal 
averaging device. This unit allows us 
to sum repetitive scans of an image, 
thereby improving the signal-to-noise 
ratio and at the same time reducing the 
total number of picture elements that 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



147 



need to be stored at the expense of re- 
duced resolution in one dimension. 



Acknowledgments 

We thank the Directors of the Cerro 
Tololo Inter-American Observatory, 
Kitt Peak National Observatory, and 
National Radio Astronomy Observat- 
ory for making telescope time avail- 
able, and the Director of the Hale Ob- 
servatories for granting permission to 
reproduce the National Geographic 
Society-Palomar Observatory Sky 
Survey prints. One of us (CJP) wishes 
to thank Drs. R. Racine and S. van den 
Bergh of David Dunlap Observatory 
and H. Swope of Hale Observatories for 
their hospitality during visits to these 
institutions. We also wish to thank the 
Office of Naval Research for the con- 
tinuing loan of the lead ballast used in 
the counterweights of our Derwood 
telescope (Contract No. N0014-71-C- 
0211). 



each of one month duration. These 
trips are intended for personnel of the 
IAR to cooperate in the design, assem- 
bly, testing, and data processing for the 
joint project. 

Five parametric amplifiers at 21 cm 
have been received from the Nether- 
lands Foundation for Radioastronomy. 
These amplifiers were used in the Wes- 
terbork Synthesis Radiotelescope dur- 
ing its earlier observations. They are 
going to be used in the near future in 
the radio interferometer of the IAR, 
which is planned as an aperture syn- 
thesis instrument. 

The scientific staff has increased this 
year with two young members: Drs. T. 
E. Gergely and M. Zales de Caponi, 
both of whom received the Ph.D. at the 
University of Maryland. 

Since July, E. Bajaja has been the 
Director of the IAR. 



Technical Activities 



Activities of the Instituto 
Argentino de Radioastronomia 

For the IAR as well as for most of the 
Argentine observatories, 1975 was a 
difficult year because of the economic 
situation. Nevertheless, some major 
projects have been carried out. One of 
them was the relocation of the 21-cm 
line receiver to a new control building. 
Except for the necessary interruption 
during the movement of the receiver, 
observations have continued an aver- 
age of 16 hours a day. 

The joint project between CIW and 
IAR for the new 21-cm receiver was 
started after the agreement between 
the Ajencia Ejecutiva Argentina and 
the National Science Foundation was 
finalized. Under this agreement the 
Consejo National de Investigaciones 
Cientificas y Tecnicas de la Republica 
Argentina will support the expenses 
for three round trips to Washington, 



The main task of the Technical De- 
partment this year was the installation 
of the 21-cm receiver in the new build- 
ing. This building was built between 
the two 30-m dishes and equipped with 
an effective air conditioning system. 
There is enough room for the receivers 
and for the operation facilities. 

The electrical wiring, cable ducts, air 
conditioning, lighting, and appliances 
were ready around March. The move- 
ment of the receiver started in April. 
At the same time, some parts were re- 
placed with new ones designed and 
built at the IAR; for example, the low 
frequency section of the LO and the old 
SOLA AC regulators were replaced by 
small units with harmonic rejection 
and better stability. A complete set of 
interconnected boxes allows easy 
maintenance of the whole system; and 
individual power distribution was 
added to each rack. Some of the old IF 
amplifiers were replaced by new ones 
with better stability. A more reliable 
temperature controller was installed in 






148 

the filter boxes, and some of the chassis 
were rewired and readjusted. First 
tests of the system started in June, and 
in July the receiver was put into nor- 
mal operation. 

The new set-up is much more reli- 
able, and stability has been improved 
by a factor of 5, allowing a better 
front-end adjustment. The system- 
noise temperature is about 180°K. 

At the end of the year some work was 
also done on the second 30-m dish, such 
as painting, and construction of cable 
ducts, interconnection house, and part 
of the electrical installation. 



Summary of Current Research 

E. Bajaja, F. R. Colomb, and R. Mor- 
ras have extended their observations at 
intermediate galactic latitudes and are 
now analyzing the results of the region 
200 ssZ ^315; -10 ^b ^ -32. 

Bajaja, Colomb, G. Dubner, and T. 
Gergely are completing the study of 
four supernova remnants: G 296.3 + 10, 
G 261.9 + 5.5, G 327.6 + 14.5 and G 
330.7 + 15. Other supernova remnants 
of small angular diameter have also 
been observed. 

Colomb, W. Poppel, and C. Heiles of 
the University of California continued 
the observations for the hydrogen sur- 
vey at low velocities (-50 =5 V *£ 50 



CARNEGIE INSTITUTION 



km/s) of the Southern Hemisphere. 
They completed about 60% of the ob- 
servations and produced a radio map of 
the Southern sky every 2° in 8. 

Colomb and I. Mirabel observed 
emission profiles in the direction of 23 
southern pulsars. 

Bajaja, Colomb, and Poppel began 
observations of spiral and irregular 
galaxies with m< 9. Thirty wide filters 
(AV = 25 km/s) and the multichannel 
integrator have been used in this pro- 
gram. 

Poppel and M. Franco observed the 
region 0^ / ^ 20; -3° 5* b ss -17° every 
1° in I and b, and continued with the 
analysis of the region 348" «s I «£ 372°; 
3° =£ b *£ 17° (Atlas of Galactic Hydro- 
gen, Poppel and Vieira, CIW, 1974). 

Poppel and D. Vota completed the 
observation of the galactic clusters 
NGC 5460, Rn 106, and G 394. 

E. Arnal completed the observations 
of the galactic clusters NGC 2243, 
3688, 2345, 2204, 4755, 3680, 6611, 
and 6994, IC 2391, Mel 227, and A 
(Blanco) as a general survey of HI in 
the direction of galactic clusters. 

Gergely and Arnal completed the ob- 
servations of the globular clusters B46, 
B67a, B87, and B63. 

Arnal completed the observations of 
the association CMaOBl. 

Gergely and Mirabel completed the 
observations of globular clusters with 
known reddening. 



DEPARTMENT OF TERRESTRIAL MAGNETISM 

THE EARTH AND THE SOLAR SYSTEM 



149 



L. T. Aldrich, C. Brooks, J. Cleary, J. R. Evans, M. Feigenson, S. E. Forbush, S. R. Hart, A. W. 

Hofmann, D. E. James, A. T. Linde, M. Magaritz, S. Rajan, I. S. Sacks, M. Schneider, C. M. Seber, 

J. A. Snoke, K. Suyehiro, D. J. Whitford, and S. Weidenschilling 

Collaborators 

C. Aguirre, J. Bannister, R.J. Beauan, K. Bloomfield, M. Bracamonte,R. Cabre,A. Cuyubamba, G. L. 

Davis, S. del Pozo, D. W. Evertson, A. Flores, L. Gelinas, A. A. Giesecke, N. Girardin, G. C. P. King, 

Y. Motoya, L. Ocola, H. Okada, G. Olafsson,R. W. Page, G. Poupeau, A. Rodriguez B., D. Simoni, 

R. Stefansson, S. Suyehiro, L. Tamayo, J. G. Williams, Y. Yamagishi, and E. Zinner 



Gas-Solid Interactions in the 
Solar Nebula 

S. J. Weidenschilling 

It is widely believed that the planet- 
ary system was formed from matter in 
a cloud of gas and dust which sur- 
rounded the sun. The origin and struc- 
ture of this solar nebula are disputed, 
but it is generally depicted as a rotat- 
ing, flattened disklike structure. It is 
reasonable to assume that it was 
somewhat centrally condensed, with 
the gas pressure and density in the 
central plane decreasing with distance 
from the axis. Such a pressure gradient 
would partially support the gas against 
the sun's gravity, requiring its rotation 
rate at any radius to be less than the 
local Kepler orbit velocity. 

Solid bodies are not supported by the 
pressure gradient; in the absence of 
gas, they would pursue Kepler orbits. 
A balance of gravitational, centrifugal, 
and drag forces requires that a particle 
always have some velocity relative to 
the gas. The radial component of its 
motion is always in the direction of in- 
creasing pressure, i.e., inward. We de- 
fine the "stopping time," t e , as the time 
for a particle's velocity to be reduced by 
a factor e by drag in the absence of 
other forces; t e depends on the prop- 
erties of the particle and the gas, and 
on the velocity. The particle motion is 
characterized by the ratio of t e to the 
orbital period, t p . Two limiting cases 
were examined quantitatively by 
Whipple (1973); 

1. Small body (t e /t p « 1). The parti- 
cle is constrained to move with the an- 



gular velocity of the gas. It feels a re- 
sidual central attraction. In a frame 
rotating with the gas, it falls directly 
inward. The terminal velocity in- 
creases with particle size. 

2. Large body (t e /t p » 1). The parti- 
cle pursues a Kepler orbit, which is 
perturbed by drag. It loses angular 
momentum and spirals inward. The 
larger the body, the smaller is the ef- 
fect of drag. 

In the limits of very large and very 
small bodies, the radial velocity ap- 
proaches zero. Somewhere between 
these cases (when t e lt„ ~ 1), it must 
reach a maximum, as shown schemati- 
cally in Fig. 23. This case, potentially 
the most interesting, had not been 
examined in detail before the present 
study. In general, the equations of mo- 
tion for the strongly perturbed case 
must be solved numerically. However, 
some interesting properties of the solu- 
tion can be shown analytically: 

1. There is an upper limit to the ra- 
dial velocity, which depends only on 
the nebular structure. It is indepen- 
dent of the drag law and the particle 
properties. 

2. The radial and transverse compo- 
nents of the "wind" velocity felt by the 
particle are uniquely related. They also 
depend only on the nebular structure. 

3. If the gas is assumed ideal, the 
maximum radial velocity is indepen- 
dent of the gas density. The results for 
any case are therefore insensitive to 
the nebular mass. 

The particle properties and drag law 
determine the velocity components cor- 
responding to a given particle size. The 



150 



CARNEGIE INSTITUTION 



Upper Limit (%-V.) 



t e «t 
Radial Drift 



Perturbed Kepler 
Orbit 




particle radius 



Fig. 23. Radial velocity as a function of particle size (schematic). The shape of the curve depends 
on the drag laws, but the value of the upper limit does not. 



solution is complicated by the fact that 
any of four drag laws may be appli- 
cable, depending on the Reynolds 
number and the ratio of mean free path 
to particle size. Each drag law must be 
tried until a self-consistent solution is 
found. I have written a computer pro- 
gram that performs this task, solving 
the equations of motion numerically. 
The velocity components can be deter- 
mined for a (spherical) particle of any 
size and density in a nebula of arbi- 
trary structure. Results to date must be 
considered preliminary, but the follow- 
ing conclusions appear to be justified 
for a wide range of "reasonable" nebu- 
lar models: 

(1) The maximum radial velocity 
can be large. In the inner nebula, it 
may exceed 100 m/s ] at 1AU, decreas- 
ing by about an order of magnitude in 
the outer part. (2) These velocities gen- 
erally correspond to particle of radii ~ 
10-100 cm (Fig. 24). (3) Characteris- 
tic lifetimes for spiraling into the sun 
can be very short for such particles, 
ranging from a few tens of years in the 
inner nebula to 10 4 years in the outer 
part. The loss of solids from any region, 



or mixing between regions, could have 
been significant, depending on the par- 
ticle size distribution. (4) For small 
particles (t e «t p ), larger and/or den- 
ser bodies are removed more rapidly. 
For large particles (t e »t p ), the oppo- 
site is true. Fractionation by composi- 
tion, e.g., Fe from silicates, could occur 
in either direction. The rate of mass 
loss is greatest in the inner nebula; 
Mercury's high Fe content may be ex- 
plainable by this effect. (5) Radial 
and transverse velocities are strongly 
dependent on particle size. It is gener- 
ally assumed that gas drag would 
damp their relative motions. However, 
if the particles vary in size, they may 
have large relative velocities because 
of the gas. It is not clear whether the 
growth of large bodies would be aided 
or hindered. High relative velocities 
would inhibit gravitational instability 
(Goldreich and Ward, 1973), but if par- 
ticles stick together on impact, the in- 
creased rate of collisions might be con- 
ducive to growth. 

These and many other consequences 
remain to be investigated in greater 
detail. 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



151 






I0 : 



10 - 



10- 



-[ — I I I I 1 1 1 1 1 — I I II 1 1 

Peer -3 

T = (600/r)°K 

P„ = 3 g cm-3 




r(AU) 

Fig. 24. Radial velocities of particles of vari- 
ous radii (S) as functions of location in a typical 
model nebula. Particle density = 3 g cm -1 . Nebu- 
lar gas pressure varies as r~\ density as r~ 2 . 



References 

Goldreich, P., and Ward, W. R., The forma- 
tion of pbanetesimals, Astrophys. J. 183, 
1051, 1973. 

Whipple, F. L., Radial pressure in the solar 
nebula as affecting the motions of 
planetesimals, NASA SP-319, 355, 
1973. 



Interplanetary Bodies and 
Solar System History 

George W. Wetherill 

Studies of the solar system and its 
origin have been primarily directed 
toward the largest bodies — the sun and 
the planets. However, during the past 
few decades it has become increasingly 
clear that equally significant informa- 
tion can be obtained from the smaller 
bodies of the solar system: the satel- 



lites of the planets including our moon, 
and the asteroids, comets, meteoroids, 
and meteorites. The small size of these 
bodies has permitted their record of 
primordial events in solar system his- 
tory to be preserved to a much greater 
extent than is the case for hot and in- 
ternally active planets. The inter- 
planetary orbits of comets and many 
asteroids produce a cratering record on 
the moon and planets which reveals 
their size and number throughout the 
history of the solar system. Study of 
meteorites and lunar samples permits 
direct chemical investigation of the 
record of events dating back to the for- 
mative period of the solar system. Re- 
cent evidence for presolar grains in 
meteorites (Clayton and Mayeda, 1975) 
provides a link between planetary for- 
mation and the presolar galactic his- 
tory of the matter from which it was 
formed. 

Interpretation of the data obtained 
from studies of these small bodies re- 
quires understanding of their sources 
in the solar system and the evolution of 
their orbits. Very significant advances 
in these areas have followed the devel- 
opment of new techniques for tracing 
their orbital evolution over billions of 
years (Opik 1951, Arnold 1965), for 
understanding the detailed structure of 
the asteroidal regions (Williams 1969, 
1971), and for calculation of the devel- 
opment of the swarm of interplanetary 
bodies which preceded the formation of 
the planets themselves (Safronov 
1969). Examples of application of these 
achievements are given in the follow- 
ing three subsections, which deal with 
the role of interplanetary bodies in dis- 
tinct periods in solar system history: 
(1) the formation of the planets and the 
moon (4.5 billion years ago); (2) the late 
heavy bombardment of the moon and 
terrestrial planets which produced the 
heavily cratered terrains preserved on 
the lunar highlands, Mercury, and 
Mars (4.0 billion years ago); and (3) the 
earth-crossing Apollo objects that have 
dominated the cratering record of the 
earth and moon during the past three 
billion years. 



152 



CARNEGIE INSTITUTION 



The Role of Large Impacting Bodies in 
the Formation of the Earth and Moon 

Introduction. Over the years many 
processes have been suggested which 
involve the existence of large (> 10 24 g, 
~ 500 km radius) planetesimals during 
the formation of the solar system. 
These include lunar-sized meteorite 
parent bodies (Urey, 1959), capture 
theories for the formation of the moon 
(Gerstenkorn, 1969; Singer, 1972), dis- 
integrative capture by passage of 
bodies within the earth's Roche limit 
(Opik, 1972; Smith, 1974; Mitler, 
1975), impact heating of the earth (Saf- 
ronov, 1969) and moon (Ruskol, 1974), 
tilting of the earth's rotation axis by a 
small number of large impacts (Saf- 
ronov, 1960), formation of the moon by 
"splashes" resulting from grazing im- 
pacts with the earth (Hartmann and 
Davis, 1975; Cameron and Ward, 
1976), predifferentiation of lunar 
planetesimals (Kaula and Bigeleisen, 
1975), and the existence of large post- 
accretional residual bodies needed to 
produce the late heavy bombardment 
which continued for —600 m.y. after 
the end of the accretion of the planets 
(Wetherill, 1975b). An evaluation of 
the plausibility and relative impor- 
tance of these processes requires a 
theory that predicts at least semiquan- 
titatively both the size and the velocity 
distribution of the planetesimals that 
accumulated to form the terrestrial 
planets and the moon. Considerable 
progress toward such a theory has been 
made by Safronov (1969) and his co- 
workers (Zvyagina et al., 1973). Other, 
less comprehensive treatments of this 
problem have been given by Marcus 
(1965, 1969), Hallam and Marcus 
(1974), Hartmann and Davis (1975), 
and Weidenschilling (1974). A charac- 
teristic of these theories is that ac- 
cumulation of the planetesimals into 
the terrestrial planets and the moon by 
random encounters is slow (10 5 to 10* 
years) compared with the time scale of 
~10 :i yr proposed for accumulation 
from a gaseous solar nebula rather 
than from a protoplanetary swarm of 



planetesimals (Cameron, 1973). Seri- 
ous difficulties with the latter type of 
theory have been discussed (Safronov, 
1975). In spite of these difficulties, 
gravitational accretional heating as- 
sociated with very rapid accumulation 
has been the starting point of most dis- 
cussions of the geochemical and 
geophysical evolution of the moon, and 
it is commonly stated that such dynam- 
ically unlikely rapid accretion is re- 
quired if one is to understand the evi- 
dence for early chemical differentiation 
of the moon. 

As discussed by Safronov (1969), ap- 
preciable accretional heating does not 
require rapid accretion, provided that a 
sufficiently large number of the ac- 
cumulating bodies are massive and 
energetic enough to deposit a signifi- 
cant quantity of energy deep beneath 
the accretional surface and thereby to 
preclude loss by surface radiation into 
space. 

Calculation of the size distribution of 
the largest bodies is not readily amen- 
able to coagulation theory (Safronov 
and Zvyagina, 1969) even though con- 
siderable insight is provided by this 
approach to the size distribution of 
smaller bodies, especially when the 
combined effects of accumulation and 
fragmentation are considered 
(Zvyagina et al., 1973). One reason for 
the failure of the theory for the largest 
accumulating bodies is that for these 
bodies the gravitational field of the 
growing body can augment its effective 
cross section in such a way that the 
cross section becomes nearly propor- 
tional to the fourth power of its radius 
rather than to the geometrically de- 
termined square of the radius. This ef- 
fect is partially included in the only so- 
lutions to the coagulation equation 
which have been reported, i.e., for a 
coagulation kernel linear in the mass 
and hence proportional to the cube of 
the radius. This approximates the situ- 
ation over much of the mass range but 
not at the uppermost end of the range, 
in which a small difference in radius 
might, at least in principle, lead to a 
great difference in the rate at which 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



153 



the radius increases. Numerical exam- 
ples of such "runaway accretion" have 
been given by Hartmann and Davis 
(1975). 

Safronov (1969) has made use of 
runaway accretion to predict a "second 
largest mass" in the earth's accumula- 
tion zone of 10~ ! earth masses, i.e., 6 x 
10 24 g. In the present work a variant of 
this model is proposed which leads to 
the second largest body being more 
massive, and thereby also implies sig- 
nificantly greater accretional heating 
of the moon. 

Runaway accretion. The rate of 
growth of the ratio of the radii of the 
two largest bodies (R = rjr. z ) is by the 
elementary rules of differentiation: 



dR 

dt 



L d 



dr x 
df 



R 



dr, 
dt 



(1) 



In a simple accumulation model in 
which a body of radius r sweeps up 
smaller bodies with relative velocity V, 



dr_ 
dt 



p,.(t) 



V 



(i + Jfc) 



(2) 



where p c (t) = the spatial density of 
mass in the form of smaller bodies, p„ = 
the density of the larger body, and Ve 
= the escape velocity of the larger 
body. 

In Safronov's accretion theory the 
mean relative velocity is related to the 
escape velocity of the largest body by 
use of the dimensionless ratio 6: 



V 1 = 



2H ~ 



and 



= 



GM„ 

R,y 2 



(4) 



where G is the gravitational constant, 
and the subscript p refers to the escape 
velocity, mass, and radius of the 
largest body in the earth's zone. Saf- 
ronov, following earlier work of 
Gurevich and Lebedinskii (1950), pre- 
sents calculations that support his con- 
clusion that 6 varies slowly (i.e., by a 
factor of ~2), while M p increases by a 



factor of ~10 12 during accretion. This 
implies that the mean relative velocity 
"keeps step" with the growth in radius 
and escape velocity during accumula- 
tion. The numerical value of 6 depends 
on the balance between acceleration by 
mutual perturbations by planetesimals 
and dissipation of energy in collision of 
planetesimals. Under a variety of con- 
ditions in which accretion occurs after 
loss of most of the nebular gas (appro- 
priate to earth's zone), the value of 6 is 
estimated to range from about 7 down 
to about 1. Therefore, the mean veloc- 
ity with which smaller bodies collide 
with the largest body is always less 
than its escape velocity. Calculations 
such as those reported by Marcus 
(1969) using experimental data of 
Gault et at. (1963) then lead to the con- 
clusion that at these velocities the 
largest bodies will grow in radius 
rather than fragment. Smaller bodies 
may either accrete or erode, depending 
upon their mass. 

Substitution of 6 into (2) permits (1) 
to be developed to: 



dR 

dt 



\/ p c (t)\ (2d-R)(R-l)\ 



(5) 



where G is the gravitational constant. 
For constant 6, the solution of (5) 
may be written as: 



1 +28 



(3) R 



L 20-R J v 26 + 1 y 



I- 2Q-R... J \ 9,ft+-[ / 



(6) 



where it! (l is the initial value of/?, 

A- (20+1) Pr (o)\ I- —1 V2 
L 3 6p„ -I 



and f is a time-like variable: 

f i: =—^~ f p ( .(t)dt. 
p r (o) J " H 



154 



CARNEGIE INSTITUTION 



Equation (6) shows that the extent of 
the runaway accretion is limited, as 
discussed earlier by Safronov (1969). 
As t becomes indefinitely large, R—>26, 
which is therefore the maximum ratio 
of the final radii. For nonzero values of 
the initial radius 

R<26 (7) 

The final ratio of the masses of the two 
largest bodies is therefore less than 
(20) 3 . By use of 6 = 5 and assuming t* 
to be infinite, Safronov obtained the re- 
sult mentioned earlier, that the second 
largest body in the earth's zone had a 
mass of ~ 6 x 10 24 g. 

As will be seen later, in this case the 
number of bodies surviving to the ter- 
minal stages of growth of the moon will 
be too small to produce much accre- 
tional heating. 

However, there is no reason to sup- 
pose that 6 must be as large as 5. In 
fact, Safronov (1969, Table 8, Chapter 
7) gives 6—2 during the stage of 
planetesimal growth prior to the time 
in which most of the mass is in a single 
largest body, the planetary embryo. 
This value limits the mass ratio to less 
than 64. It is very difficult to know the 
correct value of 6 to be used at this 
stage, as direct knowledge of the ap- 
propriate dissipation to be assumed in 
protoplanetary planetesimal collisions 
is not available. However, the values 
assumed by Safronov to obtain 6 — 2 
appear plausible. During later stages 
of growth of the planetary embryo, Saf- 
ronov shows that the value of 6 in- 
creases to ~5, implicitly assuming that 
the remaining planetesimals cross the 
orbit of only one dominant planetary 
embryo, moving in a circular orbit. In- 
sofar as this assumption is correct, this 
is a consequence of the fact that succes- 
sive encounters with the embryo will 
always occur at the same value of the 
mean geocentric velocity, provided that 
no perturbations by a third body occur 
between these successive encounters. 
This in turn is a consequence of the 
conservation of energy in these en- 
counters in the geocentric reference 
frame, related to the constancy of the 



Tisserand criterion and the Jacobi in- 
tegral in the restricted three-body 
problem. 

The fact that when the earth has 
reached —10% of its final mass, bodies 
with geocentric velocities of 2.5 km/s 
will cross the orbit of Venus invali- 
dates the above assumption. The case 
where a small body is crossing the orbit 
of both Earth and Venus has been in- 
vestigated both by analytical and 
Monte Carlo techniques, and it has 
been found that Venus-crossing results 
in 6 decreasing to the range 2 to 0.5 
during the final stages of planetary 
accretion, rather than increasing to 
~5, as is obtained when this effect is 
not considered. This leads to mean ve- 
locities of ~8 km/s for objects crossing 
the orbits of both Earth and Venus. Re- 
sults of calculations of the masses of 
the second largest body are given in 
Table 2. 

Elimination of the residual planetes- 
imals. These lower values of 6 lead to 
much larger objects in the zone of 
Earth and Venus during the final 
stages of accumulation. In the case of 
runaway accretion the overwhelming 
dominance of the planetary embryo 
was established by the much more 
rapid growth of the largest body. For 
these lower values of 6, this dominance 
is established by an alternative effect: 
the greater stability of the embryo dur- 
ing the regime of fragmentation which 
exists during the final stage of accumu- 
lation of the embryo. This fragmenta- 
tion primarily results from two pro- 
cesses: disruption following approach 
within the Roche limit of the Earth or 
Venus, or collisional fragmentation. 

The largest residual planetesimals 
(~10 2,i g) will probably be immune to 
destruction by collision and are likely 
to be continuing to accrete until they 
are disrupted by tidal forces. Their 
mass is greater than that of the present 
moon, which is near equilibrium with 
respect to accretion vs. erosion by crat- 
ering (Gault et al., 1963; O'Keefe and 
Ahrens, 1975, 1976) at an average im- 
pact velocity about twice as great. At a 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



155 







TABLE 2. 






A. Mass of the Second Largest Body When the Earth Embryo is 2 
Initial Mass Ratio atM, = 10 21 g: 


x 10 27 g 


6 

1 
2 
3 
4 
5 


1.16 
1.1 x 10 27 g 

4.0 x 10 2,i 
1.9 x 10 2,i 

1.1 x 10 2,i 

7.2 x lO 25 


2.20 

4.2 x 10 2li g 
7.4 x 10 25 
2.4 x 10 25 

1.1 X lO 25 

6.4 x 10 24 


8.00 

2.0 x 10 2,i g 
3.9 x 10 25 
1.2 x 10 25 
5.5 x 10 2J 
2.9 x 10 2J 




B. Mass of the Second Largest Body When the Largest Body is 4 


x 10 27 g 


1 
2 
3 
4 
5 


2.2 x 10 27 g 
7.0 x 10 2B 

3.0 x 10 2,i 
1.6 x 10 2 « 

1.1 x 10 2li 


6.4 x 10 2,, g 
1.3 x 10 2,i 
4.3 x lO 2 ' 

2.0 x 10 25 

1.1 x 10 25 


5.0 x 10 2,i g 
7.7 x 10 25 
2.3 x 10 25 
1.0 x 10 25 
5.5 x 10 2J 



mean impact velocity of 8 km/s, bodies 
less than ~ 2 x 10 24 g in mass will be 
subject to fragmentation rather than 
continuing to accrete. The lifetime for 
collisional destruction of these bodies 
by smaller residual bodies in the zone 
of Earth and Venus may be estimated 
by methods used earlier (Wetherill, 
1967) to be in the range of 10" to 10 7 
years. Smaller bodies of mass less than 
10 22 g will be very short lived (<10 5 
yr), and a collisional mass distribution 
may be expected to be established 
among these smaller bodies. The popu- 
lation of small bodies will be main- 
tained by supply of ~10 2i g bodies re- 
sulting from Roche limit destruction of 
bodies <10 24 g in mass. Fragments will 
be removed from this population of 
fragmental material by accretion onto 
those bodies large enough to accrete 
(i.e., < 2 x 10 24 g), which will ulti- 
mately be only the Earth and Venus 
embryos. Therefore, the final result 
will be the same as was the case for the 
alternative of runaway accretion. All 
the mass will be in the planetary em- 
bryos (except for a small fraction accel- 
erated to Jupiter-crossing). The dif- 
ference will be that large bodies, up to 
~3 x 10 2,i g in mass will have had a 
transient existence during the final 
stages of accretion. The time scale for 
sweep-up of all the bodies will be 
somewhat extended by the higher ve- 



locities during the final stages but will 
still be ~10 8 yr. 

Heating of the moon by impact of 
large planetesimals. As discussed ear- 
lier (Safronov 1969, Ruskol 1974, 
Wetherill 1975a), large high-velocity 
impacts will deposit their kinetic en- 
ergy beneath the lunar surface, which 
will be buried by fallback of crater 
ejecta into the transient cavity. This 
trapping of energy will be particularly 
pronounced for the largest impacts in- 
asmuch as in this case most of the 
fragmented material will fail to escape 
the cavity because of the large gravita- 
tional energy required. 

In an accreting body, the heat trap- 
ped in this manner will be further 
buried by deposition of newly accreted 
material onto the floor of the crater, 
thereby minimizing the radiation into 
space which limits the temperature in 
most accretion models (i.e., Hanks and 
Anderson, 1969; Mizutani et al., 1972). 
This effect has been estimated by a 
simple model in which the depth of bu- 
rial of the heat increases as a result of 
accretion and decreases by conduction 
of heat to the surface. 

Under the conditions of accretion, 
the dominant mode of heat transfer to 
the surface will not be ordinary lattice 
conduction but mixing of previously 
buried material to the surface by sub- 
sequent impacts. The appropriate 



156 



CARNEGIE INSTITUTION 



value of "eddy conductivity" to be used 
is quite uncertain. Values of —2 cm 2 /s 
are calculated by Safronov (1969), and 
similar values can be estimated from 
the work of Blake and Wasserburg 
(1976) carried out in a different con- 
text. Use of this value indicates that 
bodies with radii larger than —40 km 
in radius, i.e., with masses <10 21 g will 
have a large fraction of their kinetic 
energy permanently trapped beneath 
the accreting surface. At first the frac- 
tion of impact melt produced in any 
single impact will not be large. Much of 
the melted material will be mixed with 
pulverized unmelted crater debris; it 
will merely heat it slightly and produce 
little igneous differentiation. However, 
the cumulative effects of further im- 
pacts will cause the temperature of the 
multiply reworked debris to approach 
and exceed the melting point. Very 
close estimates are difficult, but pre- 
liminary calculations suggest that dur- 
ing the addition of the final half of the 
moon's mass (outer 360 km) about 10% 
of the impacting mass was in the form 
of bodies more massive than 10 2 " g. 
This leads to an average kinetic energy 
trapped within the outer 360 km of ~8 
x 10 9 ergs/g. This is about 40% of the 
energy required to produce complete 
melting of this region. If these figures 
are correct, the outermost portion of 
the moon will be heterogeneously 
melted and differentiated at the end of 
accretion. The individual large accret- 
ing bodies will be sufficiently energetic 
to completely melt many times their 
own volume of material, which has al- 
ready been heated much of the way to- 
ward melting point. Regions which by 
chance failed to be impacted by a very 
large body would contain only small 
regions of total melt and much fine ma- 
terial mixed with quenched glass. The 
resulting state of primordial dif- 
ferentiation may be quite different 
from a "magma ocean" resulting from 
uniformly distributed heat sources. 

In the alternative model, in which 
runaway accretion causes the second 
largest body to be much smaller, it 
seems unlikely that lunar heating 



would be sufficient to account for the 
evidence for primordial lunar dif- 
ferentiation, especially if the larger 
bodies are still destroyed by encounters 
within the Roche limit. The resulting 
trapped energy in the outer 360 km is 
estimated to be ~ 3 x 10 7 ergs/g, suffi- 
cient to melt only —0.1% of this region. 
Because of many uncertainties in these 
calculations, the absolute values of 
these numbers should probably not be 
taken too seriously. However in the al- 
ternative model, the much smaller 
number of large bodies, impacting at 
less than half the velocity, must in- 
evitably result in much smaller quan- 
tities of trapped energy. 

Concluding remarks. This rather 
sketchy account of a chain of events 
within the framework of the Safronov 
accretion theory should at least show 
that significant primordial differentia- 
tion of the moon could have occurred 
even on a long — 10 8 time scale. The 
subsequent magmatic evolution of a 
heterogeneously melted and dif- 
ferentiated moon will be more complex 
than that of the "magma ocean." Al- 
though this complexity may be an in- 
convenience, in no way does complexity 
or simplicity bear on the question of 
whether the moon was uniformly or 
heterogeneously differentiated. The 
possibility of heterogeneous dif- 
ferentiation was not introduced as an 
ad hoc complexity to complicate the 
lives of petrologists; it is merely a 
probable consequence of an equally 
simple, and dynamically more plausi- 
ble, model of accretion. It may be that 
exploration of the petrological conse- 
quences of this variant may result in 
its seeming more satisfactory than the 
magma ocean, or vice versa. This re- 
mains to be done; some speculations 
bearing on this have been presented 
(Wetherill 1975a). 

Primordial heating of the terrestrial 
planets would also result from this 
mechanism. The differences in the ini- 
tial temperatures between the larger 
bodies such as the earth and the moon 
would be less than in the case of heat- 
ing by the very rapid accretional mod- 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



157 



els that have been published (e.g., 
Hanks and Anderson, 1969). This 
arises from the fact that in the theory 
presented in this work the energy 
input depends primarily upon the kine- 
tic energy of the heliocentric planetes- 
imals, the acceleration by the gravita- 
tional field of the accreting planet 
being of secondary importance. How- 
ever, somewhat higher initial tempera- 
tures may be expected for the earth, if 
only as a consequence of all the impacts 
being caused by heliocentric bodies 
rather than largely geocentric bodies, 
as is the case for a coaccreting moon. 
Insofar as planetesimals in the range of 
10 25 - 10 2,i g are. still accreting prior to 
tidal disruption, significant heating 
and even igneous differentiation of 
their interiors are possible but difficult 
to estimate with any degree of cer- 
tainty. 

As mentioned in the introduction, 
numerous other authors have invoked 
large bodies in connection with various 
problems of lunar origin. The accre- 
tional model outlined herein can, when 
more fully worked out, lead to fairly 
definite predictions concerning the size 

(distribution, orbits, and evolution of 
large bodies in the early solar system. 
In this context these suggestions by 
others represent processes which are 
possible, rather than being contrived, 

I as has sometimes been alleged. It re- 
mains to be seen to what extent they 
are probable rather than simply possi- 
ble. 
It is also of interest to inquire into 
the ultimate fate of the large bodies 
that were initially present in the inner 
solar system. Most of them simply ac- 
creted to form the moon and terrestrial 
planets. A small percentage of them 
were ejected from the solar system. 
However, the Monte Carlo calculations 
mentioned earlier indicate that if Mars 
was present during the accretion of the 
earth, about 1% of the earth planetes- 
imals remaining after the growth of 
the earth was essentially complete 
would be removed from earth-crossing 
by Mars perturbations. Most of these 
would have been deep Mars-crossers in 



orbits similar to the present Amor ob- 
jects and returned to earth-crossing on 
a time scale of ~2 x 10* years. The role 
of these bodies in the late heavy bom- 
bardment of the terrestrial planets has 
been discussed elsewhere (Wetherill 
1975b). Of greater interest are the 
smaller number ( — 10%) of these which 
evolve into shallow Mars-crossers with 
lifetimes comparable to the age of the 
solar system. Of the ~10 2li g which re- 
mained when the growth of the earth 
was 98% complete, perhaps 10 2i g was 
"implanted" into these long-lived or- 
bits. If Mars accumulated on the same 
time scale as the earth, those bodies 
would be outnumbered by similar re- 
sidua from the formation of Mars. On 
the other hand, the formation of Mars 
is difficult to understand unless it rep- 
resents an "aborted" earth-size planet, 
the growth of which was terminated by 
the early heavy bombardment of Jupi- 
ter planetesimals, as suggested by Saf- 
ronov (1969) and Weidenschilling 
(1975). In this case the Mars residua 
would have been destroyed before the 
implantation of the earth planetesi- 
mals, and this region of the asteroid 
belt would now be largely populated 
with residual material from the forma- 
tion of Earth and Venus. The dif- 
ferentiated meteorites, such as the 
basaltic achondrites, are likely to arise 
from this region of the asteroid belt, 
and it is possible that these objects are 
fragments of earth planetesimals that 
have now returned home after an ab- 
sence of 4.5 billion years. When viewed 
from this standpoint it is possible that 
the dynamic and chemical heterogene- 
ity of the present asteroid belt is a con- 
sequence of this region of the solar sys- 
tem being a "sink" for residual mate- 
rial from both the inner and the outer 
solar system. 

Late (4.0 b.y.) Bombardment of the 
Moon and Terrestrial Planets 

Introduction. Prior to the recent pro- 
gram of manned and unmanned explo- 
ration of the moon and terrestrial 
planets, there was a tendency to sepa- 



158 



CARNEGIE INSTITUTION 



rate earth history into two discrete and 
nonoverlapping periods of time. These 
were the last ~ 3.5 x 10 9 yr, revealed 
by the terrestrial geological record, and 
the primordial nebular and accretional 
periods, 4.6 x 10 9 years ago. Evidence 
concerning the latter was inferred from 
theoretical considerations and observa- 
tional and meteoritic data. It was 
commonly believed that the cratered 
surface of the moon represented bom- 
bardment during the terminal phase of 
this primordial period. 

Following the Apollo 14 mission to 
the Fra Mauro region of the moon and 
dating of these rocks at 3.9-4.0 x 10 9 
years, it became apparent that the 
period of intensive lunar cratering and 
mare basin formation continued long 
after the completion of the — 10 8 -year 
formational period of the moon and 
planets. In addition to the "early heavy 
bombardment" which more or less ac- 
companied the accretion of the planets, 
there was a "late heavy bombardment" 
which continued for hundreds of mil- 
lions of years and may well have been 
episodic in character (Tera etal., 1974). 

The Mariner 10 spacecraft photo- 
graphed —40% of the surface of Mer- 
cury to a resolution of 10 km and 
selected regions to a resolution of 200 
m. Photogeological study of these data 
indicates a remarkable similarity be- 
tween the impact flux on Mercury and 
the moon (Murray et al., 1975). Both 
planets contain densely cratered re- 
gions containing more than 200 
craters/10" km 2 larger than 10 km in 
diameter. The lunar regions of this 
kind represent a steady-state crater 
population, whereas Mercury contains 
both steady-state regions of this kind 
and poorly understood "intercrater" 
terrains in which a high density of 
craters larger than 50 km in diameter 
appears to be superimposed on an older 
relatively uncratered surface (Trask 
and Guest 1975). Both planets contain 
ringed impact basins and associated 
ejecta deposits ranging up to ~ 1000 km 
in diameter. In both cases postbasin 
surfaces have crater densities of —120 
craters/10" km 2 greater than 10 km 



diameter. Finally, both planets contain 
younger basin-filling surfaces presum- 
ably of volcanic origin with lower cra- 
ter frequencies of —40 craters/10" km 2 
greater than 10 km in diameter. Al- 
though the Martian crater record is 
obscured by erosional and depositional 
processes, photogeological studies 
suggest a similar impact history for 
that planet (Wilhelms, 1973; Chap- 
man, 1974; Soderblome^a/., 1974). 

These similarities between the three 
planetary bodies could be coincidental 
or could be the result of their experi- 
encing a very similar impact flux his- 
tory. If the latter is the case, the rela- 
tively lightly cratered postbasin sur- 
faces with crater counts of —120/10" 
km 2 represent a "marker horizon" 
throughout the inner solar system cor- 
responding to a time of 3.9±0.1 b.y. 
ago, as indicated by the lunar 
radiometric chronology. 

The purpose of this work is to quan- 
titatively explore alternative models 
for the interplanetary flux during the 
first — 10 9 years of solar system history 
in order to evaluate whether any of 
these are compatible with the hypothe- 
sis of a uniform flux history for these 
planets. The other consequences of 
these models must also be examined in 
order to understand the extent to 
which they can be reconciled with 
other features of the solar system, its 
origin, and history. 

Size distribution of the impacting 
bodies. Insofar as the crater density is 
not so high as to represent a steady 
state between crater production and 
destruction (Gault 1970), the size dis- 
tribution of craters can be used to infer 
the size distribution of the impacting 
bodies. Although the most densely cra- 
tered regions of the lunar highlands 
are in such a steady state, the more 
lightly cratered plains regions such as 
the Fra Mauro and Cayley plains are 
well below a steady-state density. 

Detailed investigations of lunar cra- 
ter size distributions (Neukum et al., 
1975) show that the pre-Imbrium, 
plains, and mare surfaces contain the 
same size distribution of craters. This 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



159 






distribution cannot be described by a 
single-power law exponent over the en- 
tire range of crater sizes. For the 
largest craters (>10 km diameter) the 
slope of the distribution is rather flat 
and approximately corresponds to a 
cumulative distribution: 

N = N D ~ a 

where a is in the range 1.4 to 1.6. This 
is in agreement with the results of pre- 
vious authors (Shoemaker et al., 1962; 
Baldwin, 1963) and represents a pro- 
jectile mass distribution in which the 
total mass is strongly concentrated in 
the largest projectiles. 

If the size range of bombarding pro- 
jectiles is continued to include the Im- 
brium planetesimal, this concentration 
of mass in the largest bodies is at least 
equally pronounced. The projectile 
mass required to produce the large 
mass craters can be estimated by com- 
bining mare crater counts (Shoemaker 
et al., 1962) with a reasonable energy- 
scaling law based on nuclear explo- 
sions and which relates diameter to 
impacting mass: 

D = 0.115 M m 

where D is the crater diameter in me- 
ters, M is the impacting mass in grams, 
and an impact velocity of 15 km/s is 
used. This calculation gives 3 x 10 
as the total flux since the filling of the 
mare basins. The integral flux on the 
Fra Mauro and Cayley plains forma- 
tions is about three times as great, cor- 
responding to a mass flux of ~ 10 20 g. A 
similar calculation for the mass of the 
Imbrium planetesimal gives a mass of 
~10 21 g. In the same way, the mass of 
the Orientale projectile comes out to be 
—2 x 10 2<) g. Therefore, up to and in- 
cluding projectiles of Imbrium size 
(—50 km in radius), the large pro- 
jectiles are characterized by the fact 
that the largest body has a mass 
greater than that of the remaining ob- 
jects. 

This is not the kind of size distribu- 
tion expected for objects that are in a 
steady state resulting from mutual col- 
lisional comminution. It more closely 



resembles the end product of an accre- 
tional process in which larger bodies 
have grown at the expense of smaller 
ones. In the treatment of specific mod- 
els for the sources of the late heavy 
bombardment, one relevant considera- 
tion should be whether or not a colli- 
sional mass spectrum should be ex- 
pected under the conditions assumed 
for the model. 

Insofar as there is some propor- 
tionality between the quantity of im- 
pact breccias and the extent of impact 
resetting of measured ages, a mass dis- 
tribution dominated by a few large im- 
pacts will produce an episodic age dis- 
tribution. Therefore the ~ 30-fold de- 
crease in mass flux between the time of 
the Imbrium impact and the older 
mare surfaces need not be entirely ex- 
plained in terms of a uniformly decay- 
ing mass flux but will be in a major 
way a consequence of a statistical fluc- 
tuation in this flux. Therefore, to a cer- 
tain extent episodicity will result sim- 
ply from the "lumpiness" of the size 
distribution. This effect in itself is in- 
sufficient to produce simultaneous 
episodes on several planets. However, 
approximate simultaneity of the cessa- 
tion of the late heavy bombardment 
could result from a flux decreasing 
with a characteristic half-life of —50 



g m.y. 



Relative bombardment rates on the 
moon and terrestrial planets. The 
hypothesis that the density time scale 
of the lunar craters can be extended to 
the other terrestrial planets requires 
that there have existed one or more 
populations of bodies in the inner solar 
system which had a nearly equal prob- 
ability of impacting a given area on 
any of the planets. Candidate popula- 
tions will be characterized by their or- 
bital element and mass distributions 
and by their total mass. 

The two principal classes of orbits 
which should be considered are those of 
bodies in heliocentric orbit and those 
orbiting as satellites of a terrestrial 
planet. Apart from the possibility that 
a constant crater density may arise by 
coincidence, the latter class must 



160 



CARNEGIE INSTITUTION 



evolve into heliocentric orbits in order 
to impact planets other than the one 
they are initially orbiting. No detailed 
discussion of this evolution from 
planetocentric to heliocentric orbits 
exists, although treatments of tidal 
evolution of satellites (reviewed by 
Kaula, 1971) and geocentric swarms 
(Ruskol, 1960, 1963, 1972; Ruskol et 
al., 1975; Kaula and Harris, 1975) are 
relevant to this problem. This class of 
initial orbits will not be discussed here. 
However, this possibility should not be 
forgotten. 

The evolution of various classes of 
heliocentric interplanetary orbits has 
been discussed previously (Wetherill 
1974). This treatment included a dis- 
cussion of the modifications made in 
the Monte Carlo techniques which 
permitted an adequate statistical sam- 
ple of impacts on small bodies such as 
the moon and Mercury. The results of 
these calculations are given in Table 3. 

The first two entries in this table 
represent bodies evolving from initial 
orbits similar to those required to ex- 
plain the orbital distribution of the 
Prairie Network fireballs, and the ob- 
served time of fall and radiants of 
chondritic meteorites (Wetherill 1968, 
1971). Such orbits can evolve from 
those of short-period comets. The sec- 
ond entry in Table 3 is such an object, 
Comet Temple 2. These data are nor- 



malized so that the lunar impact prob- 
ability per unit area is assigned the 
value of unity. The impact probability 
per unit area on the four terrestrial 
planets is seen to agree with the lunar 
value within a factor of about 3. This 
result is largely independent of the 
large difference in the efficiency with 
which these bodies impact the planets, 
shown in the third column. Objects ini- 
tially in orbits similar to those of the 
first entry will very frequently strike 
an inner planet; 24% of them will hit 
the earth. In contrast, those in orbits 
similar to that of the second entry will 
more often be perturbed into Jupiter- 
crossing and ejected by that planet into 
interstellar orbits. Only a fraction of a 
percent of these bodies will impact a 
terrestrial planet. 

The next three entries represent ini- 
tial orbits that penetrate more deeply 
into the inner solar system, penetrat- 
ing the perihelia of Venus or Mercury. 
The orbits calculated are those of ac- 
tual bodies in the present solar system, 
but this is done only for illustrative 
purposes and not to suggest that these 
modern objects have any relation to the 
late heavy bombardment. The objects 
with the smaller perihelia will cause a 
greater crater density on Mercury than 
on the moon and Mars. Strictly speak- 
ing, these initial orbits are not consis- 
tent with a uniform flux history. How- 



TABLE 3. Relative Impact Probability/Unit Area* 





Initial Orbit 


Earth 
















Impacts 




















Aphelion 


Perihelion 


(%) 


Mercury 


Venus 


Earth 


Mars 


1 


1.01 


4.00 


24.0 


0.30 


0.94 


2.26 


1.15 


2 


1.36 


4.67 


0.11 


0.69 


1.26 


1.78 


0.71 


3 


0.34 


4.18 


6.0 


4.21 


2.37 


1.23 


0.37 


4 


0.19 


1.97 


18.5 


7.50 


1.97 


1.08 


0.45 


5 


0.70 


3.61 


5.2 


1.60 


2.34 


1.34 


0.27 


6 


1.27 


1.89 


27.8 


1.08 


2.58 


2.82 


11.9 


7 


0.98 


0.99 


45.0 


1.05 


3.02 


3.99 


0.96 


8 


0.67 


0.82 


29.6 


1.08 


2.44 


1.36 


0.18 


9 


0.33 


0.41 


<0.01 


>1.5 x 10 4 









* Normalized to Moon = 1.0. 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



161 



ever, an admixture of such orbits with 
others with perihelia nearer to that of 
the earth (the first two entries) would 
lead to an approximately uniform flux 
history as an average result. 

The last four entries represent initial 
orbits of sufficiently low eccentricity to 
cross the orbit of only one planet. It 
might be thought that this would 
greatly favor impacts on that planet. 
To some extent this is true. The Mars- 
crossers are about ten times more 
likely to strike Mars (per unit area) 
than the moon. Mercury-crossers only 
very rarely evolve into even Venus- 
crossing before they strike Mercury. 
The results for these two planets are 
largely a consequence of these small 
planets having insufficient mass to 
readily "pump" the eccentricity to high 
enough values to cross another planet- 
ary orbit. In contrast, the larger ter- 
restrial planets, Venus and the earth, 
perturb bodies into highly eccentric or- 
bits that have a good chance of striking 
the moon or any of the terrestrial 
planets. 

The general result of these calcula- 
tions is that planetary perturbations 
tend to spread interplanetary bodies 
throughout the inner solar system, re- 
sulting in a comparable flux on the 
moon and terrestrial planets. There are 
exceptions to this; those which initially 
cross only Mercury are a particularly 
striking exception. Nevertheless, it 
will usually be difficult to confine an 
interplanetary population primarily to 
a particular planet, and many distribu- 
tions of initial orbits will lead to a simi- 
lar flux on the moon and Mercury. 

To compare crater densities on the 
various planets it is necessary to con- 
sider the impact velocity as well. These 
have been calculated by the same tech- 
niques used to obtain the data of Table 
3. Examples of these calculated veloc- 
ity distributions are given in Figs. 25 
and 26. Those orbits with initial 
perihelia well beyond Mercury will im- 
pact Mercury with about two to three 
times the velocity they impact the 
moon. When this result is combined 
with the size distribution discussed in 



Initial Orbit: (Mars Grosser) 
Aphelion = 1.90 A.U. 
Perihelion =1.27 A.U. 



" 




— 










Inclination = 6° 


- 










Mercury 
















\—V— r— ■ , , 



Moon 



Lti 



10 20 30 40 
RELATIVE VELOCITY 



50 



60 ?0 
KM/SEC 



Fig. 25. . Distribution of impact velocities on 
the moon and Mercury for a swarm of bodies ini- 
tially in Mars-crossing orbit. The velocity is 
given as V a , i.e., prior to the additional accelera- 
tion resulting from the gravitational field of the 
planet. For small planets like these this effect is 
small. 

the previous section, it turns out that 
the effect of velocity is to increase the 
crater density by a factor of about 2.5. 
For objects such as those given in the 
first two entries of Table 2, this effect 
offsets the factor of 2 to 3 lower impact 
probability on Mercury relative to the 
moon, which arises from the rather in- 
effectual "Earth and Venus barrier" 
these bodies must surmount. This ve- 
locity effect is less pronounced for or- 
bits that are initially Mercury- 
crossing, as shown in Fig. 26. These 
bodies produce a greater flux on Mer- 
cury than on the moon. The effect of 
velocity is thus seen to partially offset 
the effect of the initial perihelia, tend- 
ing to result in more uniform crater 
densities. 

Storage places. Much of the earlier 
discussion of this problem has centered 
on the question of whether there are 
places in the solar system in which 
large bodies can be "stored" for hun- 
dreds of millions of years prior to their 
impacting the moon or planets. Typical 
lifetimes, given by Monte Carlo calcu- 
lations, characterizing the stability of 
various types of initial orbits, are given 
in Table 4. It may be seen that two re- 
gions of the solar system will store 



162 



CARNEGIE INSTITUTION 



150 



50 



Initial Orbit: (Comet Encke) 

Aphelion =4.18 A.U. 
Perihelion = 0.34 A.U. 
Inclination =D° 



Mercury 



rr 

£ 50 



Moon 



10 20 30 40 50 60 70 80 90 
RELATIVE VELOCITY (V^) KM/SEC 

Fig. 26. Distribution of impact velocities for bodies initially in orbits equal to that of periodic 
Comet Encke. 



bodies for the required length of time. 
They are the inner Mars-crossing edge 
of the asteroid belt and the outer por- 
tion of the solar system in the vicinity 
of Uranus and Neptune. 

Breakup within the Roche limit. 
There are no known storage places 
which lead to a simultaneous sharp 
peak in the flux on the moon and ter- 
restrial planets. Insofar as the photo- 
geological and radiometric age data do 
indicate a peak flux or "cataclysm" at 
— 3.9 b.y. and possibly at other discrete 
earlier times, some other explanation 
must be found. Several authors have 
proposed the breakup of a large body in 
the asteroid belt at that time. This is 
possible but improbable. 

There is another, more probable way 
in which ~ 200-km bodies in earth- 



crossing orbits may be fragmented. As 
discussed by Opik (1951), secular per- 
turbation of the argument of perihelion 
will cause planet-crossing orbits to 
evolve on a time scale of ~10 4 years 
into orbits that intersect those of the 
crossed planets. Collisions with the 
planets can occur during the time 
interval for which this intersecting 
condition exists. Quantitative evalua- 
tion of the problem leads to the well- 
known collision formula of Opik that 
forms the basis for subsequent Monte 
Carlo calculations of orbital evolution. 
The same formula can be used to calcu- 
late the probability of close encounter 
within a given distance of a planet 
simply by use of the encounter radius 
rather than the planetary radius. The 
probability of encounter within a dis- 



TABLE 4. Typical Lifetimes of Planet Crossing Orbits 



Type of Orbit 



Approximate Half-Life 

(10" yr) 



Earth crosser, aphelion —3 A.U. 
Earth crosser, aphelion —4.50 A.U. 
Mars crosser, perihelion —1.2 A.U. 
Mars crosser, perihelion — 1.5 A.U. 
Jupiter crosser 
Saturn crosser 
Uranus crosser 
Neptune crosser 



40 

10 

200 

1200 

1 

2 

100 

200 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



163 



tance of R e planetary radii is simply 
R 2 e times the probability of impact. A 
consequence of this is that close 
planetary encounters do not in any 
sense constitute an improbable or ad- 
hoc assumption but are a natural con- 
sequence of the same dynamical pro- 
cesses that have produced the observed 
craters on the moon and planets. 

When a small body passes suffi- 
ciently close to a planet, differential 
gravitational attraction by the planet 
will generate a stress field within the 
body which, if strong enough, can lead 
to its disruption. Roche (1847) consid- 
ered this problem for the case of a self- 
gravitating liquid satellite and ob- 
tained the result: 



R. 



2.44 ipjp) 



1/3 



When the encounter distances (in 
planetary radii) are less than R e , dis- 
ruption will occur; p and p are the den- 
sities of the planet and the small body, 
respectively. This expression must be 
modified in the case of a solid body with 
nonzero rigidity. The latter problem 
has been treated by Jeffreys (1947), 
Opik (1950, 1966), and Sekiguchi 
(1970). This earlier work has been re- 
viewed and extended by Aggarwal and 
Oberbeck (1974). The latter authors 
show that a 200-km radius body with a 
reasonable tensile strength of 10 7 
dynes/cm 2 will undergo tensile failure 
along a v complete plane passing 
through the center of the body within 
an encounter distance of 1.18 (p /p) 1/3 
planetary radii. For a "rocky" body 
with p = 3 encountering the earth (p () 
= 5.5) this distance is 1.44 planetary 
radii, whereas for a predominantly 
"icy" body with p = 1, the correspond- 
ing encounter distance will be 2.08 
planetary radii. Somewhat smaller dis- 
tances will be found for Venus encoun- 
ters. Because of these assumptions, 
very close approaches are required for 
disruption. Under these circumstances 
the number of disruptive encounters 
will be similar to the number of Earth 
or Venus impacts. Because of the 
greater surface area of Earth or Venus, 
impacts with these planets are much 



more frequent than lunar or Mercury 
impacts (Table 3). Consequently a 
large body is more likely to undergo a 
disruptive encounter with Earth and 
Venus and to distribute fragments to 
the moon and Mercury than it is to im- 
pact these smaller planetary bodies di- 
rectly. For the observed mass distribu- 
tion law (equation 8), and disruption 
into —100 fragments of more or less 
equal mass, lunar impacts of ~10 21 g 
fragments of a 10 2:! g body will out- 
number direct impacts of 10 21 g bodies 
by a factor of about 5. Therefore, even 
in this extreme case, disruption can be 
expected to produce episodes of heavy 
bombardment distributed throughout 
the inner solar system. 

The discussions given lead to the 
conclusions that a modest total mass of 
~10 2:i g will suffice to produce the late 
heavy bombardment, that storage 
places with sufficiently long lifetimes 
exist, and that there is at least one 
mechanism for producing simultane- 
ous episodes of bombardment in the 
inner solar system. 

In work presented elsewhere a dis- 
cussion of alternative specific models 
for the late heavy bombardment is pre- 
sented (Wetherill 1975b). The most 
satisfactory of these are derivation of 
the impacting bodies from residual 
planetesimals in the vicinity of Uranus 
and Neptune, possibly the proto- 
cometary cloud, and derivation from 
residual Mars-crossing bodies. Both of 
these models are compatible with 
probable concentrations and size dis- 
tributions of mass in these regions of 
the solar system 4 billion years ago. In 
both cases Roche limit disruption pro- 
vides a mechanism for producing a 
simultaneous late heavy bombardment 
similar to that suggested by the crater- 
ing record. 

The Mars-crossing source has the 
property of requiring a different crater- 
ing time scale for Mars than for the 
moon and Mercury. A difficulty with 
the Mars-crossing model is the re- 
quirement of a rather special initial 
distribution of Mars-crossing orbits to 
obtain the sharp cut-off in lunar bom- 



164 



CARNEGIE INSTITUTION 



bardment about 3.9 billion years ago. 
Further detailed treatment of the 
Uranus-Neptune source requires de- 
velopment of new theoretical tech- 
niques for proper handling of dynami- 
cal problems in the outer solar system. 

Concluding remarks. The central 
question of whether it is dynamically 
permissible to extend to other planets 
the late heavy bombardment observed 
and dated on the moon can be an- 
swered. The answer is yes: Bodies in a 
wide class of orbits will have very simi- 
lar probabilities for impacting the 
moon and terrestrial planets; there are 
at least two "storage places" that are 
sufficiently long-lived; the mass re- 
quirements are not excessive; and 
there is a dynamically plausible mech- 
anism for producing simultaneous 
episodes of heavy bombardment on all 
the terrestrial planets. 

Attention can then be shifted from 
the question of whether this hypothesis 
of simultaneous late heavy bombard- 
ment is permissible to the much more 
difficult question of whether or not it 
actually occurred. 

Positive evidence for the hypothesis 
may some day come from radiometric 
dating of impact-related rocks from 
other terrestrial planets. At present 
the only bodies in the solar system 
other than the earth and the moon 
which can be dated in this way are the 
parent bodies of the meteorites. As re- 
viewed elsewhere (Wetherill, 1974), 
there is considerable uncertainty con- 
cerning the location of these parent 
bodies in the solar system. It seems 
likely that at least most differentiated 
meteorites-achondrites, irons, and 
stony-irons are of asteroidal origin, 
even though skepticism on this matter 
would prove difficult to refute. The 
projectiles of the late heavy bombard- 
ment should have spent much of their 
time in the asteroid belt and thereby 
increased the impact rates on the par- 
ent bodies of these differentiated 
meteorites, and this should be reflected 
in the meteorite age measurements. 

Even if it occurred, this increased 
impact rate in the asteroid belt may 



not have been very pronounced because 
of the high background collision rate in 
this region of the solar system. For 
example, there are at present about 30 
asteroids equal to or larger than the 
Imbrium planetesimal (R = 50 km). If 
the late heavy bombardment involved 
an additional 100 projectiles of this 
size, the number of such bodies would 
be increased by only a factor of 4. The 
duration of the bombardment would be 
~ 30-100 m.y., and the integral flux on 
the asteroids would represent primar- 
ily impacts unrelated to the late heavy 
bombardment. It may also prove dif- 
ficult to distinguish between impact 
metamorphism and melting, and simi- 
lar rocks resulting from internal heat- 
ing of the meteorite parent bodies. At 
present there is some evidence for dif- 
ferentiated meteorite ages in the vicin- 
ity of 3.8 b.y. (e.g., Burnett and Was- 
serburg, 1967; Papanastassiou et al., 
1974). However, it is premature to say 
whether these data argue for or against 
a simultaneous late heavy bombard- 
ment in the inner solar system. 

The Earth-Crossing Apollo Objects 

The study of meteorites is central to 
geochemical research. The remarkable 
similarity in the concentration of the 
abundant nonvolatile elements (e.g., 
Mg, Si, Fe, Na, Al, Ca) in carbonaceous 
and chondritic meteorites and the sun 
shows that the chondrites represent 
solar system material that has under- 
gone only limited chemical processing. 
Therefore, it is likely that chondritic 
ratios such as K/Rb, Fe/Ni, and Mg/Fe 
represent an approximation to funda- 
mental solar system values for these 
quantities rather than being more or 
less random numbers associated with 
special types of rocks. Also, the parti- 
tion of trace elements between silicate, 
metal, and sulfide phases in meteorites 
is a principal basis for the geochemical 
classification of the elements as 
lithophile, chalcophile, and sidero- 
phile. The primitive nature of meteor- 
ites is indicated by their —10* yr I-Xe 
formation intervals, and therefore the 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



165 



age of the solar system can be inferred 
from the —4.6 x 10 9 yr age of most 
meteorites. Consequently, it is quite 
likely that isotopic inhomogeneities 
such as those found in neon (Black and 
Pepin, 1969), oxygen (Clayton and 
Mayeda, 1975), and magnesium (Lee 
and Papanastassiou, 1974) can be at- 
tributed to nebular or even presolar 
system phenomena. 

For these and other reasons, our un- 
derstanding of solar system history de- 
pends heavily on meteoritic data. In- 
valuable as these data have been, their 
full importance cannot be realized 
until we identify the actual bodies in 
the solar system of which the meteor- 
ites are fragments. Our understanding 
of this problem has advanced consid- 
erably during the past decade. To a 
large extent these advances have made 
more clear the intricacies of the prob- 
lem and have shown that simple solu- 
tions, whether or not they are "gener- 
ally accepted," may be incorrect or at 
least inconclusive. The present status 
of the general problem has been re- 
viewed (Wetherill 1974). It is con- 
cluded therein that the present sources 
of meteorites are probably multiple: It 
is plausible that both comets and as- 
teroidal bodies of several kinds con- 
tribute to the flux of meteorites on the 
earth. 

During the past year attention has 
been focused on a particular class of po- 
tential meteorite source, the kilome- 
ter-sized interplanetary bodies known 
as Apollo objects. These are earth's 
nearest neighbors in space; one will 
pass within the orbit of the moon about 
once in 50 years. The name of the first 
of these bodies to be discovered, 1862 
Apollo, has been used to give the name 
"Apollo objects" to the entire class. The 
common association of the term "as- 
teroid" with this class may be mislead- 
ing. Rather than being related to the 
true asteroids, primarily confined to 
orbits between Mars and Jupiter, these 
Apollo "asteroids" may actually be the 
exposed nonvolatile cores of comets, 
remaining after their icy mantles have 



been evaporated by solar radiation 
(Opik, 1963). 

There is no doubt that both Apollo 
objects and their fragments impact the 
earth. Their elliptical orbits extend 
into the asteroid belt, and while in this 
portion of their orbits they cannot es- 
cape collisions with the asteroidal col- 
lision debris present in that region 
(Wetherill, 1967). The larger collision 
fragments derived from the Apollo ob- 
jects will typically have velocities less 
than 100 m/s relative to the body of 
which they are fragments (Gault et al., 
1963). They will travel in orbits very 
similar to that of their Apollo object 
source, and therefore the orbits of the 
fragments will also cross the orbit of 
the earth. Precession of the argument 
of perihelion will cause orbits that 
cross earth's orbit to evolve on a time 
scale of ~10 4 years into orbits which 
intersect earth's orbit. The earth will 
be actually present at the point of in- 
tersection a calculable fraction of the 
time, and an impact with the earth will 
result. Whether or not we have meteor- 
ites in our collections which are frag- 
ments of Apollo objects depends upon 
whether the total annual number of 
fragments impacting the earth is suffi- 
ciently large to ensure their being rep- 
resented among the —2000 meteorites 
that have been found. In addition, it is 
necessary that the fragments have suf- 
ficient strength to survive passage 
through the earth's atmosphere at the 
entry velocities of typical meteorites 
(11-20 km/s). 

In contrast, fragments of most belt 
asteroids cannot impact the earth. 
Whereas the Apollo object fragments 
when produced are already in earth- 
crossing orbit, fragments of typical as- 
teroids ejected at —100 m/s following 
collisions will remain in the asteroid 
belt. Only if they are ejected at very 
high velocity ( — 5 km/s) will they be 
placed directly into earth-crossing. 
However, any reasonable partition of 
this kinetic energy into internal de- 
grees of freedom will melt or pulverize 
the fragment, and it will no longer re- 



166 



CARNEGIE INSTITUTION 



semble typical relatively unshocked 
meteorites. The absence of lunar rocks 
in our meteorite collections contributes 
empirical evidence for this conclusion. 
Special mechanisms have been pro- 
posed which may permit removal of a 
small quantity of collision fragments 
from the asteroid belt without these 
shock effects. The most promising of 
these involve proximity of the asteroid 
to an orbit that is resonant with the 
motion of Jupiter (Williams, 1973; 
Zimmerman and Wetherill, 1973). 
These also constitute promising 
meteorite sources, particularly for dif- 
ferentiated meteorites such as irons. 

In recognition of their prima facie 
candidacy as meteorite sources a quan- 
titative evaluation of the mass yield, 
lifetime, and orbital characteristics of 
meteorites derived from Apollo objects 
was carried out a few years ago 
(Wetherill and Williams, 1968). This 
work showed that if the mass yield is 
calculated from the approximately 20 
observed Apollo objects alone, it is 
much too low. Estimates were given of 
the number of unobserved similar ob- 
jects required in order for the mass 
yield to be adequate to produce a major 
part of the observed meteorite flux. Al- 
though the fairly large number re- 
quired could not be ruled out on obser- 
vational grounds, an assumption that 
there actually were so many would 
have been excessively conjectural. 

Extension of an approach first used 
by Whipple (1967, 1973) now shows 
that the total number of these bodies 
larger than about 500 meters in radius 
is about an order of magnitude higher 
than most previous estimates. This ap- 
proach makes use of the fact that no 
Apollo object has been accidentally dis- 
covered twice, even though the mode of 
discovery approximates a random 
search procedure. This would obviously 
be an improbable result if there were 
only a few more bodies than actually 
observed. More quantitatively, this 
fact has been inverted to estimate the 
most probable number of Apollo ob- 



jects. This procedure requires estab- 
lishing an upper limit on the number of 
Apollo objects; otherwise the most 
probable number would be arbitrarily 
large. Rather good upper limits can be 
set in two ways. Shoemaker et al. 
(1975) have used completeness of 
search calculations to show that it is 
unlikely there are more than 1200 of 
these bodies. Impacts of these bodies 
will produce lunar and terrestrial crat- 
ers about 10 km in diameter. The small 
number of these craters on surfaces 3 
billion years in age shows that 
Shoemaker's upper limit is if anything 
too high. Thus bounded, the inverse 
problem can be solved, with the result 
that it is 95% probable there are more 
than 200 of these bodies; a value com- 
patible with all the evidence is about 
600. This is in agreement with the re- 
cent estimate of 800 ±400 given by 
Shoemaker et al. based on complete- 
ness of search. 

Use of this higher abundance, to- 
gether with more recent results on as- 
teroid diameters and cratering me- 
chanics leads to a revision of the esti- 
mates of meteorite yield given previ- 
ously. The total quantity of earth- 
impacting Apollo fragmentation debris 
in the meteorite size range ( 10 2 - 10 ,; g) 
is now estimated to be about 2 x 10 8 
g/yr. This is in good agreement with 
estimates of the actual impact rate, as 
determined by observations of fireballs 
with the Prairie Network (McCroskey, 
1968) and lunar seismometry (Duen- 
nebier et al., 1975). Thus, provided 
these fragments have sufficient 
strength to penetrate the atmosphere, 
it may be that most meteorites are de- 
rived from these bodies. 

If so, what kind of meteorites are 
they likely to be? Good spectrophoto- 
metric data have been obtained on one 
Apollo object, 1685 Toro (Chapman et 
al., 1973). They indicate a surface com- 
position similar to the most abundant 
class of meteorites: the metamorphosed 
ordinary chondrites. Albedo and lower 
resolution spectral data suggest this is 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



167 






the case for several other Apollo ob- 
jects. 

Discrepancies exist between the ob- 
served distribution of meteorite 
radiants and time of fall and that pre- 
dicted for fragments of Apollo objects. 
Possible but by no means assured ways 
to remove these discrepancies have 
been studied and discussed (Wetherill, 
1976). 

The short (~10 7 yr) dynamical 
lifetime of Apollo objects requires a 
source to maintain the supply through- 
out solar system history. Various pos- 
sible asteroidal sources have been in- 
vestigated. Several mechanisms for 
removing kilometer-sized asteroid 
fragments from the asteroid belt are 
qualitatively acceptable. However, 
quantitatively these mechanisms fail 
by at least an order of magnitude. An 
adequate theory of asteroidal origin 
requires a mechanism by which ~5% of 
all the Apollo-sized fragments being 
produced in the asteroid belt undergo 
the necessary ~5 km/s velocity change 
to cause them to become earth- 
crossing. No such efficient mechanisms 
are known at present. An alternative is 
that most Apollo and many Amor ob- 
jects are the cores of comets that have 
lost their volatiles by solar heating 
(Opik, 1963). Observational evidence 
exists that there actually are a suffi- 
cient number of such cores, and it is 
believed that this is a likely possibility. 
The principal uncertainty in relating 
chondritic meteorites to comets is the 
possibility that the Apollo material 
does not have the mechanical strength 
characteristic of ordinary chondrites. 

If the foregoing chain of reasoning is 
followed, then ordinary chondritic 
meteorites are fragments of comets. In 
spite of the fact that this conclusion 
does not fit well with our present 
thinking regarding the chemical his- 
tory of the early solar system, these in- 
vestigations indicate that there is a 
problem to which this conclusion may 
be a solution. 



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170 



CARNEGIE INSTITUTION 



Particle Tracks and 

Micro-Impact Features in 

Grains from Gas-Rich 

Meteorites 

G. Poupeau* and R. S. Rajan 

Introduction 

Gas-rich meteorites are extraterres- 
trial breccias made of single crystals 
and lithic fragments once exposed to 
free space before compaction, as evi- 
denced by the presence in individual 
grains of tracks from solar flare, iron 
group nuclei (Lai and Rajan, 1969; Pel- 
las et al., 1969) and solar wind 4 He 
(Poupeau et al., 1974). From the aniso- 
tropy of solar flare irradiation effects in 
single crystals, it was inferred that the 
irradiation occurred in a regolith on 
the surface of a parent body (Poupeau 
and Berdot, 1972; Rajan, 1974). From 
analogy with the lunar soil samples, 
we should expect that impact features 
such as microcraters and glassy 
splashes will also be found on crystal 
surfaces of gas-rich meteorites. This 
expectation was indeed confirmed 
(Brownlee and Rajan, 1973) when such 
features were found on solar flare ir- 
radiated glass balls of the gas-rich 
howardite Kapoeta. Similar observa- 
tions have recently been reported for 
carbonaceous chondrites (Goswami et 
al., 1976). Radiometric studies of indi- 
vidual clasts from a few gas-rich 
meteorites have shown that these brec- 
cias could have been compacted as 
early as —3.5 AE ago in the case of 
Kapoeta (Papanastassiou et al., 1974; 
Rajan et al. 1976), -3.7 AE for Malvern 
(Rajan et al, 1975) to -4.3 AE for 
Bununu (Rajan et al., 1975). Fission 
track evidence also suggests that the 
carbonaceous chondrite Murchison was 
probably compacted —4.2 AE ago 
(Kothari and MacDougall, 1975). 

So far, extensive studies of the solar 
flare tracks/microcraters record in 
single grains of extraterrestrial re- 

* McDonnell Center for the Space Sciences of 
Washington University, St. Louis, Missouri 
63130. On leave from Centre National de la Re- 
cherche Scientifique, Paris, France. 



golith have been limited to the lunar 
soils (Poupeau et al., 1975; Poupeau 
and Johnson, 1976) and the carbonace- 
ous chondrite Murchison (Goswami et 
al., 1976). We are extending these 
studies to a number of gas-rich meteor- 
ites, with the main purposes of ( 1) de- 
fining the micrometeorite mass distri- 
bution over different geological times 
as inferred from microcrater meas- 
urements, and (2) studying the tem- 
poral variations of the micrometeorite 
flux relative to the solar flare flux and 
ultimately, in collaboration with E. 
Zinner, to the solar wind flux. The 
solar wind record can be studied in 
single grains with the help of an ion 
probe (Zinner et al., 1976). Finally, the 
combined study of glassy splashes as 
well as microcraters and tracks on 
single meteoritic grains should give 
some insight into the regolith 
dynamics of their parent body. 

In this report we present results of a 
search for microcraters and other im- 
pact features (glassy discs/splashes) as 
well as the measurement of the 
cosmic-ray track records in the gas-rich 
meteorite Kapoeta. 

Sampling and Experimental 
Procedures 

Gas-rich meteorites are known in 
several groups of stony meteorites: or- 
dinary and carbonaceous chondrites as 
well as achondrites such as howardites 
and aubrites. We have selected for our 
work, on the basis of their high content 
of solar wind gases and abundance of 
track-rich grains, one meteorite from 
some of these groups: the howardite 
Kapoeta and the aubrite Khor Temiki. 
For comparison, we have also included 
the carbonaceous chondrite Murchison. 
The samples of Kapoeta and Murchison 
were single fragments and weighed 
— 0.5 and —0.8 grams, respectively. 
Our sample of Khor Temiki is an 
aliquot of a 92-150 ,um sieved powder 
previously used by Eberhardt et al. 
(1965) for rare gas analysis. 

In order to study the impact record of 
micrometeorites on single grains, we 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



171 



have disaggregated the fragments of 
Kapoeta and Murchison into their orig- 
inal grains. It is very important that 
during this process the "external" sur- 
faces of the crystals and chondrules are 
preserved. Although gentle crushing 
may partly preserve the external sur- 
face of crystals, we have increased the 
recovery of original surfaces by ther- 
mally induced disaggregation. 

For this purpose at DTM we have de- 
signed and built an automated device 
for gentle disaggregation of the 
meteorite samples. The sample is first 
evacuated to ~1 fx and filled with 
deionized water. The tube containing 
the sample is then alternately cycled 
between liquid nitrogen and water at 
45°C. A schematic diagram of the de- 
vice is shown in Fig. 27. The method is 
very effective in recovering grains with 
their original surfaces intact for grains 
greater than 50 /x, as deduced from 
scanning electron microscope (SEM) 
observations. 

The Khor Temiki powder was pre- 
pared by Eberhardt et al. (1965) from 
gentle crushing of a fragment of this 
meteorite. The low degree of compac- 
tion of this meteorite allows an easy 
separation of its constituent crystals. It 
has been shown from the presence of 



both solar wind 4 He and solar flare 
VH-ion tracks in single crystals that 
original surfaces have in fact survived 
the crushing process at least partially 
(Poupeau^ a/., 1974). 

Impact features on the surface of ex- 
traterrestrial grains consist of micro- 
craters and a variety of glassy splashes 
(cf. Blanford et al., 1974). The presence 
of microcraters, in either meteorites or 
lunar material, seems to be limited to 
grains irradiated by the solar flares 
(Brownlee and Rajan, 1973; Goswami 
et al., 1976; Poupeau and Johnson, 
1976). On the contrary, it has been 
shown that in lunar soils, glassy 
splashes were present on almost all 
crystals (Poupeau and Johnson, 1976). 
We therefore applied different proce- 
dures for the search of microcraters 
and glassy splashes on meteoritic crys- 
tals. For glassy splashes, a series of 
crystals chosen randomly among those 
exhibiting euhedral to nearly euhedral 
shapes were observed in an SEM. For 
microcraters, as the abundance of solar 
flare irradiated grains is usually 
rather low, a selection had to be made 
on the basis of nuclear track data. 
Therefore, series of crystals were 
mounted in epoxy, polished, and 
etched. Only crystals having solar flare 



TIMER 
0- 300 SEC 



LEFT LIMIT 
+ REV. SWITCH 



LIQUID N SUPPLY 




1 10 VAC 



1 10 VAC 



REVERSIBLE 
MOTOR 



DEIONIZED WATER 




RIGHT LIMIT 
+ REV. SWITCH 



IMMERSION HEATER 



: -H« 



45°C WATER 



Fig. 27. An automated device for gentle disaggregation of meteorites (and lunar samples). 



172 



CARNEGIE INSTITUTION 



tracks were extracted from the epoxy 
and their remaining external surface 
studied for microcraters in an SEM. 

Results and Discussions 

We have studied for impact features 

— 70 crystals and glassy objects from 
the three meteorites Kapoeta, Khor 
Temiki, and Murchison. The search for 
microcraters and glassy splashes was 
carried out under a standard 5000 x 
magnification scan (Poupeau et al., 
1975) in order to make a meaningful 
comparison with the results from the 
lunar samples. The results are reported 
in Table 5, where they are compared 
with other available lunar and 
meteoritic data. All our measurements 
(including the data on lunar soils) have 
been obtained on crystals in the 

— 100-150 /xm range. 

Of the 45 crystals studied from 
Kapoeta, Khor Temiki, and Murchison, 
none had any detectable microcraters. 
Brownlee and Rajan (1973) had found 
microcraters in 2 out of the 9 glassy 
spherules from Kapoeta. We scanned 
16 more spherules, but no additional 
microcraters (^0.2 /xm) were detected. 



Lunar and meteoritic microcraters 
present the same general morphology 
illustrated in Fig. 28. Figure 28(a and 
b) shows cratered glass balls from the 
lunar breccias 15086 and Kapoeta, re- 
spectively. The largest microcraters 
found in the meteoritic spherule (Fig. 
28b) have diameters of 9 and 6 fxra, re- 
spectively Brownlee and Rajan, 1973). 
One of the above craters is shown in 
Fig. 28(d), and a crater found in a lunar 
glass sphere from the regolith breccia 
15086 is shown for comparison in Fig. 
28(c). Both craters on Fig. 28(c and d) 
present characteristic spall zones sur- 
rounding the central pits. Submicron 
craters in both lunar and meteoritic 
grains are typically characterized by 
raised rims around the pits (Fig. 28, e 
and f). By analogy to the lunar soil 
studies, we have classified the thin 
discs of 5£2 jam diameter (the "pan- 
cakes" of Blanforde^ al., 1974) from the 
thicker glassy splashes on the surface 
of crystals (Fig. 28f). No glassy discs 
have yet been found on any meteoritic 
crystal, but some are possibly present 
on the surface of glassy spherules. 

Thicker glassy splashes are defi- 
nitely present on glassy spherules in 



TABLE 5. Cosmic-Ray Tracks, Microcraters, and Glassy Discs on Crystals from 
Lunar Soils and Gas-Rich Meteorites 









Fraction of Crystals with: 






Track 






Glassy 






Mineral 


Densities 




Microcraters 


Discs 




Sample 


Phase 


3= 10*/cm 2 


(N)t 


ssO.Lum 


s*2/um dia. 


(AD* 


Lunar Soils'" 














12033 


Ft 


0.03 


(156) 





0.07 


(14) 


14141 


F 


0.35 


( 60) 


0.14 


0.74 


(19) 


Mature Soils 


F 


2*0.8 


O200) 


0.35-0.67 


&0.90 


O200) 


Meteorites 














Kapoeta 


F.Pyt 


0.13 


( 65) 








(15) 




"glass" balls 


0.20 


( 15) 


0.08 


-0.04 


(25) 


Khor Temiki 


Py 


0.12 ,2) 


(1203) 








(15) 


Murchison 


Olt 












(15) 




01 


0.01-0.05 I: " 


(3000) 


-0.001 


N.D. 


(20) 



*(/V) = total number of observed crystals. 

tF = Feldspar; Py = pyroxene; 01 = olivine; N.D. = not determined. 

(1) Poupeau and Johnson, 1976. 

(2) Poupeau et al., 1974. 

(3) Goswami et al., 1976. 



DEPARTMENT OF TERRESTRIAL MAGNETISM 

Lunar Meteoritic 



173 




21L 



Fig. 28. Microcraters and glassy splashes on meteoritic and lunar glassy spherules and crystals, 
as observed under a scanning electron microscope (SEM). (a),(c) Glassy spherule from the Apollo 15 
regolith breccia 15086. (b),(d) Glassy spherules from Kapoeta meteorite, (e) A feldspar crystal from 
the lunar soils. Flat glassy discs ("pancakes") are indicated by arrows, (f) A glassy splash on the 
surface of a spherule from Kapoeta. A submicron crater is indicated by the arrow. 






Fig. 28(e). On crystal surfaces, we have 
not yet found any features that can be 
unambiguously attributed to glassy 
splashes. This may be in part due to the 
large background of adhering grains on 
the crystal surface, many of which are 
in fact embedded. 

Our track density (= P) meas- 
urements on 40 feldspar grains from a 
single centimeter-sized fragment give 



well-defined peak in the range of 1.5 ± 
0.8) x 10'Vcm 2 , containing 29 crystals. 
The above peak is well understood as 
due to the stopping iron nuclei from the 
galactic cosmic rays during the 
cosmic-ray exposure age of the meteor- 
ite. An example of such a crystal is 
shown in Fig. 29(c). Six crystals had p 
values in the range of 2.4-4.0 x 10 ( V 
cm 2 , which is possibly due to a some- 



174 



CARNEGIE INSTITUTION 



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j i L 



J UJ- 



20 60 100 \ 200 4QQ 600 / 700 740 780 

Notice change in scale 

DEPTH IN MICRONS 

(d) 

Fig. 29. Cosmic-ray tracks in Kapoeta feldspars. (a),(b) "Track-rich" crystals with track density 
gradients from the edges, as viewed under an optical (a) or in a transmission electron microscope (b), 
using a C-Pt replica technique (Rajan, 1974). The scale bars are 10 /j. and 2 /x and track density values 
at the edges are -2 x 10>7cm 2 and ~2 x 10 ! 7cm 2 . (c) Crystal with a low homogeneous track density, 
accumulated from galactic cosmic rays during the cosmic-ray exposure age of the meteorite. The track 
density is ~2 X 10'Vcm 2 . 



what closer location to the preatmo- 
spheric surface. The remaining five 
crystals have significantly higher 



track densities, indicative of a precom- 
paction irradiation history. While one 
crystal had p = 1.5 x 10 7 /cm 2 , the re- 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



175 






maining four crystals were "track- 
rich", i.e., they had track density at the 
edges of 2= 10 8 /cm 2 and had character- 
istic steep gradients toward the center 
of the crystal, with p values at the cen- 
ter of the crystal being ~2 x 10 7 /cm 2 . 
An example of a track-rich grain is 
shown in Fig. 29 (a and b), and the 
measured track-density profile across 
the crystal is shown in Fig. 29(d), indi- 
cating the dramatic gradient. 

Finally, we removed the track-rich 
crystals from the epoxy mounts and ob- 
served their remaining external sur- 
faces under an SEM. No microcraters 
larger than 0.15 fim (our detection lim- 
it) were found in any of these four crys- 
tals. 

It is known from lunar soil studies 
that for a grain to be cratered, being 
track-rich is a necessary but not suffi- 
cient condition (Poupeau and Johnson, 
1976). The probability of a track-rich 
grain being cratered increases with the 
maturity of the soil, being essentially 
zero for immature soils, evolving in a 
complex way, and approaching unity 
for very mature soils. Interestingly, 
from the present work we find that the 
meteorites lie on the extrapolation of 
the same trend line exhibited by lunar 
soils. Such a correlation would be sig- 
nificant when confirmed with more ob- 
servations on meteorites and would 
suggest a common dynamical mecha- 
nism operative on all the regoliths. 
However, we do not yet understand the 
scarcity of glassy splashes on meteori- 
tic grains, since they are quite abun- 
dant even on rather immature lunar 
soils (see Table 5). 

References 

Blanford, G. E., R. M. Fruland, D. S. 
McKay, and D. A. Morrison, Lunar sur- 
face phenomena: solar flare track gra- 
dients, microcraters and accretionary 
particles, Proc. 5th Lunar Sci. Conf., 3, 
2501, 1974. 

Brownlee, D. E., and R. S. Rajan, Discovery 
of micrometeorite craters on chondrule- 
like objects from Kapoeta, Science, 182, 
1341, 1973. 

Eberhardt, P., J. Geiss, and N. Grogler, 
Ueber die Verteilung der Uredelgase im 



meteoriten Khor Temiki, Tschermaks 
Mineral. Petrogr. Mitt., 10, 535, 1965. 

Goswami, J. N., I. D. Hutcheon, and J. D. 
MacDougall, Microcraters and solar 
flare tracks in crystals from carbonace- 
ous chondrites and lunar breccias, Proc. 
7th Lunar Sci. Conf., in press, 1976. 

Kothari, B. K., and J. D. MacDougall, 
Meteoritics, 10, 428, 1975. 

Lai, D., and R. S. Rajan, Observations on 
space irradiation of individual crystals 
of gas-rich meteorites, Nature, 223, 269, 
1969. 

Papanastassiou, D.A., R. S. Rajan, J. C. 
Humeke, and G. J. Wasserburg, Rb-Sr 
ages and lunar analogs in a basaltic 
achondrite: implications for early solar 
system chronologies, Lunar Science V 
(C. Watkins, ed.) Lunar Science Insti- 
tute, Houston, 583, 1974. 

Pellas, P., G. Poupeau, J. C. Lorin, H. 
Reeves, and J. Audouze, Primitive low- 
energy particle irradiation of meteorite 
crystals, Nature, 223, 272, 1969. 

Poupeau, G., and J. L. Berdot, Irradiation 
ancienne et recente des aub rites, Earth 
Planet. Sci. Lett., 14, 381, 1972. 

Poupeau, G., and J. Johnson, Micrometeor- 
ite and solar flare maturation of lunar 
soils, Earth Planet. Sci. Lett., in press, 
1976. 

Poupeau, G., T. Kirsten, F. Steinbrunn, and 
D. Storzer, The records of solar wind and 
solar flare in aubrites, Earth Planet. Sci. 
Lett., 24, 229, 1974. 

Poupeau, G., R. M. Walker, E. Zinner, and 
D. A. Morrison, Surface exposure history 
of individual crystals in the lunar re- 
golith, Proc. 6th Lunar Sci. Conf, 3, 
3433, 1975. 

Rajan, R. S., On the irradiation history and 
origin of gas-rich meteorites, Geochim, 
Cosmochim. Acta, 38, 111, 1974. 

Rajan, R. S., J. C. Huneke, S. P. Smith', and 
G. J. Wasserburg, 40 Ar- 39 Ar chronology 
of isolated phases from Bununu and 
Malvern howardites, Earth Planet. Sci. 
Lett., 27, 181, 1975. 

Rajan, R. S., J. C. Huneke, S. P. Smith, and 
G. J. Wasserburg, 40 Ar- 39 Ar chronology 
of individual clasts from the Kapoeta 
howardite and the origin of howardites, 
submitted to Earth Planet Sci. Lett. 

Zinner, E., R. M. Walker, J. Chaumont, and 
J. C. Dran, Ion probe analyses of artifi- 
cially implanted ions in terrestrial sam- 
ples and surface-enhanced ions in lunar 
sample 76215,77, Proc. 7th Lunar Sci. 
Conf, 1976. 



176 



CARNEGIE INSTITUTION 



Cosmic-ray Research 

Scott E. Forbush 

Cosmic-ray diurnal anisotropy and 
the reversals of the sun's poloidal mag- 
netic field. Variations with a "period" 
of two solar cycles in the cosmic-ray 
diurnal anisotropy component in the 
asymptotic direction 128° east of the 
sun which are related to the reversals 
of the sun's poloidal magnetic field 
have been discussed in previous annual 
reports. The component of the diurnal 
anisotropy in the asymptotic direction 
90° east of the sun varies with 
geomagnetic activity and exhibits a 
solar cycle variation. Together these 
two components account for essentially 
all the variations of the annual means 
of the diurnal anisotropy from 1937 to 
1974, as was reported by Forbush at 
the 14th International Cosmic-Ray 
Conference in Munich, August 1975. 
These results could only have been de- 
rived from data covering the long 
interval since 1937; such data are 
available from three stations with 
Carnegie Institution of Washington 
ionization chambers. Nevertheless, 
this interval is less than that covered 
by two "cycles" in the variation of the 
sun's poloidal magnetic field. Thus, it 
is desirable to extend the observational 
series for at least another two or three 
years (circa 1978), which fortunately is 
being achieved by the unselfish, whole 
hearted, gratuitous cooperation of in- 
dividuals at three stations, 

Observations and reduction of data. 
Cosmic-ray ionization chambers were 
operated throughout the report year at 
Huancayo, Peru; at Fredericksburg, 
Virginia, U.S.A.; and at Christchurch, 
New Zealand. It is anticipated that by 
the end of 1976 the scaling and reduc- 
tion of records will again be on a cur- 
rent basis. 

Cooperation in operation of cosmic- 
ray meters. Appreciation is expressed 
to the U.S. Geological Survey and the 
U.S. Department of the Interior and its 
staff at the Fredericksburg Geomagne- 
tic Center for efficient operation of the 



cosmic-ray meter there during the past 
report year, and to the Government of 
Peru and the Director and staff of the 
Instituto GeofTsico del Peru for making 
cosmic-ray records from Huancayo 
available. Appreciation is also ex- 
pressed to the Director and staff of the 
Geophysical Observatory at Christ- 
church, New Zealand, for the excellent 
maintenance of the equipment there 
and for the fine records obtained. 



Rb-Sr Mantle Isochrons 



C. Brooks, D. E. James, S.R. Hart, 
and A. W. Hofmann 

INTRODUCTION 

The use of isotopic abundances of 
strontium in volcanic rocks as tracers 
of the source composition has long been 
an important geochemical tool for the 
study of the earth's mantle. Because of 
their uniquely low 87 Sr/ Hti Sr ratios, it 
has been shown, for example, that the 
mid-ocean ridge volcanics must tap a 
mantle source that is different from 
virtually all other volcanic rocks in- 
cluding those found on oceanic islands. 
A difficulty encountered in the in- 
terpretation of the isotopic data is that 
these other groups of volcanic rocks, 
though nearly always higher in K7 Sr/ 
K,; Sr than mid-ocean ridge basalts, tend 
to have much more variable isotope 
ratios, thus obscuring the isotopic dis- 
tinction even between crustal and 
mantle derived material. 

In the two sections of this report we 
attempt to obtain additional informa- 
tion by using both the 8T Sr/ H,i Sr ratio 
and the Rb/Sr ratio of these rocks in 
conventional isochron diagrams. Be- 
cause isolated occurrences of apparent 
Rb-Sr isochrons with "ages" greatly in 
excess of the age of the volcanic erup- 
tions have been reported before, we 
undertook a systematic search of the 
literature and our own data for such 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



177 



pseudoisochrons. We found that pseu- 
doisochrons are indeed very common in 
individual groups of volcanic rocks. 
Moreover there is a well-defined single 
pseudoisochron for a world-wide sam- 
pling of oceanic tholeiitic basalts. 

We believe that these pseudoiso- 
chrons are inherited from the mantle 
and that the volcanic rocks therefore 
contain much more specific informa- 
tion about the local and world-wide 
evolution of the mantle than had been 
realized. For example, the world-wide 
Rb-Sr isochron age of 1.6 ±0.2 b.y. for 
tholeiites from oceanic islands and 
mid-ocean ridges may date a major 
event in the evolution of the earth's 
mantle. Similarly, the more variable 
and mostly younger ages derived from 
isochron correlations for more localized 
groups of continental volcanic rocks 
are interpreted as inherited from old 
subcontinental lithosphere. 

To account for this heterogeneous 
structure of the mantle we propose a 
model for the mantle whereby diapirs 
or "blobs" rise from the lower mantle 
(mesosphere) through a partially mol- 
ten upper mantle (asthenosphere) until 
they encounter the completely solid 
uppermost mantle (lithosphere). In 
oceanic regions the lithosphere is rela- 
tively thin (100 km or less), and the 
blobs rise directly to the surface to form 
oceanic islands. The world-wide mantle 
isochron for these rocks is generated by 
mixing of mesosphere and asthenos- 
phere in different proportions, and the 
age of 1.6 b.y. may date the chemical 
fractionation and segregation of these 
largest-scale heterogeneities of the 
mantle. In continental regions the 
lithosphere is thicker (up to 400 km), 
and the mesospheric material does not 
necessarily penetrate directly to the 
surface. Rather, much of it remains 
near the base of the lithosphere and is 
gradually incorporated by the slowly 
thickening lithosphere which grows by 
freezing out of partially molten as- 
thenosphere. Continental volcanism is 
caused by reactivation of this hetero- 
geneously accreted lithosphere, and 



the mantle isochrons contain a record, 
possibly distorted, of the accretion his- 
tory of this lithosphere. 

PART I: CONTINENTAL REGIONS 

In this report we discuss apparent 
Rb-Sr isochron relationships of vol- 
canic (and a few intrusive igneous) 
rocks, believed to be derived from the 
subcontinental mantle. Most of the 
rocks included in this discussion are 
basalts and andesites, but others are 
more exotic rock varieties, mostly of 
strongly alkalic affinity (phonolites, 
nephelinites, syenites, ijolites, and 
shoshonites). The intrusive rocks in- 
clude two suites of granites from 
California. In most cases the age of the 
continental crust is greater than 10 9 
years, so it is reasonable to expect that 
at least some of the underlying litho- 
sphere is also quite old. In contrast 
with the results reported in Part II (for 
oceanic volcanic rocks), we have not 
averaged any of the data by geographi- 
cal (or any other) groupings because we 
do not attempt to show (nor would we 
expect to find) any meaningful world- 
wide correlations, except perhaps that 
from a statistical point of view rocks 
with high Rb/Sr ratios would tend to 
have high * 7 Sr/ Ki; Sr ratios. 

The terminology adopted is as fol- 
lows: Lithosphere is the rigid outer 
shell of the earth which includes the 
.crust and upper mantle; asthenosphere 
is mobile (probably incipiently melted) 
mantle material underlying the litho- 
sphere; mesosphere is the rigid mantle 
material underlying the asthenosphere 
and constituting the bulk of the man- 
tle. Subcontinental mantle is a general 
term used primarily to mean litho- 
spheric mantle but not necessarily ex- 
cluding asthenosphere and meso- 
sphere. Undepleted refers to a mantle 
system with Rb/Sr and Sr-isotopic 
ratios compatible with closed system 
evolution for 4.5 b.y. Enriched and de- 
pleted refer to systems in which present 
Rb/Sr ratios are higher and lower, re- 
spectively, than those required for 
closed system evolution. Thus, the typ- 



178 



CARNEGIE INSTITUTION 



ical mid-ocean ridge tholeiite comes 
from partially depleted mantle and is 
therefore much lower in K, Rb, Sr, Ba, 
and Rb/Sr ratio than tholeiites of 
otherwise similar composition from 
oceanic islands and from continental 
regions. Homogeneous implies that 
chemical composition is uniform from 
place to place, provided that the sample 
is large enough to average out mineral 
scale compositional variation. Hetero- 
geneous is used to refer to composi- 
tional variations on any scale. A man- 
tle rr age" refers to the time of some 
geological event in the mantle, such as 
the development of large-scale 
heterogeneities. 

Pseudoisochrons. Because mantle- 
derived magmas in continental areas 
have traversed some 30-40 kilometers 
of ancient radiogenic sialic crust, the 
high and variable Sr-isotope ratios ob- 
served in these magmas are commonly 
ascribed to crustal contamination (e.g., 
Bell and Powell, 1970; Compston et al, 
1968; Compston, 1974; Doe etal, 1969; 
Ewart and Stipp, 1968; Faure et al., 
1972; Green and Ringwood, 1967; Hal- 
pern et al., 1974; Hedge, 1966; Hurley 
etal., 1966; Kudo et al., 1971; Laughlin 
etal, 1972; Manton, 1968; Montignye^ 
al, 1969; Pushkar, 1968; Pankhurst, 
1969; Scott et al, 1971). It is also possi- 
ble, however, that these higher Sr 
ratios are primary, inherited from sub- 
continental mantle possessing higher 
and more variable abundances of 
radiogenic Sr. The observed range in 
values would reflect lateral and/or ver- 
tical heterogeneities within this man- 
tle. The recent literature increasingly 
favors the latter possibility, mostly due 
to studies of alkalic rocks (e.g., Powell 
and Bell, 1974), and volcanics of west- 
ern U.S.A. where subcontinental litho- 
sphere and asthenosphere are appar- 
ently playing a major role in surface- 
reaching magmatism (Hedge and No- 
ble, 1971; Leeman, 1974, 1975; Leeman 
and Manton, 1971; Mark et al, 1975; 
Peterman et al, 1970). The possibility 
that the Sr-isotopic ratios are inherited 
directly from subcontinental mantle is 



further strengthened by the presence of 
ubiquitous pseudoisochrons. (We note 
here that we use the term pseudoiso- 
chrons or mantle isochron for any posi- 
tively correlated array of data, as long 
as this correlation is statistically sig- 
nificant. In the conventional usage of 
the term isochron, one usually requires 
the data to define a straight line within 
the limits of analytical error. The im- 
plication of our more relaxed usage is 
that the initial K7 Sr/ K,i Sr need not be 
perfectly correlated for all samples). 

Prior to discussing the data upon 
which this paper is based, it is essential 
to distinguish between the simple iso- 
chron plot and the pseudoisochron plot 
utilized throughout our data presenta- 
tion. The isochron diagram for a group 
of igneous rocks plots their individual 
K7 Sr/ 8,i Sr composition as measured 
today (termed present-day ratios) ver- 
sus their respective Rb/Sr ratios. The 
pseudoisochron diagram, however, 
plots the initial 87 Sr/ 8,; Sr ratios of the 
samples versus their respective Rb/Sr 
ratios. In other words, on a pseudoiso- 
chron plot the K7 Sr/ K(i Sr of the samples 
have been individually "corrected 
back" to their time of crystallization, 
using independent information for the 
age of crystallization. Ideally, a group 
of volcanics 100 m.y. old would define a 
line on an isochron diagram whose 
slope corresponds to an age of 100 m.y. 
The same volcanics on a pseudoiso- 
chron plot, however, should define a 
horizontal line (i.e., zero age) whose 
intercept is the unique 87 Sr/ H(i Sr compo- 
sition of the parent magma. When 
these volcanics do not plot as a horizon- 
tal array on the pseudoisochron plot, 
then there is a hint that some precrys- 
tallization history in H7 Sr/ K,i Sr has been 
preserved in the rocks. If the data ac- 
tually define a good linear array, the 
hint is strengthened, and there is a 
strong possibility that the information 
given in the pseudoisochron plot per- 
tains directly to the source region of 
the volcanics. For many volcanic rock 
associations of continental regimes this 
source region is the mantle. Therefore, 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



179 



we searched the literature for pseudo- 
isochrons with the aim of evaluating 
the possible preservation of mantle 
isotopic properties in surface-reaching 
igneous rocks. 

The results of the literature survey 
are given in Table 6 and Table 7. Here, 
each measured 87 Sr/ 8(i Sr ratio has been 
corrected for the true age of the rock, 
and the whole-rock initial 87 Sr/ 8li Sr 
ratios then plotted vs. Rb/Sr on a 
pseudoisochron diagram. Correlation 
theory and regression analysis indicate 
that most of these pseudoisochron have 
slopes significantly different from zero 
at confidence levels up to 95% (in some 
cases up to 99.9%) and that they define 
excess "ages" ranging from 70 m.y. to 
more than 3000 m.y. Some examples 
are shown in Fig. 30. 

For the most part the pseudoiso- 
chrons come from young volcanic ter- 
ranes in which no age correction of the 
measured present-day Sr isotopic com- 
position is necessary. Noteworthy are 
the numerous examples from western 
U.S.A., where pseudoisochrons are ob- 
served in rock associations of widely 
differing chemical composition. Fur- 
thermore, pseudoisochrons are not re- 
stricted to young volcanics. Examples 
are also encountered in plutonic and 
hypabyssal rocks of Mesozoic age (the 
California batholith, the Jurassic dol- 
erites of Antarctica), Caledonian basic 
igneous rocks of Scotland (Haddo 
House mass), and in ancient, layered, 
mafic intrusions (Usushwana Complex 
of South Africa). 

There are a variety of mechanisms 
that could produce the pseudoisochrons 
given in Table 6. These mechanisms 
can be divided into crustal and mantle 
types. The crustal-related processes 
simply involve mixing of mantle- 
derived magmas with crustal material 
of higher 87 Sr/ 8,i Sr (crustal contamina- 
tion). The criteria for evaluating this 
process as a potential means of raising 
the 87 Sr/ 8,i Sr of mantle-derived magmas 
are well illustrated in studies of young 
island arc volcanics (Ewart and Stipp, 
1968; Pushkar, 1968). In these studies, 



a hyperbolic relationship between 
87 Sr/ 8,i Sr and Sr concentrations is taken 
to be indicative of contamination. 
Clearly, however, many forms of dis- 
equilibrium melting or mixing of het- 
erogeneous mantle may produce simi- 
lar curves and otherwise be indistin- 
guishable from crustal contamination. 
Hence such relationships cannot be 
considered diagnostic. 

On the other hand, there is often in- 
dependent evidence against crustal 
contamination of mafic volcanics. We 
include in this category such pertinent 
features as: 

1. Limited range of Sr isotopic com- 
position of the volcanic rocks despite 
the very large range in isotopic compo- 
sition of the supposed contaminating 
materials. 

2. Limited range of Sr isotopic com- 
position in many volcanic associations 
despite a wide range in Sr concentra- 
tions. 

3. Persistence of normal rock com- 
positions (e.g., tholeiites) despite sig- 
nificant variability in isotopic composi- 
tion and hence presumed wide varia- 
tion in the nature and quantity of con- 
taminant required. 

4. Constancy of composition of dif- 
ferentiates, irrespective of the nature 
and amount of the supposed contamin- 
ant. 

5. Lack of correlation between 
thickness or age of continental sialic 
crust and the 87 Sr/ 86 Sr ratios observed 
at the surface. 

6. High "initial" 87 Sr/ 86 Sr ratios 
that ought not to be influenced by crus- 
tal contamination, assuming linear 
mixing relations. 

7. Lack of geographic correlation of 
isotopic compositions for many tectonic 
environments. 

8. Lack of correlation between Sr 
and Pb isotopic ratios. 

9. 206 Pb/ 204 Pb ratios in basalts and 
rhyolites which commonly show the 
same range of values. 

10. Paucity of included material 
such as crustal xenoliths. 



180 



CARNEGIE INSTITUTION 



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• CD 
co 13 

II 

« s 

.2 &D 

CO CD 





co co 




c 


o?^ 
O.S 


Ec CD 


o 

_C0 


S CD 


o 


O -P 
C CO 
CO *h 

CJ —■ 


T3 ® 


c 


*S -2 


CD " 

T3 C 


o 

CO 

CO 
CD 


3 ."S 

be a 


a <D 



CN .2 
» 13 

05 K 



M CO 
>^ ^ CO 

>H CD HH 

=> HO ^ 

CO 3 



a be > 



>, 


co ^r 

<B CD 


CO 

u 


*" CD 

g 2 


09 


^^ 


CO 


cc o 


CO 


nr^ 


a 

CO 


.S'S 



.2 -c 

CO 

CD CO 

CD CO 

0) CO 

CO & 

3 B 



C 

„ co CD 

cd -e 

■ t_l +J CO 

2 8*3 -a 

H) CO -H ^ 
O Cj ..-P 

PQ g^ 2 

I ! lis 

g co C 53 

> o ob 0) 

> -rt CD O 

c2ii a) o 
c > 5* co 
• J T3 3 g ■ 

>5ou 



■ S § 

T3 co -Q a, 
^ CD C JH 

co -a 3 co 
t3 pfr ,3 s° 

CPZ Ph 
CO - « « 



.3 (3 
> <D 

CD H 



cd H 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



181 



TABLE 7. Reference Key to Table 6 



Key No. 


Reference 


1 


Bell and Powell (1970) 


2 


Church and Tilton (1973) 


3 


Compston et al. (1968) 


4 


Davies eial. (1970) 


5 


Ewart and Stipp (1968) 


6 


Faure etal. (1972) 


7 


Halpern etal. (1974) 


8 


Hedge et al. (1970) 


9 


Heier etal. (1970) 


10 


Jameses al. (1976) 


11 


Kistler and Peterman ( 1973) 


12 


Leeman (1970) 


13 


Leeman(1974) 


14 


Leeman and Manton ( 1971) 


15 


Nobel etal. (1973) 


16 


Pankhurst(1969) 


17 


Peterman et al. (1970a) 


18 


Peterman et al. (19706) 


19 


Powell and Bell (1970) 


20 


Stipp (1968) 



In a detailed study on the cause of 
high 87 Sr/ 8,i Sr ratios in Cenozoic ande- 
sites and da cites of southern Peru, 
criteria of this nature led us to reject 
crustal contamination as a plausible 
mechanism for either the high 87 Sr/ 8,i Sr 
ratios or the pseudoisochrons observed 
there (James et al., 1976). This conclu- 
sion is especially remarkable when it is 
noted that the Andean crust through 
which these magmas have passed is 
about 70 km thick, thicker than any 
other known crustal section in the 
world save that of the Himalayas. 

Based on this synthesis, we advocate 
that the hitherto durable and facile 
crustal contamination explanation for 
high 87 Sr/ 8i; Sr in continental volcanic 
rocks be considered improbable, and 
that other possible mechanisms lead- 
ing to the observed isotopic composi- 
tions be evaluated. We believe that 
these mechanisms are all mantle re- 
lated, implying that both the high Sr 
ratios and the pseudoisochrons are the 
result of mantle processes. This view- 
point is supported by the observation 
that some pseudoisochrons and high 
87 Sr/ 8(i Sr ratios are observed in oceanic 
regions where little or no possibility of 
sialic contamination exists (Brooks et 
al, 1976). 



Mantle- related processes that could 
lead to Rb-Sr pseudoisochrons include: 
(1) mixing of heterogeneous mantle 
material; (2) selective melting of heter- 
ogeneous mantle material of the same 
or different age; and (3) disequilibrium 
melting of homogeneous mantle in 
which individual mineral phases are in 
isotopic disequilibrium. 

Distinguishing between these pos- 
sibilities is difficult, as none of the pro- 
posed mechanisms leads to a uniquely 
identifiable compositional property in 
the rocks, other than that of Pb and Sr 
isotopic variation; nonetheless, it is 
important to inquire whether the 
pseudoisochrons do contain meaningful 
age and isotopic information about the 
mantle. 

One means of evaluating pseudo- 
isochrons is to determine whether they 
involve identifiable mixing-line 
chemistry. Rb-Sr variation diagrams 
and plots of 87 Sr/ 8,i Sr versus 1/Sr can be 
used for this purpose. Examination of 
such plots for the data used to build 
Table 6 reveals that a good pseudoiso- 
chron is commonly accompanied by a 
good positive correlation between 
( 87 Sr/ 8,i Sr) and 1/Sr but no correlation 
at all between Rb and Sr. The absence 
of a Rb-Sr correlation is inconsistent 
with simple magma mixing. Thus, 
while we cannot discount mixing of 
heterogeneous mantle material, the 
data in general seem to preclude sim- 
ple two-component magma mixing as 
an important factor in most 
pseudoisochrons. The volcanics of Peru 
are among the few examples that dis- 
play good correlations between ( 87 Sr/ 
8,; Sr) and 1/Sr, and Rb and Sr (James et 
al., 1976). In this case, the pseudoiso- 
chrons could result either from mixing 
or selective melting of a heterogeneous 
source region of uniform age or from 
disequilibrium melting of homogene- 
ous mantle. 

Selective withdrawal of magma from 
mantle material that is heterogeneous 
(and of uniform or nonuniform age) 
will result in the generation of 
pseudoisochrons that do not necessar- 



182 



CARNEGIE INSTITUTION 



720 



CO 



CO 



715 - 



710 



705 




u im 


B 








706 




• 


• 


\^ 


0704 




• 






702 




i i 


i 


i 



002 0.04 006 0.1 



706 


c 






• 




0.704 




• 
• 




J^i 


• - 




• 


• ^^^ 


• 






702 




• 

• 
1 1 


1 


I 





0.02 04 06 0i 




02 04 06 



720 



716 



712 



708 



704 




05 0.10 015 20 



Fig. 30. Rb-Sr mantle isochrons (initial 87 Sr/ 86 Sr versus Rb/Sr) for some representative igneous rock 
associations of continental affinity (for more detail, see Table 6). (A) Quartz and olivine norites from 
the Arnage (open circles) and Haddo House (closed circles) igneous masses, Scotland. (B) Alkali basalt 
series of the Western Grand Canyon, U.S.A. (C) Mafic rocks of the Archean Usushwawa Complex, 
South Africa. (D) Colorado Plateau basalts, U.S.A. (E) Snake River Basalts, U.S.A. Closed Circle, 
Craters of the Moon basalts; Open circle, King Hill basalts. The field is for olivine tholeiites (N = 34) 
with the cross showing the average composition. (F) Peruvian Cenozoic volcanics, Southern Peru. 
Closed circle, Barroso volcanics; open circle, Arequipa volcanics. 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



183 





G 








\\05i- 


0.706 
704 


08 

o 

o • O- ^ 

•3>® 


o 

% 


"~~~~ o 









0702 


1 


1 






1 



0712 



0710 



0.10 020 0.30 040 



708 - 



0706 



0.704- 



0702 



H 














_ 












• • 


- - 


^•__ 


• 


• 


• 


»^ 




- 


i 




i 


i 




i i 



0.10 0.20 0.30 040 50 



720 



715 



710 



0.705 -, 



700 




(Rb/Sr! 



Fig. 30. (G) New Zealand East Arc volcanics. 
Closed circle, basalts; open circle, andesites. (H) 
Mesozoic granites of California, U.S.A. (initial 
* 7 Sr/" H Sr > 0.706). (I) Jurassic dolerites of Ant- 
arctica (includes the Ferrar Group and Queen 
Maud Land dolerites). 



ily reflect the average age of the source 
region; however, we cannot avoid the 
conclusion that such pseudoisochrons 
reflect a' long history of isolated 
isotopic evolution of the subcontinental 
mantle. Thus, whatever the specific 
origin of pseudoisochrons, they do 
imply a significant age for mantle 
heterogeneities, and for this reason we 
refer to them as mantle isochrons, a 
general term designed to emphasize 
the unique nature of the ancient 
isotopic imprint borne to the surface in 
mantle-derived magmas. 

Accepting that pseudoisochrons are 
mantle isochrons places major con- 
straints on the nature of the mantle 
source regions. First, the age informa- 
tion embodied in mantle isochrons de- 
mands the existence of mantle that has 
long preserved a distinct chemical 



identity. This in turn demands that the 
mantle source region be sufficiently 
immobile as to be static and resistant 
to reworking and mixing processes dur- 
ing most of its history. Second, the ele- 
vated Sr-isotopic ratios indicated by 
the mantle isochrons suggest that the 
mantle source region is not chemically 
depleted. Third, the wide range in ob- 
served Sr-isotopic composition requires 
that the mantle be grossly heteroge- 
neous, probably on both regional and 
local scales (Hofmann, 1975; Hofmann 
and Hart, 1975). 

What sort of mantle fits these re- 
quirements? Asthenosphere most 
probably does not, leaving either sub- 
continental lithosphere or mesosphere 
as the likely source region. The linear 
arrangement of continental island arc 
volcanoes associated with magmatic 



184 



CARNEGIE INSTITUTION 



activity derived from above subducted 
lithosphere would seem to preclude 
mesosphere as a likely direct source, 
even though its nature and composi- 
tion are probably compatible with the 
mantle isochrons. Is continental 
lithosphere compatible with these re- 
quirements, however? In the following 
section we will show that there is a 
substantial amount of evidence that al- 
lows an independent case to be built for 
thick, ancient, heterogeneous and 
chemically undepleted continental 
lithosphere with an isotopic composi- 
tion consistent with that implied by 
mantle isochrons. 

Nature of continental lithosphere. 
Most of our knowledge concerning the 
nature of continental lithosphere 
comes from three sources: seismic 
studies, heat-flow measurements, and 
the analysis of ultramafic nodules be- 
lieved to be samples of lithosphere 
rafted to the earth's accessible regions 
in mantle-derived magmas. These 
broad areas are so interdependent that 
it is impractical to isolate the indi- 
vidual contributions originating in 
each. Consequently, in this discussion 
we synthesize the evidence from these 
sources by treating successively the 
questions of thickness of continental 
lithosphere, chemical composition of 
lithosphere (and the resulting implica- 
tions for heat flow), isotopic heteroge- 
neity of lithosphere, and the age of 
lithosphere. 

Although there is no consensus 
among geophysicists on the thickness 
of continental lithosphere, with esti- 
mates ranging between about 200 and 
400 km (Dziewonski, 1971; Kanamori, 
1970; Jordan, 1975; Okal and Ander- 
son, 1975; Sipkin and Jordan, 1975; 
Toksoz and Anderson, 1966), the lack 
of agreement is largely one of defini- 
tion and not substance. In particular, 
the lower estimates of lithosphere 
thickness are based on the determina- 
tion of the minimum depth below 
which oceanic and continental velocity 
structures need not differ. But depths 
so calculated provide no information on 
actual lithosphere thickness; that 



thickness is determined by the depth at 
which shear-wave velocities or Q de- 
crease rapidly or discontinuously to as- 
thenospheric values. By this criterion, 
few seismic studies successfully iden- 
tify the base of the lithosphere. They do 
show, however, that lithosphere be- 
neath cratons may be at least 300-400 
km thick (Alexander and Sherburne, 
1973; Goncz and Cleary, 1976); that is, 
the continental mantle is subsolidus to 
that depth, exhibiting no significant 
low velocity or low Q zone at lesser 
depths. While there may be a low- 
velocity channel at greater depths, the 
poor resolving power of the seismic 
data precludes its unequivocal identifi- 
cation. One important study of Q be- 
neath the Andes of South America, 
however, clearly demonstrates the 
existence of a low-rigidity (and proba- 
bly low-velocity) channel at a depth of 
350 km beneath a high-rigidity lithos- 
phere (Sacks, 1969; Sacks and Okada, 
1974). This result is of considerable 
general importance because the An- 
dean orogen evolved atop the ancient 
South American shield, and the integ- 
rity of the underlying lithosphere ap- 
pears to be intact (James, 1971). 

Additional evidence favoring thick 
continental lithosphere comes from an 
analysis of global heat-flow data by 
Pollack and Chapman (Pollack and 
Chapman, 1976; Chapman and Pol- 
lack, 1975). Although the results of 
their study are not conclusive, they in- 
dicate that the lithosphere beneath an- 
cient continental interiors may attain 
thicknesses in excess of 300 km. This 
estimate is based upon regional varia- 
tion in geothermal gradient and the 
corresponding depths at which the 
geotherm intersects an empirically 
postulated temperature of incipient 
melting. Pollack and Chapman (1976) 
take the temperature at the litho- 
sphere-asthenosphere boundary to be 
0.85 the temperature of dry melting of 
peridotite. This value is obtained em- 
pirically by considering the thickness 
of oceanic lithosphere as a function of 
varying oceanic geothermal gradients. 
The general form of continental geo- 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



185 



thermal gradients inferred from heat 
flow is corroborated by the results of 
geochemical studies in which experi- 
mentally determined phase relation- 
ships and mineral compositions of 
coexisting pyroxenes as functions of 
temperature and pressure are applied 
to nodules to obtain estimates of their 
equilibration temperatures and 
pressures (e.g., Boyd and Nixon, 1975). 
The persistence of subsolidus tempera- 
tures to great depth is further attested 
to by the absence of interstitial glass in 
ultramafic nodules (e.g., Kudo et al., 
1972). The Pollack and Chapman 
model, as in most previous thermal 
models (Clark and Ringwood, 1964; 
Sclater and Francheteau, 1970), as- 
sumes that the continental lithosphere 
is somewhat depleted in heat- 
producing elements. Yet, one of the 
principal conclusions to be drawn from 
the interpretation of mantle isochrons 
is that continental lithosphere is essen- 
tially undepleted. Since chemical com- 
position is the input to the thermal 
model, our conclusion must be consis- 
tent with both the observed heat-flow 
data and with what is known about the 
composition of continental lithosphere 
as revealed by studies of ultramafic 
nodules found in kimberlites and 
basalt (see Ringwood, 1975, pp. 100- 
114 for general discussion and exten- 
sive bibliography) . 

Kimberlites crystallize from vola- 
tile-rich magmas that appear to have 
originated at depths greater than 200 
km (Dawson, 1972; Boyd and Nixon, 
1970; Ringwood and Lovering, 1970; 
Ringwood, 1975). They apparently as- 
cended rapidly, and in so doing incor- 
porated many ultramafic xenoliths 
which are believed to represent sam- 
plings of different depth zones of the 
mantle (Wagner, 1928; Dawson, 1972; 
Harris et al., 1972; Harris and 
Middlemost, 1969; MacGregor, 1975; 
Boyd and Nixon, 1975; Lovering and 
Tatsumoto, 1968). Although there is 
debate about the source region of the 
kimberlite magma and no assurance 
that it lies in the lithosphere, there is 
general consensus that many of the 



xenoliths are subsolidus mantle frag- 
ments showing little reequilibration to 
lower temperature and pressure as- 
semblages (MacGregor, 1968; Boyd and 
Nixon, 1975; Dawson, 1972; O'Hara et 
al., 1975). The nodule compositions can 
therefore be used as guides to real 
mantle compositions. Studies of 
nodules in kimberlite indicate that the 
subcontinental mantle has high Rb/Sr 
ratios, correspondingly high 87 Sr/ 8<i Sr 
ratios, and is otherwise undepleted in 
its basaltic fraction (Rhodes and Daw- 
son, 1975; Barrett, 1975; Ridley and 
Dawson, 1975; Hutchinson and Daw- 
son, 1970; O'Hara etal., 1975). The un- 
depleted character of the mantle from 
which the nodules were derived is at- 
tested to by the dominance of the fertile 
garnet peridotite (especially the 
sheared variety) over other more de- 
pleted nodules (O'Hara et al., 1975). 
Thus, nodule studies corroborate our 
conclusions based on Sr-isotopic com- 
positions and argue for the existence of 
a continental lithosphere that is far 
from depleted and which, under favor- 
able thermal conditions, would be a 
ready source of basaltic magma. 

Steady-state thermal models based 
on undepleted mantle compositions can 
be constructed that are entirely consis- 
tent with measured heat flow and pos- 
tulated geothermal gradients. Consid- 
er, for example, a surface heat flux for 
a continental craton of 1.05 heat flow 
units (HFU), a standard 40-km crust 
which produces 0.43 HFU, and a 
lithospheric mantle 310 km thick that 
consists of undepleted peridotite con- 
taining 1000 ppm K, 0.06 ppm U, and 
0.23 ppm Th. Crustal heat production 
and steady-state heat flow are taken 
from Sclater and Francheteau (1970), 
and are consistent with the relation- 
ship between heat flow and surface 
heat production proposed by Roy et al. 
(1968). K/U and U/Th ratios are taken 
from Clark and Ringwood (1964). This 
mantle material will contribute 0.38 
HFU to the steady-state heat flux, 
leaving 0.24 HFU to be derived from 
below 350 km, a deep-mantle contri- 
bution that is only about one half the 



186 



CARNEGIE INSTITUTION 



value assumed in the static earth 
model of Clark and Ringwood. Our 
thermal model, in which most surface 
heat flow originates in the lithosphere 
rather than at greater depth, results in 
slightly lower geothermal gradients for 
the same surface heat flux; this in turn 
forces the point of incipient melting to 
even greater depths, thereby making 
thick continental lithosphere even 
more feasible. This example illustrates 
that steady-state thermal models based 
on undepleted mantle compositions are 
entirely consistent with observed heat 
flow and postulated geothermal gra- 
dients. 

The study of ultramafic nodules pro- 
vides evidence not only that the conti- 
nental lithosphere is chemically unde- 
pleted but that it is ancient, isotopi- 
cally enriched, and grossly heteroge- 
neous. Kimberlite nodules typically 
show 87 Sr/ 8( 'Sr ratios that vary between 
0.703 and 0.710 (average, 0.707) (Bar- 
rett, 1975; Allsoppeia/., 1969; Mitchell 
and Crocket, 1971; Ridley and Dawson, 
1975; Hutchinson and Dawson, 1970; 
Harris et al., 1972). The consistent and 
wide variation in isotopic ratio cannot 
be attributed to contamination and 
must reflect mantle inhomogeneity 
(Barrett, 1975; Ridley and Dawson, 
1975). Furthermore, the existence of 
isotope disequilibrium between min- 
eral phases in individual nodules 
suggests that the heterogeneity is pre- 
sent even on the scale of mineral grains 
(Kudo et al, 1972; Paul, 1971; Com- 
pston and Lovering, 1969; Steuber and 
Ikramuddin, 1974; Burwell, 1975; Bar- 
rett, 1975; Peterman et al, 1970; Har- 
rises al, 1972; Dasch and Green, 1975). 

The high average 87 Sr/ 8,i Sr ratio 
suggests significant prehistory for con- 
tinental lithosphere, long isolated from 
less enriched asthenosphere. More di- 
rect evidence that the subcontinental 
mantle is very old comes from Sr and 
Pb isotope studies on ultramafic 
nodules. Measurement of apparent 
ages indicates mantle ages up to 3 b.y. 
or more (Burwell, 1975; Manton and 
Tatsumoto, 1971; Paul, 1971; Steuber 
and Ikramuddin, 1974; Dasch and 



Green, 1975; Cooper and Green, 1969; 
Harris et al, 1972). Other studies 
showing wide variation in Sr isotopic 
ratios between nodules provide indirect 
evidence that parts of the mantle have 
remained in isotopic isolation over very 
long periods of time (Barrett, 1975; 
Kudo et al., 1972; Compston and Lover- 
ing, 1969). Thus while no single geo- 
chemical study yet provides definitive 
evidence of ancient mantle, the inte- 
grated result of all studies argues for 
an aged and heterogeneous subconti- 
nental lithosphere. 

Implications. Because continental 
lithosphere exists at relatively low, 
subsolidus temperatures, it is protected 
from transport into subduction zones 
by the buoyancy of the overlying conti- 
nental capping; as a result, parts of the 
lithosphere may be as old as the 
coupled sialic crust (e.g., early Pre- 
cambrian for continental cratons). Any 
sections of lithosphere that are not de- 
pleted will, by virtue of their sheltered 
existence, possess distinctive isotopic 
compositions that result from both re- 
gional and local variation in Rb/Sr 
ratios. If this ancient lithosphere par- 
ticipates in younger magmatism, then 
those volcanic products will contain 
isotopic evidence of the source region's 
"pre-history." This evidence, manifest 
as mantle isochrons, raises two major 
questions: first, how does this litho- 
sphere form, and second, how does such 
lithosphere, isolated for long periods of 
time, become involved in young mag- 
matism? 

The range in initial 87 Sr/ 8(i Sr of 
mantle-derived magmas (Table 6 and 
Figure 30) provides strong evidence 
that the continental lithosphere is 
characterized by a degree of regional 
isotopic heterogeneity that is more 
pronounced than that observed in other 
mantle provinces. Specifically, the sub- 
continental mantle has Sr isotopic 
ratios ranging from as low as 0.703 to 
more than 0.710, a range of values 
nearly identical to that found for 
mantle-derived ultramafic nodules. By 
comparison, ocean-ridge basalts, taken 
to represent oceanic asthenosphere, 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



187 



display a quite limited range of values 
(about 0.7027) (Hofmann and Hart, 
1975), indicating that this astheno- 
sphere is comparatively homogeneous 
isotopically as might be expected in 
rapidly convecting, partially molten 
material. On the other hand, oceanic 
islands, taken to represent at least part 
mesosphere, exhibit a considerable 
range in Sr-isotopic values (from about 
0.703 to 0.706), which suggests that the 
mesosphere has substantial regional 
isotopic heterogeneities, although not 
as pronounced as those of continental 
lithosphere (Brooks et al., 1976; and 
Sun and Hanson, 1975). 

The upper limits of these ranges in 
87g r /8«g r ra tios lead to an important in- 
ference concerning the relative en- 
richment of the different mantle pro- 
vinces. Continental lithosphere consis- 
tently has Sr-isotopic values that are 
significantly higher than those of oce- 
anic asthenosphere, indicating that 
some parts of the subcontinental man- 
tle are considerably more enriched in 
radiogenic components and LIL (large- 
ion-lithophile) elements than sub- 
oceanic mantle. Mesosphere, as man- 
ifest in oceanic islands, can be regarded 
as a nondepleted base to oceanic and 
continental regions alike, locally pos- 
sessing a degree of enrichment in 
radiogenic components comparable to 
but possibly somewhat smaller than 
that of continental lithosphere. 

Our review of mantle isochrons and 
related Sr-isotopic data indicates that 
continental lithosphere is in part both 
more enriched and more heterogeneous 
in its radiogenic components (and LIL 
elements) than other mantle provinces. 
Such lithosphere enrichment and 
heterogeneity could result from a vari- 
ety of processes operating either indi- 
vidually or in concert, and possible 
models include: 

(i) Thermal accretion by "freezing" of 
enriched asthenosphere. Since litho- 
sphere begins its formation soon after 
crustal formation, the lithosphere 
under old continental areas may have 
started its formation at a time when 
the asthenosphere was enriched or 



nondepleted (as contrasted with its 
present state of chemical depletion). 

(ii) Trapping of an enriched as- 
thenosphere boundary layer (formed by 
progressive crystallization of the 
lithosphere with concomitant concen- 
tration of LIL elements into the 
partial-melt zone of the asthenosphere 
directly beneath the lithosphere boun- 
dary). 

(iii) Upward percolation of a LIL- 
element and volatile-rich phase (not 
miscible with the silicate partial melt) 
from mesosphere or asthenosphere into 
the subcontinental lithosphere. 

(iv) Diapiric rise of LIL-element en- 
riched mantle diapirs or blobs from the 
mesosphere, which underplate and be- 
come incorporated into the thickening 
subcontinental lithosphere. The mech- 
anism of rise of diapirs from a buoyant 
mantle layer has been considered at 
length by Ramberg (1972). The forma- 
tion of oceanic island volcanism by as- 
cent of chemically distinct blobs or 
plumes forms a background to our 
model and has been discussed by Schil- 
ling (1975). 

Model (i) would produce a zoned 
lithosphere grading from enriched at 
the top to depleted at the base (since 
present-day asthenosphere is de- 
pleted). Derivation of continental 
magmas from such a lithosphere might 
reflect this zoning, with magmas 
formed at the greatest depths 
(youngest lithosphere) carrying a de- 
pleted signature. There is no convinc- 
ing evidence, however, for the exis- 
tence of significant quantities of "de- 
pleted" magmas in continental regions, 
and hence operation of this model re- 
quires either that no magmas are de- 
rived from this depleted zone of the 
continental lithosphere, or that such 
magmas reequilibrate with shallower, 
more enriched lithosphere. 

Model (ii) has been proposed by Kay 
(1975) for the formation of het- 
erogeneities in the suboceanic mantle; 
however, consideration of the rates of 
diffusion of LIL-elements in silicate 
melts (Hofmann, 1975; Hofmann and 
Hart, 1975), shows that any such en- 



188 



CARNEGIE INSTITUTION 



riched boundary- me It layer will be 
very thin (—10 m) and quantitatively 
insignificant for the later production of 
enriched magmas. The main difficulty 
in this model (for both oceanic and con- 
tinental regions) arises because litho- 
sphere accretes at a velocity larger 
than the scale lengths of LIL-element 
diffusion in silicate melts, so that the 
steady-state composition of the litho- 
sphere would be close to that of the as- 
thenosphere from which it forms, and 
only trivial amounts of "enriched" par- 
tial melt could result from this process. 

Model (iii) requires a mobile phase 
that is not miscible in the partial melt 
phase of the asthenosphere at the 
pressures and temperatures character- 
istic of the asthenosphere. Such a 
phase might be a hydrous, carbonate- 
rich fluid that separates from the 
mesosphere and rises buoyantly 
through the asthenosphere to be trap- 
ped against the advancing lithospheric 
front. Later partial melting of litho- 
sphere containing this enriched phase 
could produce magmas showing en- 
riched characteristics. In extreme 
cases, this enriched phase might pro- 
gress directly through the lithosphere, 
forming members of the kimberlite- 
carbonatite suite at the surface. While 
this model has many plausible charac- 
teristics (and is closely related to model 
iv with which it could operate in con- 
junction), we note that it also implies 
the presence of kimberlite-carbonatite 
magmas throughout the geologic rec- 
ord. They are in fact rare in Precamb- 
rian time, a time when presumably 
thinner crust and lithosphere would 
have greatly enhanced the likelihood of 
such magmas reaching the surface. 

Model (iv) involves a convecting 
mesosphere containing thermal or 
chemical heterogeneities (Anderson, 
1975), such that blobs or plumes of ma- 
terial can become separated, rise 
through the asthenosphere and become 
plastered-on and slowly incorporated 
into the thickening subcontinental 
lithosphere. This mesospheric material 



is presumed to be enriched in the LIL- 
elements relative to asthenosphere so 
that the resulting subcontinental 
lithosphere will be a heterogeneous 
combination of frozen-out astheno- 
sphere and included mesosphere- 
derived blobs. This model is attractive 
because it can be applied equally well 
in oceanic regions to explain the exis- 
tence of oceanic islands (variously re- 
lated to hot spots or plumes) which in- 
variably are built of volcanic material 
enriched in LIL-elements and Sr and 
Pb isotopes relative to the depleted, 
asthenosphere-derived spreading-ridge 
basalts. 

In oceanic regions, the mesospheric 
blobs or plumes (a plume may be sim- 
ply a more or less continuous train of 
blobs) do not necessarily go through a 
two-stage process of lithosphere under- 
plating and subsequent remelting. In- 
stead, they may diapirically penetrate 
the thinner oceanic lithosphere and di- 
rectly supply the partial melt leading 
to ocean island volcanism. If the blobs 
do not penetrate the lower boundary of 
oceanic lithosphere, they will produce 
heterogeneity similar to that of the 
continental lithosphere. By this con- 
cept, then, oceanic regions provide us 
with a recent (<200 m.y.) record of the 
continuous upward migration of en- 
riched blobs from the mesosphere, a 
process which has been going on under 
continents for billions of years. 

The main difference between mantle 
isochrons of oceanic and continental 
regimes is that higher Rb/Sr and * 7 Sr/ 
8,i Sr ratios are found for continental 
volcanics. This implies that there must 
be an additional fractionation (in- 
crease) of Rb/Sr ratio associated with 
the two-stage continental case (under- 
plating and subsequent remelting) re- 
lative to the single-stage oceanic case 
(direct penetration). We explain this as 
follows: At some point in the rise of 
mesospheric blobs, they become par- 
tially molten; this silicate melt, with a 
higher Rb/Sr than the initial value of 
the blob, becomes partially segregated 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



189 



from the crystalline residue as the blob 
ascends and is plated onto the bottom 
of the continental lithosphere. Later 
partial melting of this material will 
favor the previously partially segre- 
gated melt, leading to a second stage of 
Rb/Sr enrichment. The relatively high 
87 Sr/ 8( 'Sr ratios observed in continental 
volcanics (relative to oceanic-island 
volcanics) arise because of the isotopic 
aging that follows the initial Rb/Sr 
fractionation. Cases of more or less 
complete melt-crystal segregation 
prior to or during the underplating 
stage would result, on further cooling, 
in an eclogite solid (frozen melt) and 
ultramafic residue. The eclogite nod- 
ules commonly brought to the surface 
in kimberlitic rocks may be evidence 
for such a segregation process. 

The combined weight of the evidence 
discussed in this paper suggests that 
continental lithosphere accretes 
through a combination of processes (i) 
and (iv). Such a model provides a satis- 
factory explanation for mantle iso- 
chrons from both continental and 
oceanic regions. A schematic view of 
this model is given in Fig. 31A, and 
although it is recognized that some as- 
pects of the other models discussed may 
be of local importance, we have re- 
stricted our figure to a portrayal of the 
main features of continental litho- 
spheric growth. 

The data of Table 6 show that mantle 
isochrons are associated with a number 
of specific tectonic environments where 
ancient lithosphere can become reacti- 
vated and involved in partial melting 
processes. These environments are 
created where, (1) subcontinental 
lithosphere overlying a subduction 
zone is activated by proximity to that 
zone; (2) continental-scale rifting forms 
fractures that penetrate deep, un- 
loaded lithosphere; and (3) a 
continent-bearing plate over-rides an 
oceanic rise or plume. 

Schematic representation of these 
reactivation environments is illus- 
trated in Fig. 3 IB. 



Recognition that ancient lithosphere 
can play a major role in younger mag- 
matism in these tectonic environments 
allows us to formulate hitherto un- 
treated models of magma genesis. 

As an example of subduction zone 
reactivation, consider the well-studied 
Cenozoic volcanic rocks of the Andean 
region of southern Peru. These provide 
documentation of the role of ancient 
continental lithosphere in producing 
both high * 7 Sr/ Kli Sr ratios and well- 
defined mantle isochrons in subduction 
zone environments (James et at., 1976). 
Here, the ancient lithosphere of the 
South American shield extends be- 
neath the Andes to a depth of at least 
200 to 300 km and forms, throughout 
the depth range over which magma is 
generated, a continuous lithosphere- 
lithosphere boundary with the de- 
scending Nazca plate. The model which 
most plausibly explains the isotopic 
and trace element data is one in which 
andesitic magma is produced by sub- 
duction zone melting of the ancient 
South American lithosphere. The 
isotopic imprint of that aged litho- 
spheric mantle material is preserved in 
the mantle isochrons and anomalously 
high 87 Sr/ 8,i Sr ratios of the young vol- 
canics of the Andean region. 

The rifting-related reactivation en- 
vironment is represented by the alkalic 
complexes of Africa and the Jurassic 
tholeiitic flood basalts (and dolerite 
sheets) associated with the break-up of 
Gondwanaland. The latter define 
mantle-isochrons both on a local (Ant- 
arctica) and regional (Antarctica, 
Tasmania, and Africa) scale (Table 6), 
and document the existence of very un- 
depleted mantle in these regions. Man- 
tle isochrons are also common in re- 
gions where a continent appears to 
have over-ridden an ocean rise or man- 
tle plume. Our examples all come from 
western U.S.A., where many rock asso- 
ciations have mantle isochron ages in 
the range 600 to 1000 m.y. This par- 
ticular region is probably the most 
complex of those discussed, and despite 



190 



CARNEGIE INSTITUTION 



LITHOSPHERE 




ASTHENQSPHERE 



KJ . Oj v .,.• MESOSPHERE 



© 



(D 




Fig. 31. Diagram showing development (A) and subsequent reactivation (B) of blob-enriched 
continental lithosphere (vertical exaggeration about 5 = 1). (A) Blobs are released from mesosphere 
by buoyancy and convection, ascend through convecting asthenosphere, and penetrate through either 
under-plate thickening continental lithosphere or oceanic lithosphere. Ascending blobs (diagonally 
ruled pattern) may be partially molten, with some segregation of liquid occurring before they become 
subsolidus in the lithosphere (solid pattern). Dashed "age" lines in lithosphere represent progressive 
positions of the boundary representing accretion of lithosphere from asthenosphere. Note that oceanic 
lithosphere is shown to be younger than 200 m.y. An oceanic "plume" is indicated schematically as a 
continuous train of blobs. (B) Various reactivation environments, where partial melts (stippled pat- 
tern) are formed in continental lithosphere: (1) above a subducting oceanic plate; (2) over a thermal or 
upwelling anomaly in the asthenosphere (plume or overridden ridge, arrows); and (3) during conti- 
nental rifting (initial stage shown without causal mechanism). 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



191 






numerous studies a definite pattern of 
ancient lithosphere consumption is not 
yet forthcoming. However, the possible 
participation of the lithosphere in the 
magmatism of the region is discussed 
in the literature (Hedge and Noble, 
1971; Leeman, 1974, 1975; Leeman 
and Manton, 1971; Mark et al, 1975; 
Petermane^a/., 1970). 

An important aspect of Table 6 is 
that mantle isochrons are not re- 
stricted to young volcanics but are also 
encountered in young hypabyssal and 
plutonic intrusives (Jurassic dolerites 
of Antarctica; California Mesozoic 
batholiths), metamorphosed basic 
rocks of early Palaeozoic age (Caledo- 
nian basic-igneous Province, Scotland), 
and in ancient layered-intrusions 
(Usushwana complex of South Africa). 
These examples are encouraging be- 
cause they demonstrate that mantle 
isochrons are not necessarily masked 
by either differentiation or meta- 
morphism, thereby considerably ex- 
tending their usefulness as petrogene- 
tic indicators. 

One consequence of the mantle iso- 
chron model is that crystallization ages 
determined on basic igneous rocks by 
the Rb-Sr whole-rock technique can be 
in excess of the true age by many hun- 
dreds of millions of years. This problem 
of inherited age is more serious for 
younger rocks, and there are well- 
documented instances of conflicts be- 
tween stratigraphic age and Rb-Sr age 
in the literature (e.g., Compston, 1974; 
Cormier, 1969). 

Effective use of the mantle isochron 
concept requires knowledge of actual 
crystallization ages (so that the meas- 
ured isochron can be divided into its 
pre- and postcrystallization compo- 
nents), and determination of isochron 
parameters on rocks that have been 
subjected to minimal postmelting pro- 
cesses (fractional crystallization, wall- 
rock contamination, etc.). The concept 
of Rb-Sr mantle isochrons as we have 
presented it provides a new tool for un- 
derstanding the petrogenesis of 
mantle-derived igneous rocks. 



Acknowledgments 

This study has been supported in 
part by the National Science Founda- 
tion (Grant No. GA 36094) and the Na- 
tional Research Council of Canada 
(Grant No. A 5581 to C.B.). One of us 
(C.B.) gratefully acknowledges the 
Carnegie Institution of Washington for 
providing financial support and re- 
search facilities during his sabbatical 
year. 

PART II: OCEANIC REGIONS 

Introduction 

Tatsumoto (1966) showed that 
basalts from oceanic areas preserved a 
correlation between U/Pb ratio and 
lead isotopic ratio, and concluded that 
this correlation, presumably inherited 
from the mantle source of the basalts, 
suggests a mantle in which U/Pb 
heterogeneities have existed for a bil- 
lion years or more. A similar general 
correlation was noted between * 7 Sr/ H,i Sr 
and Rb/Sr for oceanic basalts (Hart, 
1971; Allegre, 1972). Recently, this 
systematic Rb-Sr isochron correlation 
was emphasized by Sun and Hanson 
(1975), who showed that Sr isotope 
data for alkali basalts (and nepheli- 
nites) from 14 oceanic islands defined a 
correlated array which, if interpreted 
as an isochron, suggested an age for 
mantle heterogeneities of about 2000 
m.y. We were impressed with these re- 
lationships and undertook a careful 
screening of published data to look at 
the Rb-Sr systematics of both tholeiitic 
and alkali basalts from oceanic re- 
gions. We found striking regularities 
in almost all cases and present these 
results because of their importance in 
constraining the space and time evolu- 
tion of mantle chemistry. 

Compilation of Data 

The data were compiled from the lit- 
erature, using only those analyses of 
relatively high precision which could 



192 



CARNEGIE INSTITUTION 



be confidently normalized to an Eimer 
and Amend standard value of 0.7080. 
For some islands the published 87 Sr/ 
8, 'Sr was not accompanied by Rb/Sr 
data. In these cases, powders of the 
original sample were requested (and 
are here acknowledged with thanks), 
and the Rb/Sr ratio was measured 
using conventional x-ray fluorescence 
techniques. In other cases, unpublished 
data were made directly available to 
us, and these are also gratefully ac- 
knowledged. 

During the oceanic-island compila- 
tion, we attempted to retrieve data for 
basaltic rocks only, because dif- 
ferentiation effects might distort the 
Rb-Sr systematics. This screening, to- 
gether with the identification of asso- 
ciation (tholeiitic or alkali basalt) was 
based on published assignments; for 
some islands earlier literature was 
searched for chemical analyses and de- 
scriptions as an aid to proper evalua- 
tion. The rock types from some islands 
were classed as transitional, possessing 
chemical compositions between 
tholeiite and alkali basalt. Such rocks 
were grouped with the alkali basalts in 
order to keep the potentially "primi- 
tive" tholeiitic association as unclut- 
tered as possible. 

Results and Treatment 

* 7 Sr/*' ; Sr- Rb/Sr analyses were re- 
trieved for 167 alkali basalts and 109 
tholeiites. There are three principal 
ways in which correlations between 
Rb/Sr and H7 Sr/ 8,1 Sr can be evaluated: 
(1) On a local scale using individual 
data. Wherever the number of samples 
permits, intra-island (or island group) 
relationships can then be examined. (2) 
On a regional scale using individual 
data. The islands from a given ocean 
can be grouped and the relationships 
between the individual data examined. 
(3) On a regional scale using averaged 
data. The data for each island (or is- 
land group) can be averaged to give a 
single representative data point, and 
the relationships between these aver- 



age data points examined. Obviously 
treatments (2) and (3) will provide the 
same result only if each island is repre- 
sented by a comparable number of in- 
dividual analyses. However, many is- 
lands are represented by a few 
analyses, and a few islands by many 
analyses, and it is the latter that then 
dominate the relationships obtained by 
use of treatment (2). This is well por- 
trayed in a trial comparison of the 
Pacific and Indian Ocean tholeiites. 
The averaged island data are well cor- 
related (r = 0.90) and form an 1100- 
m.y. isochron. The individual data 
however are uncorrelated (r = 0.12) 
and do not define an isochron. In this 
example, the difference is due to the 
sample weight of the Hawaiian Islands 
(n = 30), an island group that displays 
no internal correlation between Rb/Sr 
and K7 Sr/ 8H Sr on even an intra-island 
scale. The manner in which the data 
are treated is essential to the discus- 
sion that follows, and we are confident 
that the average island treatment (3) 
provides an insight to the regional 
Rb-Sr systematics of mantle-derived 
volcanics that otherwise would be lost 
in the more conventional individual- 
data treatments. Consequently we will 
use only treatments (1) and (3) in our 
appraisal of the data. 

The averaged data for the oceanic is- 
lands (or island groups) are given in 
Table 8, together with the number of 
samples and the appropriate source(s) 
of the data. When plotted on an Rb/Sr 
versus * 7 Sr/ K(i Sr (isochron) diagram 
(Fig. 32) the averaged data show two 
striking features: Tholeiites and alkali 
basalts occupy distinct and essentially 
nonoverlapping fields; and each field 
indicates a pronounced positive corre- 
lation between K7 Sr/ K,i Sr and Rb/Sr. The 
tholeiite data appear more closely cor- 
related than the alkali basalt data, as 
there are four alkali-basalt data points 
which appreciably extend that field 
toward higher Rb/Sr ratios. We note 
that two of these island points (Gough 
and Rodriquez) contain only one and 
two individual analyses, respectively, 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



193 



TABLE 8. Average Rb/Sr and K7 Sr/ Hli Sr for Basalts from Oceanic Islands 
(or Island Groups) and the Ocean Floors 











Graph 


Data 


Location 


Rb/Sr 


"Sr/ Wi Sr 


N* 


No.t 


Sources^ 


Oceanic island tholeiites 












Bouvet 


0.016 


0.70369 


4 


1 


1 


Hawaiian Islands 


0.017 


0.7038 


30 


2 


2,3,4,5,6 


Iceland 


0.014 


0.70307 


13 


3 


1,7,(A) 


Kerguelen 


0.033 


0.7048 


5 


4 


8 


Kolbeinsey 


0.009 


0.70290 


1 


5 


1 


Reunion 


0.033 


0.7042 


8 


6 


8,9,(B) 


Samoa 


0.049 


0.7053 


5 


7 


10,11 


St. Pauls 


0.039 


0.7045 


7 


8 


8,12 


Ocean floor tholeiites 












Indian 


0.015 


0.70314 


8 


9 


13,14 


Pacific 


0.011 


0.70265 


15 


10 


13,15 


Atlantic 


0.008 


0.70264 


13 


11 


13,(A) 


Oceanic island alkali basalts 












Amsterdam 


0.045 


0.7039 


4 


12 


8 


Ascension 


0.066 


0.7028 


4 


13 


1,16 


Azores 


0.046 


0.7035 


26 


14 


1,17,18 


Canary 


0.032 


0.7033 


7 


15 


1,5 


Cape Verdes 


0.028 


0.7031 


4 


16 


19 


Crozet 


0.060 


0.7041 


2 


17 


8 


Easter 


0.019 


0.7030 


7 


18 


20,(C) 


Eniwetok 


0.048 


0.7038 


2 


19 


21,(C) 


Gough 


0.059 


0.7033 


1 


20 


16 


Guadeloupe 


0.024 


0.7033 


4 


21 


20,(C) 


Hawaiian Islands 


0.028 


0.7033 


32 


22 


4,5,6,(D) 


Iceland 


0.040 


0.7032 


10 


23 


1,7,(A) 


Jan Mayen 


0.074 


0.7036 


9 


24 


1,22 


Kerguelen 


0.069 


0.7052 


4 


25 


8 


Madiera 


0.036 


0.7031 


1 


26 


5 


Rodriguez 


0.077 


0.7038 


2 


27 


9,(B) 


Samoa 


0.067 


0.7054 


14 


28 


11,23 


St. Helena 


0.034 


0.7030 


18 


29 


10,24 


Tahiti 


0.047 


0.7042 


9 


30 


20,(C) 


Tristan de Cunha 


0.062 


0.7052 


7 


31 


1,5 



*N = number of samples averag 
t Reference number for figures. 
tSee Table 9. 



and that the Jan Mayen point may be 
anomalous because of the possibility 
that the island is built on continental 
crust. 

When individual samples are 
examined, only two islands, Kerguelen 
and Samoa, show an intraisland corre- 
lation with a positive slope (Fig. 33). 
The other islands show either horizon- 
tal data arrays (constant H7 Sr/ 8li Sr, 
variable Rb/Sr, e.g. Iceland, Tristan da 



Cunha, the Azores), equidimensional 
fields with significant variation in both 
directions (e.g., St. Helena, Hawaii), or 
correlations with a negative slope (Ca- 
nary Islands, Jan Mayen, Cape Verde 
Islands). We find it interesting that the 
only two islands which internally show 
positively correlated data are the two 
which have the highest average K7 Sr/ 
H,i Sr (and Rb/Sr) and we shall return to 
this point. 



194 



CARNEGIE INSTITUTION 



TABLE 9. Reference Key to Table 8 



Key No. 



Reference 



1 O'Nions and Pankhurst ( 1974) 

2 Hart (1973) 

3 Heier, Compston, and McDougall 

(1965) 

4 Powel 1 and DeLong ( 1 966 ) 

5 Bence(1966) 

6 Lessing and Catanzaro ( 1964) 

7 Hart, Schilling, and Powell ( 1973) 

8 Hedge, Watkins, Hildrett, and 

Doering(1973) 

9 McDougall and Compston ( 1975) 

10 Hedge ( 1966) 

11 Hedge, Peterman, and Dickinson 

(1972) 

12 Girod, Camus, and Vialeth (1971) 

13 Hart (1976) 

14 Subbarao and Hedge ( 1 973 ) 

15 Subbarao (1972) 

16 Gast, Tilton, and Hedge ( 1964) 

17 White, Hart, and Schilling ( 1975) 

18 Hart and Brooks ( 1974) 

19 Klerkx, Deutsch, and DePaepe ( 1974) 

20 Peterman and Hedge ( 1972) 

21 Hedge and Peterman (1970) 

22 Lussia, Berdou, Polve, and 

Vidal(1973) 

23 Hubbard (1971) 

24 Grant, Powell, Walther, and 

Burkholder(1976) 

(A) S. R. Hart (unpublished data) 

(B) C. Brooks (unpublished data) 

(C) C.E. Hedge (unpublished data) 

(D) M. A. Lanphere, G. B. Dalrymple, 

and D. A. Clague (unpublished data) 



To decide if the positive correlations 
shown in Figures 32 and 33 are statis- 
tically significant, the data of Table 8 
have been appraised by calculating the 
correlation coefficient r, and testing 
the slope of the fitted line for signifi- 
cance against zero. The regression 
treatment used to obtain slopes and in- 
tercepts was that of York (1966); the 
errors in the "age" and initial ratio are 
calculated solely from the data scatter 
and are given at 1 standard deviation 
(following recommendations of Brooks 
et al., 1972). While it is convenient to 
express the slopes of these correlated 
arrays as ages, we emphasize here that 
these "ages" need not have time sig- 
nificance. This point is discussed in 
considerable detail later in the paper. 



The statistical parameters relevant 
to oceanic islands are presented in 
Table 10. The significance levels of the 
correlation coefficients range from 95% 
(e.g., for Samoa and Kerguelen) to 
greater than 99.99% (e.g., for the com- 
plete tholeiite suite including ridge 
basalts, and for the high-density por- 
tion of the alkali-basalt field). We find 
these correlations remarkable and 
suggest that these data provide impor- 
tant and hitherto unused constraints 
relating to processes of oceanic mag- 
matism. 

Discussion 

Possible causes for the positive iso- 
chron correlations presented in the 
previous section include surficial wea- 
thering and contamination by crust, 
sediments, or sea water; and inher- 
itance of these isochron parameters 
from a heterogeneous and aged mantle. 

We believe direct weathering is 
ruled out because of the fresh and unal- 
tered nature of most of the samples in- 
volved. Other contamination processes 
might include mixing of low * 7 Sr/* ,; Sr 
magma of direct mantle derivation 
(ridge basalts) with more radiogenic 
material represented by sediments or 
oceanic crust which has interacted 
with sea water. Oceanic sediments may 
have * 7 Sr/ K,i Sr values as high as 0.717 
and Rb/Sr values greater than 1.0 
(Dasch, 1969; Biscay and Dasch, 1971; 
Church, 1971). Only limited data exist 
for altered oceanic crust, but material 
has been analyzed with Rb/Sr ratios of 
0.15-0.25 (Dasch et al., 1973; Hart, 
1969), and presumably * 7 Sr/ H,; Sr ratios 
could rise to values as high as sea 
water (0.709). Thus, both sediments 
and altered oceanic crust could provide 
a mixing component that would pro- 
duce positive correlations of the kind 
shown in Fig. 33. This mechanism has 
been described in detail in earlier pa- 
pers and generally dismissed as a sig- 
nificant effect in the petrogenesis of 
oceanic basalts (Tatsumoto, 1966; 
Powell and DeLong, 1966; Gast et al., 
1964; Grant et al, 1976). The fact that 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



195 



CO 

CD 
CO 



00 

CO 






CD 


^>^ 


o 

CD 




Samoa 

i 


i i 



0.02 0.04 0.06 0.08 0.10 0.12 



(Rb/Sr) 

Fig. 32. K7 Sr/ K,i Sr- Rb/Sr isochron plot of tholeiites (filled circles) and alkali basalts (open circles) 
from oceanic islands. Each point represents an average of available data on individual samples from a 
given island or island group. Numbers refer to the data index in Table 8. 

the lead isotope regression line for 
oceanic basalts is distinctly different 
from the regression line for oceanic 
sediment (and sea water) also argues 
against such a contamination mecha- 
nism (Sun and Hanson, 1975; Chow 
and Patterson, 1962; Church and Til- 
ton, 1973; Meijer, 1976). Furthermore, 
the marked uniformity of H7 Sr/ 8li Sr 
values for some islands (O'Nions and 
Pankhurst, 1974; Hart et al., 1973), 
and the general lack of a positive iso- 
chron (mixing-line) correlation be- 
tween individual intra-island samples 
also argues against a sediment-crust 
contamination model. As we have dis- 
cussed, only Samoa and Kerguelen 
show intra-island "isochrons," whereas 
these might be expected to be ubiquit- 
ous under the operation of a contami- 
nation model. Some islands do show 
correlations of negative slope, as would 
be expected from contamination pro- 
cesses involving carbonate. 

The conventional interpretation of 
oceanic island strontium (and lead) 



0.706 


- 






0.705 


• 


^•s--^ 


o--""^ 


0.704 
n7fH 


i 


• 
i 


Kerguelen 

i i i i 



0.02 0.04 0.06 0.C 



0.10 012 

(Rb/Sr) 



Fig. 33. Isochron plot of data for individual 
samples of tholeiite (filled circles) and alkali 
basalt (open circles) from the Samoa Islands and 
Kerguelen Island. Best-fit regression lines and 
slope values are shown. 



196 



CARNEGIE INSTITUTION 



TABLE 10. Mantle Isochrons of Rb/Sr versus Present-Day 
87 Sr/ 8H Sr for Basalts of Oceanic Regions 



Location Identity 



Age (m.y.)* 



( K7 Sr/ 8,i Sr)* 



Nt 



Oceanic Islands 

Alkali basalts: all averaged island data 

Alkali basalts: all data except points 

3,20,24 and 27 
Tholeiites: all averaged island data 
Tholeiites: all averaged island data 

plus MORB§ 
Kerguelen, tholeiites: individual data 
Kerguelen, alkali basalts: individual data 
Kerguelen, all basalts: individual data 
Samoa, tholeiites: individual data 
Samoa, alkali basalts: individual data 
Samoa, all basalts: individual data 



600±210 


0.7025±4 


20 


0.56(3) 


1190±150 


0.7017±3 


16 


0.91(4) 


1430 ±180 


0.7025±2 


8 


0.96(4) 


1570±150 


0.7023±2 


11 


0.97(4) 


485±210 


0.7041±4 


5 


0.80(1) 


248 ±20 


0.7045±1 


4 


0.99(2) 


355±90 


0.7042±2 


9 


0.83(2) 


466±(250) 


0.7044±3 


5 


0.74(1) 


584±200 


0.7039±6 


14 


0.65(1) 


510±150 


0.7041±4 


19 


0.62(2) 



* Errors given at lcr 
+ Number of samples regressed. 

t Pearson correlation coefficient. Value in parentheses indicates at what level of confidence the 
fitted regression line differs from zero (given in increments of standard deviation). 
§MORB indicates mid-ocean ridge basalts. 



isotope variations ascribes these to de- 
rivation from an isotopically heteroge- 
neous mantle (a review of this model is 
given by Hofmann and Hart, Year 
Book 74, p. 195, 1975). We consider this 
the most tenable model at present, and 
advance it here to explain the "iso- 
chron" correlations shown in Fig. 32. 
We view these positive "isochron" cor- 
relations as being inherited by oceanic 
basalt during derivation from a heter- 
ogeneous suboceanic mantle. 

Rb-Sr coherence during basalt 
genesis. Figure 32 shows that oceanic 
island tholeiites and alkali basalts 
have a similar range of 87 Sr/ 8fi Sr values 
and suggests that the separation of 
data fields is due to higher Rb/Sr ratios 
in the alkali basalts. On the average, 
the alkali basalts of Table 8 have Rb/Sr 
ratios about a factor of 2 higher than 
the tholeiites (whereas the average 
tholeiite 87 Sr/ 8fi Sr, 0.7037, is essen- 
tially identical with the average alkali 
basalt 87 Sr/ 8fi Sr, 0.7038). 

We believe this is a significant ob- 
servation and propose that the ob- 
served Rb/Sr and strontium isotope 
ratios of the tholeiites are directly rep- 
resentative of the mantle source but 
that the alkali basalts are displaced 
toward higher Rb/Sr ratios relative to 



the mantle source. The following dis- 
cussion is an attempt to justify this 
proposition. 

While it is generally accepted that a 
basalt will inherit the strontium 
isotopic composition from its source 
(Hofmann and Hart, Year Book 74, pp. 
195-210), it is less obvious that a 
basalt will also preserve the Rb/Sr 
ratio of that source (because of the ef- 
fect of partial melting and fractional 
crystallization processes). This will 
happen only if the partial melting pro- 
cess leaves no residual mantle phase 
which is a significant carrier of Sr and 
Rb, and if all fractional crystallization 
occurring after melting involves only 
phases that are poor in Rb and Sr or 
that do not fractionate Rb relative to 
Sr. In the case of tholeiites, which in- 
volve large degrees of melting, phases 
that carry Rb or Sr (such as phlogopite 
and amphibole) will probably not per- 
sist after melting, so we would expect 
the partial melt to have the same Rb/Sr 
ratio as the original source material. 
Phlogopite has been shown to have a 
field of stability well above the wet sol- 
idus of peridotite (Bravo and O'Hara, 
1975; Modreski and Boettcher 1973; 
Forbes and Flower, 1974; Boettcher et 
al., 1975; Holloway and Eggler, this 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



197 



Year Book). However, the phlogopite 
content of the mantle must be quite 
small (probably less than 1% : Hofmann 
and Hart, Year Book 74, pp. 195-210), 
and phlogopite will be consumed dur- 
ing a large degree of melting even if its 
upper stability limit exceeds the tem- 
perature at which melting occurs. 

Amphibole may also be stable during 
hydrous melting of peridotite 
(Boettcher et al., 1975; Allen and 
Boettcher, 1973). However, it is less ef- 
fective than phlogopite at fractionating 
Rb/Sr during large degrees of melting. 
For example, for 25% melting of am- 
phibole peridotite, where one half of an 
original 20% of amphibole persists 
after melting, the Rb/Sr ratio of the 
liquid will be only slightly higher (6% ) 
than that of the starting material (us- 
ing partition coefficients from Phil- 
potts and Schnetzler, 1970). For 
clinopyroxene, the partition coeffi- 
cients of Rb and Sr are sufficiently 
small (D Sr -0.08, D Rb -0.003) (Hart 
and Brooks, 1974; Shimizu, 1974), that 
residual clinopyroxene produces neg- 
ligible (<3%) fractionation of Rb/Sr 
during melting. Arguments similar to 
these led Gast (1968) to the conclusion 
that tholeiites formed by large degrees 
of melting (15-25%) would contain 
most of the Rb and Sr (and many other 
trace elements) of the original mantle 
source. 

Gast (1968) also showed that frac- 
tional crystallization is relatively inef- 
fective in fractionating Rb from Sr. 
Only crystallization of plagioclase will 
be effective in changing the Rb/Sr 
ratio, as the partition coefficients of Rb 
and Sr in olivine and pyroxene are too 
low to cause any significant fractiona- 
tion of Rb from Sr. However, limits can 
be placed on the extent of plagioclase 
fractionation by noting that most 
oceanic basalts have little or no detect- 
able europium anomaly (Schilling, 
1971; Sun and Hanson, 1975b). A de- 
tectable europium anomaly (5-10%) 
will be produced by crystallization of 
15-25% of plagioclase, which in turn 
will generate an increase in the Rb/Sr 



ratio of the liquid of from 25-60% (us- 
ing partition coefficients appropriate to 
a 75-80% plagioclase (Philpotts and 
Schnetzler 1970). We conclude that in 
the case of oceanic tholeiites, the ex- 
truded basalt very closely represents 
the 87 Sr/ 8,i Sr and Rb/Sr ratios of the 
mantle source. 

Alkali basalts, on the other hand, 
which are thought to be derived by rel- 
atively small degrees of partial melting 
(Gast, 1968; Green, 1973), may offer 
more possibility of leaving behind re- 
sidual phases such as phlogopite (Bes- 
wick, 1976) or amphibole. In these 
cases, using reasonable values for par- 
tition coefficients (Philpotts and 
Schnetzler, 1970) and phase propor- 
tions, major changes in Rb/Sr ratios 
can be brought about. For example, if a 
3% melt fraction is derived from a 
source containing 15% amphibole and 
10% clinopyroxene and the amphibole 
provides all of the melt (i.e., 12% am- 
phibole and 10% cpx remain after melt- 
ing), the Rb/Sr ratio of the melt will be 
35-65% higher than that of the origi- 
nal source (the range is due to selection 
of two different sets of partition coeffi- 
cients from Philpotts and Schnetzler 
(1970). Phlogopite is surprisingly inef- 
fective at fractionating Rb/Sr at low 
degrees of melting because of the com- 
peting effect of clinopyroxene (residual 
clinopyroxene raises the melt Rb/Sr, 
whereas phlogopite lowers it). A 3% 
melt fraction of a source containing 1% 
phlogopite and 20% clinopyroxene 
(where 0.5% phlogopite and 2.5% 
clinopyroxene enter the melt) will have 
a Rb/Sr ratio about 10% higher than 
the source material. Note that liquids 
derived from a mantle source contain- 
ing only phlogopite as a Rb-Sr carrier 
will have Rb/Sr ratios lower than the 
source. 

These model calculations are not in- 
tended to include all possible varia- 
tions of parameters but only to suggest 
approximate magnitudes of certain 
limiting cases. To explain the higher 
Rb/Sr ratio in alkali basalts than in 
tholeiites (Fig. 32) we suggest that am- 



198 



CARNEGIE INSTITUTION 



phibole, not phlogopite, is a necessary 
residual phase during alkali basalt 
genesis. For melts derived from depths 
greater than the stability limit of ara- 
phibole, reequilibration between such 
melts and shallower amphibole- 
bearing mantle may be required. 

Alternative explanations for the 
Rb/Sr and Sr isotopic pattern in 
tholeiites and alkali basalts. In the pre- 
ceding section, we gave our preferred 
interpretation of the pattern observed 
in Fig. 32. Some other conceivable ex- 
planations are: (i) Alkali basalts and 
tholeiites come from chemically dif- 
ferent mantle sources, (ii) Alkali 
basalts have undergone more frac- 
tional crystallization of plagioclase (or 
clinopyroxene) than tholeiites. (iii) 
Basalt derivation from the mantle does 
not occur under conditions of chemical 
equilibrium between melt and residue 
(O'Nions and Pankhurst, 1974). (iv) 
Tholeiites and alkali basalts are gen- 
erated sequentially by fractional melt- 
ing from the same source material 
(Beswick, 1976). 

Explanation (i) is difficult to recon- 
cile with the fact that many islands 
erupt both tholeiites and alkali basalts 
and that the strontium isotope ratios of 
both are generally very similar. Com- 
pare in Table 8, for example, the 
isotope ratios of tholeiites and alkali 
basalts from Iceland (0.7031 vs. 
0.7032), Samoa (0.7053 vs. 0.7054), 
Kerguelen (0.7048 vs. 0.7052), and the 
Hawaiian Islands (0.7038 vs. 0.7033). 

Explanation (ii) would require about 
30% more plagioclase crystallization in 
alkali basalts than in tholeiites. 
Plagioclase crystallization of 30% 
would lead to a consistent and easily 
observable europium anomaly of some 
15-20% alkali basalts relative to 
tholeiites (presuming oxygen fugac- 
ities and resultant REE partition coef- 
ficients to be the same in both cases). In 
addition, there should be a significant 
"plagioclase" effect in the major- 
element data and norms of alkali 
basalts relative to tholeiites, and this is 
also not observed (CaO and A1,0, con- 



tents of oceanic tholeiites and alkali 
basalts are quite similar). Extreme and 
unrealistic amounts of clinopyroxene 
crystallization would be required to 
produce significant changes in Rb/Sr 
ratios; the ratio is increased only 50% 
after 99% crystallization. 

Explanation (iii) is not consistent 
with the relative coherence of trace 
element patterns and the observed con- 
stancy of major element composition in 
various basalt types (Yoder, 1976). 
Disequilibrium melting has also been 
considered on the basis of existent dif- 
fusion data (Hofmann and Hart, Year 
Book 74, pp. 195-210), and judged to be 
highly unlikely. 

Explanation (iv), as proposed by 
Beswick (1976), depends on the persis- 
tence of a minor phase, phlogopite, 
through a first stage of extensive par- 
tial melting (tholeiite production). By 
retaining K and Rb in residual phlogo- 
pite, this first melt would have a lower 
Rb/Sr (and higher K/Rb) ratio than the 
bulk source. The reserve of K and Rb 
stored in phlogopite is then released 
during alkali basalt production, giving 
rise to a higher Rb/Sr (and lower K/Rb) 
ratio in the melt. As noted above, the 
persistence of a minor refractory phase 
is difficult to reconcile with an equilib- 
rium involving large degrees of melt- 
ing. No one, to our knowledge, has yet 
studied a peridotite system containing 
1% phlogopite or less, where residual 
phlogopite remains after, say, 20% of 
partial melting. Beswick's model also 
requires a considerably higher K/Rb 
partition coefficient for phlogopite- 
melt (~3) than has yet been reported 
(Philpotts and Schnetzler, 1970). 

In summary, the alternative expla- 
nations outlined in this section all ap- 
pear unsatisfactory, and we conclude 
that small degrees of melting with am- 
phibole as a residual phase during al- 
kali basalt genesis is the best explana- 
tion for the differences in Rb-Sr be- 
tween tholeiites and alkali basalt. 

Comparison of Rb-Sr and U-Pb sys- 
tematics. First, while tholeiites have 
the same range of 87 Sr/* (; Sr as alkali 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



199 



basalts, it appears that tholeiites show 
a smaller range in 206 Pb/ 204 Pb than al- 
kali basalts (Tatsumoto, 1966; Sun and 
Hanson, 1975a,b; Church and Tat- 
sumoto, 1975; Sun and Jahn, 1975). 
This may in part reflect the fact that no 
lead isotope data are available for 
tholeiites from the high 87 Sr/ 8(i Sr is- 
lands such as Samoa and Kerguelen. 
Second, the alkali basalts invariably 
have higher U/Pb ratios than the 
tholeiites. Furthermore, the observed 
U/Pb ratios of alkali basalts are higher 
than the inferred U/Pb ratios of the 
mantle source (these can be calculated 
from the lead isotope ratios), whereas 
the observed U/Pb ratios of tholeiites 
are similar to the inferred U/Pb ratio of 
the source (Tatsumoto, 1966; Allegre, 
1969). Thus the lead isotope syste ma- 
tics provide strong evidence for a frac- 
tionation of U relative to Pb during the 
derivation process of alkali basalts, 
and this supports the arguments pre- 
sented above for an increase in Rb/Sr 
ratio during alkali basalt genesis. 

Finally, for many islands the lead 
isotope ratios appear to exhibit signifi- 
cant variations, whereas the strontium 
isotope ratios appear almost constant. 
For example, Iceland shows a 4% 
spread in 2<,,i Pb/ 204 Pb ratio (Sun and 
Hanson, 1975a; Sun and Jahn, 1975), 
but only a barely detectable variation 
in 87 Sr/ 8(i Sr (O'Nions and Pankhurst, 
1974; Harte^ al., 1973). This effect may 
be largely one of experimental preci- 
sion in the isotope measurements. The 
spread in Pb ratios on Iceland requires 
a 35% range in source U/Pb ratio (av- 
eraged over 1.5 billion years); a 35% 
range in Rb/Sr ratio would produce 
over the same time interval a spread of 
87 Sr/ 8,i Sr of only 0.7029-0.7031. This is 
close to the observed spread of Sr 
isotope ratios for young Icelandic 
tholeiites but is only barely significant 
in terms of analytical precision. 

Rb-Sr mantle isochrons. If the 
tholeiite correlation of Fig. 32 is inher- 
ited from a mantle source, what mean- 
ing can be attached to the parameters 
of this correlation? One model would 



invoke a mantle that is heterogeneous 
on a scale larger than the scale sam- 
pled by individual island magmatism, 
with the magmas essentially sampling 
a continuum of mantle Rb/Sr systems. 
The correlation of Fig. 32 thus would 
be in a sense a mantle isochron, and the 
slope could have definite age signifi- 
cance. If the mantle heterogeneities 
developed at a discrete time (two-stage 
process), then the slope of the tholeiite 
correlation shown in Fig. 32 suggests 
an age of about 1.6 ±0.2 billion years 
for this process. If the development of 
the Rb-Sr heterogeneity was a continu- 
ous or multiepisodic process, then any 
"age" inferred from the slope is simply 
some average age for the process as a 
whole (it is possible to construct math- 
ematical models of such processes in 
such manner that subsequent mixing 
of the appropriate reservoirs will give 
rise to linear trends or "isochrons" 
[Church and Tatsumoto, 1975]). 

Another model would consider a 
mantle that has only two discrete com- 
positions, one at each end of the 
tholeiite trend shown in Fig. 32, with 
intermediate points representing vari- 
ous mixtures of the two compositions. 
For example, the mantle that supplies 
MORB (asthenosphere) is relatively 
uniform and distinct isotopically (Hof- 
mann and Hart, 1975), and could be the 
lower end-member. A uniform meso- 
sphere might be the upper end- 
member, with mixing of these two tak- 
ing place during mantle convection and 
diapirism. In this case, the slope of the 
mixing line can only be interpreted di- 
rectly as an age if the system from 
which the two end-members developed 
had a uniform strontium isotopic com- 
position. A simple test for such a bi- 
nary mixing process might be a linear 
relationship between 87 Sr/ 8ti Sr and the 
reciprocal of Sr concentration ( no such 
linear relation is apparent in our data). 
However, such a test is valid only if it 
is applied to mixtures of melts rather 
than mixtures of mantle source mate- 
rial. The Sr concentration of melt 
extracted from a mixed mantle source 



200 



CARNEGIE INSTITUTION 



will vary inversely with the degree of 
melting, and any significant variation 
in the degree of melting would perturb 
the expected inverse mixing relation- 
ship between isotopic composition and 
concentration. 

At this point, it is important to note 
that many oceanic islands have lead 
isotopic compositions that define linear 
trends (Sun and Hanson, 1975a, b; Sun 
and Jahn, 1975). These trends are sub- 
ject to the same uncertainty of in- 
terpretation as the Rb-Sr trend of Fig. 
32. They may be secondary isochrons, 
implying the age of a geological event, 
or they may be mixing lines. Ages cal- 
culated from the slopes of these lead 
isotope trends are in remarkable 
agreement with the Rb-Sr "age" of the 
tholeiite trend in Fig. 32. For basalts 
from sea mounts and spreading ridges 
in the N.E. Pacific, the Pb isotope trend 
gives an age of 1.75 ±0.1 b.y. (Church 
and Tatsumoto, 1975). Alkali basalts 
from oceanic islands fall along this 
same trend, with a Pb isotope age of 
about 2 b.y. (Sun and Hanson, 1975a). 

We feel this general concordance of 
U-Pb and Rb-Sr systematics is not for- 
tuitous but is strong evidence against a 
random mixing process — i.e., any mix- 
ing process in which the end-members 
started with uncorrelated isotopic pa- 
rameters. Granted this point, then 
there is little essential difference be- 
tween binary (or multicomponent) mix- 
ing models and a two-stage model with 
a continuum of subsystems: both result 
in trends that have definite age signifi- 
cance. The difficulty lies in distin- 
guishing these two-stage models from 
continuous or multistage models in 
which the trends no longer have a spe- 
cific age interpretation. 

At present, the simplest model seems 
to be that of mixing between relatively 
radiogenic mantle (mesosphere) and 
unradiogenic mantle (asthenosphere), 
with the isochrons for both Pb-Pb and 
Rb-Sr systems having a distinct age 
significance. A definitive test of these 
models will depend on further refine- 
ment of U-Pb and Rb-Sr data, compari- 
son with other decay schemes (Lu-Hf or 



Sm-Nd), and analysis of materials from 
similar tectonic settings in the past 
geologic record. 

Scale of mantle heterogeneities. We 
have previously indicated that two is- 
land groups display significant positive 
intra-island Rb-Sr isochron correla- 
tions, and we would like to suggest that 
they are also derived from aged hetero- 
geneous mantle. Two facts then need 
explanation: Why do only these two is- 
land groups retain a "mantle isochron", 
and why is the apparent age of these 
isochrons younger (360-510 m.y.) than 
that of the oceanic island ensemble as a 
whole (1200-1600 m.y.) and younger 
than both intra-island and inter-island 
Pb-Pb isochrons? Our ideas on this are 
admittedly speculative, but we suggest 
that the heterogeneities in the mantle 
exist on variable size scales, and that 
the more extreme mantle compositions 
(note that Samoa and Kerguelen are 
both the most radiogenic end-members 
of the trends in Fig. 32) may only exist 
as smaller volumes. Magmatism in a 
given island group can thus sample a 
finite variety of compositions from a 
mantle in which the heterogeneities 
exist on a smaller scale. At the same 
time, the limited size of these 
heterogeneities may allow for some 
isotopic reequilibration between the 
various systems, and thus the "age" 
which would be detected by magmatic 
sampling would be in some sense par- 
tially "reset". This reequilibration or 
internal redistribution may take place 
by severe shear during diapirism or 
during the final magmatic stage, keep- 
ing in mind that diffusive transport 
distances for Sr in silicate melts can be 
of the order of 50 meters per 10 m.y. 
(Hofmann, 1975; see also Hofmann and 
Magaritz, this report). 

Rb-Sr model calculations show that 
redistribution of Sr can lead to partial 
rotation of isochrons, with relatively 
little disturbance of the linear aspect of 
the data array (Farhat, 1975). Pb-Pb 
isochrons will not be rotated by this 
mechanism, as internal redistribution 
of Pb in these cases will simply move 
subsystems up or down the existing 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



201 



isochron. An analogy could be drawn 
here between the behavior of mineral 
isochrons and whole-rock isochrons 
during the metamorphism of an igne- 
ous pluton (the mineral systems are 
smaller than the whole-rock systems 
and may undergo isotopic reequilibra- 
tion leading to clockwise rotation of 
Rb-Sr mineral isochrons and con- 
vergence of points on mineral Pb-Pb 
isochrons). Further evaluation of these 
concepts can be made when Pb isotope 
data become available from Samoa and 
Kerguelen. 

The mantle isochrons discussed here 
are compatible with the model outlined 
in the preceding companion report by 
Brooks et al. for the development of a 
heterogeneous subcontinental litho- 
sphere by the incorporation or under- 
plating of ascending plumes or blobs. 
The main difference is that there is no 
old lithosphere in oceanic regions, and 
the blobs or partial melts derived from 
mesophere penetrate the oceanic 
lithosphere directly, thus generating 
oceanic island magmatism. 

Summary and Conclusions 

Rb/Sr ratios and * 7 Sr/ 8li Sr ratios, 
when averaged by ocean basin (for 
mid-ocean ridge basalts) and by 
oceanic island or island group, yield 
significant positive correlations on an 
Rb/Sr isochron diagram. The correla- 
tion for tholeiitic basalts is especially 
strong, and we interpret it as a mantle 
isochron that may date a major event 
(or events) of mantle differentiation, 
such as the segregation of a chemically 
isolated asthenosphere 1.5-2.0 b.y. 
ago. Alkali basalts show greater scat- 
ter and are displaced toward higher 
Rb/Sr ratios relative to the tholeiites. 
We attach no direct age significance to 
the alkali-basalt correlation because 
these basalts are derived by much 
smaller degrees of partial melting than 
are tholeiites, so that the probability of 
significant fractionation between Rb 
and Sr (and therefore change of Rb/Sr 
ratio) is much greater. 



The Rb-Sr mantle isochron for 
tholeiites is identical in age (1.6±0.2 
b.y.) to ages (so-called secondary iso- 
chrons) of oceanic rocks based on the 
isotopic composition of lead (1.8±0.1 
b.y.). This is an important finding, as it 
strengthens the conclusion that the 
isochron ages represent real geological 
events rather than the result of ran- 
dom mixing processes of no age signifi- 
cance. 

Although we do not argue that the 
data could not also be explained by a 
continuous or multiepisodic dif- 
ferentiation and remixing event, the 
simpler, first-order explanation, if cor- 
rect, may date an event of fundamental 
importance in the earth's history that 
is not easily recognized from the sur- 
face geological record. 

We conclude by stating that the ap- 
parent coherence of strontium isotope 
ratio and Rb/Sr ratio in basaltic mag- 
mas lends strong support to the cur- 
rently popular ideas of a mantle in 
which chemical heterogeneities have 
existed on a long time scale (billions of 
years). This coherence can be a power- 
ful tool in mapping mantle heteroge- 
neities and in understanding their 
time evolution. 

Acknowledgments 

We acknowledge the valuable criti- 
cisms and suggested improvements of 
D. J. Whitford and C. Allegre. We also 
gratefully acknowledge the following 
people for providing either data or 
sample powders: W. Compsotn, P. 
Pushkar, C. E. Hedge, M. P. Groton, J. 
B. Gill and M. A. Lanphere. M. Strobel 
provided invaluable assistance with 
statistical theory and relevant com- 
puter programs. This study has been 
supported in part by the National Re- 
search Council of Canada (Grant No. 
5581toC.B.). 

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Geochemistry of Late Cenozoic 

Volcanic Rocks from the Nevado 

de Toluca Area, Mexico 

D. J. Whitford and K. Bloomfield* 

Introduction 

Orogenic igneous rocks are charac- 
terized by a wide diversity of composi- 
tions ranging from island arc tholeiites 
through a spectrum of calc-alkaline to 
high-K calc-alkaline (or shoshonitic) 
varieties. Although they are usually 
associated with zones of lithospheric 
subduction, they occur in a variety of 
tectonic environments, from purely in- 
traoceanic island arcs such as the 
Tonga-Kermadec island chain to conti- 
nental margins such as the Andes of 
South America. It is the purpose of this 
report to outline the geochemistry of a 
suite of orogenic igneous rocks from a 
continental margin, to compare these 
rocks with those from a typical island 
arc, and to outline some of the 
geochemical implications and limita- 
tions on petrogenetic models. 

The major and trace element and Sr 
isotopic compositions of a representa- 
tive suite of volcanic rocks have been 
determined. The rocks, which range in 
age from Miocene to Recent, are from 
an area of approximately 2500 km 2 
centered on the volcano Nevado de To- 
luca (Bloomfield and Valastro, 1974). 
This volcanic ruin lies within the Mex- 
ican volcanic belt approximately 80 km 
west^southwest of Mexico City. It is 
situated on old, typically continental 
crust, approximately 40 km thick 
(Cummings and Schiller, 1971). The 
volcano, which has been built up on a 
series of Tertiary and Mesozoic vol- 

* Overseas Division, Institute of Geological 
Sciences, London. 



208 



CARNEGIE INSTITUTION 



canics and sediments, consists of an 
eroded central strato- volcano composed 
mostly of andesitic and dacitic flows 
with lahar and fluvial deposits, often 
capped by pumice on the lower slopes 
(Bloomfield and Valastro, 1974). 

Analytical Techniques 

Major elements have been deter- 
mined by electron microprobe on di- 
rectly fused glasses following the 
methods of Nicholls (1974a). The trace 
elements Rb, Sr, Zr, and Y were deter- 
mined by x-ray fluorescence spec- 
trometry on pressed powders following 
the methods of Norrish and Chappell 
(1967). Major and trace element 
analyses were performed at the Re- 
search School of Earth Sciences, Aus- 
tralian National University. 

The Sr isotopic composition of the 
lavas was determined at DTM on a 
6-inch automated mass spectrometer. 
All 87 Sr/ 86 Sr ratios have been nor- 
malized to an 86 Sr/ 88 Sr ratio of 0.1194, 
and all analyses have been corrected to 
an E and A SrC0 3 standard 87 Sr/ 86 Sr 
ratio of 0.70800. Repeated analysis of 
the E and A standard during the time 
in which the analyses were performed 
yielded an average 87 Sr/ 86 Sr ratio of 
0.707934 ± 0.000044 (2a of popula- 
tion. 



Major and Trace Elements 

Results. Selected representative 
analyses are listed in Table 11. The 
lavas exhibit many features character- 
istic of orogenic igneous rocks, includ- 
ing generally low concentrations of 
Ti0 2 and MgO and high concentrations 
of A1 2 3 . Concentrations of K 2 0, Rb, 
and Zr and the FeO-MgO relationships 
indicate calc-alkaline affinities (Jakes 
and Gill, 1970), which are typical of the 
volcanic rocks of continental margins. 

However, in many important re- 
spects they differ from typical island 
arc volcanics such as those found in the 
western Pacific. The Mexican rocks are 
distinctly richer in Si0 2 , Na 2 0, and 
MgO relative to FeO. In these respects, 
they are similar to some Recent vol- 
canics from the Andes (Noble et al., 
1975). Some of these differences are 
summarized in Figs. 34 and 35 

Figure 34 illustrates the frequency 
distribution of Si0 2 concentrations in 
both the Mexican lavas and a represen- 
tative suite of typical island arc lavas 
from Indonesia (Nicholls and Whitford, 
1976). The Mexican rocks have Si0 2 
concentrations ranging from 55% to 
70% with a mode of about 67%. Basalts 
(< 52% Si0 2 ) are totally absent. This 
contrasts with the Indonesian rocks, 
which have a mode of approximately 



TABLE 11. Selected Representative Major and Trace Element Analyses 





157-72 


133A-72 


91-72 


183-72 


134-72 


161-72 


9-72 


'SiO, 


55.1 


57.6 


59.4 


62.4 


64.1 


66.2 


67.0 


TiO, 


0.74 


0.96 


0.84 


0.90 


0.68 


0.57 


0.50 


Al,6, 


16.4 


16.2 


17.1 


16.7 


18.3 


16.9 


16.8 


2 FeO 


7.43 


6.49 


6.08 


5.08 


4.21 


3.62 


3.41 


MO 


<0.12 


<0.12 


<0.12 


<0.12 


<0.12 


<0.12 


<0.12 


MgO 


7.94 


6.86 


4.86 


4.04 


2.20 


1.97 


1.96 


CaO 


8.18 


6.66 


6.05 


5.11 


4.44 


4.10 


3.97 


Na,0 


3.31 


3.62 


4.11 


3.71 


4.55 


4.57 


4.47 


K,6 


0.82 


1.68 


1.55 


2.07 


1.50 


2.14 


1.98 


Mg/Mg + Fe* + 


0.66 


0.65 


0.59 


0.59 


0.48 


0.49 


0.50 


'Rb 


15.7 


34.6 


28.4 


48.2 


14.2 


51.3 


42.9 


Sr 


366 


541 


537 


448 


518 


570 


516 


Zr 


100 


182 


142 


244 


158 


158 


161 


Y 


59 


23 


21 


25 


17 


16 


15 



X A11 values in wt % normalized to 100% total. 

2 Total Fe expressed as FeO. 

3 A11 trace element concentrations expressed as ppm. 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



209 




Nevado de Toluco 
Sunda arc- Indonesia 



S1O2 (wt %) 



Fig. 34. Histograms of Si0 2 (wt % ) contents of 
analyzed lavas from the Nevado de Toluca area 
and from the Sunda arc of Indonesia (Nicholls 
and Whitford, 1976). 



vado de Toluca 
j Sunda arc -Indonesia 




Mcj/(Mg+Fe Hmol ) 

Fig. 35. Histograms of the atomic ratio 
Mg/Mg + Fe 2+ in analyzed lavas from the 
Nevado de Toluca area and from the Sunda arc of 
Indonesia (Nicholls and Whitford, 1976). Before 
calculation of Mg/Mg + Fe 2 " 1 ", ferrous iron in all 
analyses was adjusted to FeO = 0.85 £ FeO (total 
Fe as FeO). This relationship is similar to that 
determined for basaltic to andesitic liquids 
equilibrated at oxygen fugacities of the 
wiistite-magnetite buffer (Fudali, 1965); similar 
oxygen fugacities are believed to prevail in the 
upper mantle (Boettcher, 1973). 



55% Si0 2 . In this respect the Mexican 
volcanics are very similar to the 
Cenozoic lavas from Peru (D. E. James, 
personal communication). 

Figure 35 shows the frequency dis- 
tribution of the atomic ratio Mg/Mg + 
Fe 2 "^ in both the Mexican and Indone- 
sian rocks. Although the division is not 
as well defined as in Fig. 34, the Mexi- 
can rocks are, on average, more mafic 
in terms of the ratio Mg/Mg + Fe 2+ 
than those from Indonesia. Experimen- 
tal data for the partitioning of Fe 2+ and 
Mg between olivine and coexisting sili- 
cate liquid (Green and Ringwood, 1967; 



Roeder and Emslie, 1970; Nicholls, 
19746) indicate that liquids in equilib- 
rium with olivine of supposed upper 
mantle peridotite must be highly 
magnesian (Mg/Mg + Fe 2+ ^ 0.61) 
(Nicholls and Whitford, 1976). Depend- 
ing upon both total pressure and water 
content, experimental evidence indi- 
cates that these equilibrium liquids 
may have up to 60% Si0 2 (Nicholls, 
19746). These results suggest that de- 
spite being relatively silicic, rocks such 
as 157-72 and 133A-72 (Table 11) may 
represent unfractionated partial melt- 
ing products of the mantle. 

The magnesian character of the Mex- 
ican volcanics together with the Y 
abundances, which suggest heavy rare 
earth concentrations 7-30 times chon- 
dritic abundances, also argues against 
any single-stage petrogenetic model 
involving partial melting of quartz ec- 
logite (Nicholls and Whitford, 1976, 
and references therein). 

Sr Isotopes 

Table 12 lists the concentrations of 
SiO,, K,0, Rb, and Sr together with the 
ratios K/Rb, Rb/Sr, and 87 Sr/ 86 Sr. Sr 
exhibits a significant spread in isotopic 
composition, with 87 Sr/ 86 Sr ratios rang- 
ing from 0.70321 to 0.70446. Because of 
uncertainties about the absolute ages 
of the older samples, no corrections 
have been applied for the 87 Sr that has 
accumulated since eruption. Assuming 
an extreme age of 25 m.y. for the oldest 
samples, the Rb/Sr ratios are suffi- 
ciently low that the differences be- 
tween the measured and calculated ini- 
tial ratios lie within the uncertainties 
of the isotopic analyses. 

It is important to note that rocks 
which are related in both space and 
time have significantly different 
isotopic compositions. The 87 Sr/ 86 Sr 
ratios are plotted as histograms accord- 
ing to age in Fig. 36. On average, there 
is a poorly defined overall decrease in 
87g r /86g r ra ti with increasing age. 

The observed range of isotopic com- 
positions is similar to that observed in 
volcanic rocks from many other island 



210 



CARNEGIE INSTITUTION 



TABLE 12. Trace Element and Sr Isotopic Results 



Age Sample 


Si0 2 (%) 


K 2 0(%) 


Rb 


Sr 


K/Rb 


Rb/Sr 


87 Sr/ 86 Sr * 


±2cr 


Miocene 91-72 


59.4 


1.55 


28.4 


537 


453 


0.0529 


0.70374 


0.00009 


221-72 


61.0 


1.57 


30.7 


667 


425 


0.0460 


0.70385 


0.00010 


172-72 


63.5 


1.79 


20.4 


500 


728 


0.0408 


0.70420 


0.00009 


125-72 


64.0 


1.90 


38.1 


621 


414 


0.0614 


0.70368 


0.00008 


t 107-72 


64.3 


2.02 


39.6 


609 


423 


0.0650 


0.70372 


0.00011 


1 116-72 


64.6 


1.57 


25.7 


754 


507 


0.0341 


0.70321 


0.00005 


45-72 


65.4 


1.87 


46.6 


432 


333 


0.108 






147-72 


69.0 


2.50 


50.5 


542 


411 


0.0930 






Pliocene 208B-72 


64.6 


1.70 


26.7 


478 


529 


0.0559 


0.70376 


0.00005 


162-72 


65.4 


2.20 


163 


668 


112 


0.244 


.70385 


0.00007 


161-72 


66.2 


2.14 


51.3 


570 


346 


0.0900 


0.70401 


0.00005 


47-72 


67.0 


2.24 


57.8 


445 


322 


0.130 


.70391 


0.00006 


Pleistocene 157-72 


55.1 


0.82 


15.7 


366 


434 


0.0429 


0.70381 


0.00009 


186-72 


55.1 


1.15 


20.5 


657 


466 


0.0312 


0.70378 


0.00005 


132A-72 


57.0 


2.37 


32.8 


1330 


600 


0.0247 


0.70419 


0.00005 


133A-72 


57.6 


1.68 


34.6 


541 


403 


0.0640 






181-72 


60.8 


1.67 


36.3 


540 


382 


0.0672 


0.70420 


0.00007 


236-72 


62.0 


1.76 


36.1 


593 


405 


0.0609 


0.70421 


0.00007 


183-72 


62.4 


2.07 


48.2 


448 


357 


0.108 






134-72 


64.1 


1.50 


14.2 


518 


877 


0.0274 


0.70446 


0.00005 


1 10-72 


65.3 


1.97 


39.0 


681 


419 


0.0573 


0.70379 


0.00008 


44-72 


66.1 


1.99 


41.2 


525 


401 


0.0785 






153-72 


66.7 


1.92 


40.7 


516 


392 


0.0789 


0.70409 


0.00009 


196-72 


66.9 


2.04 


42.2 


503 


401 


0.0839 


0.70400 


0.00005 


Recent 89-72 


60.9 


1.71 


30.3 


644 


469 


0.0470 






tVIII/A 


63.1 


1.58 


30.1 


528 


436 


0.0570 


0.70358 


0.00005 


tVIII/5 


66.1 


1.89 


41.9 


512 


374 


0.0818 


0.70400 


0.00012 


t27-71 


66.1 


1.92 


42.2 


528 


378 


0.0799 


0.70412 


0.00011 


1108-72 


66.6 


2.05 


42.3 


550 


402 


0.0769 


0.70397 


0.00004 


t9-72 


67.0 


1.98 


42.9 


516 


383 


0.0831 


0.70408 


0.00005 



t Samples used in Fig. 39. 

Corrected to £ and A 87 Sr/ 86 Sr = 0.70800. 



i r 



n 




Recent — 



H 



Miocene - 

_l I L 



0.7035 7040 7045 

87 Sr/ 86 Sr 



Fig. 36. Histograms showing the frequency distribution of K7 Sr/*"Sr ratios in the volcanic rocks 
from the Nevado de Toluca area. Average values are indicated by arrows. 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



211 



arcs and continental margins including 
the Cascades, most of the northwestern 
Pacific arcs, and Japan (Whitford, 
1975, and references therein). Despite 
some similarities in major and trace 
element composition, the Mexican 
rocks have 87 Sr/ 86 Sr ratios generally 
lower than those from the Andes 
(James e* al., 1974a, 19746). 

Figure 37 is a plot of 87 Sr/ 86 Sr versus 
87 Rb/ 86 Sr in the analyzed lavas from 
Nevado de Toluca. Overall, there is 
clearly no correlation. However, if only 
those samples from the immediate vic- 
inity of the volcano itself are chosen, 
there is a significant positive correla- 
tion (Fig. 38). Using the method of 
York (1966), an "age" of 430 ±97 m.y. is 
indicated (A 87 Rb = 1.39 x 10" n yr" 1 ). 
It may be significant that the age of 
approximately 400 m.y. is similar to 
that observed by James et al. (19746) 
for the Arequipa and Barroso volcanics 
of Peru. The fact that the rocks from 
Nevado de Toluca have been erupted 
within an area of approximately 100 
km 2 over a time interval of up to 25 
m.y. obviously has important implica- 
tions in terms of magma genesis and 
eruption. This theme has been de- 
veloped in some detail in the report by 
Brooks et al. (this Year Book). Apart 



from this relationship, the Mexican 
lavas show no correlation between 
87g r /86g r anc i e ither major or trace ele- 
ments. 

Discussion 

Without knowledge of the nature of 
the deep crust beneath Nevado de To- 
luca, quantitative evaluation of the 
possible importance of crustal melting 
and/or assimilation is not possible. 
However, it is clear that the relatively 
low 87 Sr/ 86 Sr ratios in the volcanics and 
their high Sr concentrations place very 
severe limitations on such models and 
that the ultimate origin of these rocks 
lies in the mantle. Despite higher 87 Sr/ 
86 Sr ratios in many of the Recent vol- 
canics from Peru, James et al. (19746) 
were forced to the similar conclusion 
that crustal processes were not primar- 
ily responsible for the observed 
geochemical variations. 

This immediately raises the question 
of why volcanic rocks from at least 
some continental margins differ from 
those of island arcs. Assuming that the 
process of subduction and the sub- 
ducted lithosphere itself are similar in 
each environment, the differences in 
the erupted rocks must therefore re- 



0.7045 




0.7025 



0.1 0.2 0.3 0.4 0.5 0.6 0.7 



87 Rb/ 86 Sr 



Fig. 37. Plot of K7 Sr/ KI, Sr vs. 87 Kb/ 86 Sr in all analyzed lavas from the Nevado de Toluca area. 



212 



CARNEGIE INSTITUTION 



0. 7040 



0.7035 



0.7030 



0.7025 




430±97my 



0.70265 ±0.00012 
1 



0. 



0.2 



0.3 



S7 Rb/ 86 Sr 



Fig. 38. Plot of K7 Sr/ Kli Sr vs. "Rb/^Sr for lavas from the immediate vicinity of the volcano Nevado 
de Toluca. Two u error limits in the isotopic determinations are also plotted. 



fleet differences in the chemistry 
and/or the physical state of the mantle 
wedge above the subducted litho- 
sphere. Sacks and Okada (1974) 
showed that the mantle wedge above 
the subducted lithosphere beneath the 
Andes is itself lithospheric rather than 
asthenospheric. The possibility of 
lithospheric thickening beneath older 
continental areas has been invoked by 
Brooks et al. (this Year Book) as an ex- 
planation for certain Sr isotopic effects 
such as the existence of initial iso- 
chrons similar to that shown in Fig. 38. 
Although presently unconstrained, 
lithospheric thickening beneath conti- 
nents offers a solution to the problem of 
generating differences between the 
mantle source regions beneath island 
arcs and some continental margins 

Conclusions 

1. Late Cenozoic calc-alkaline vol- 
canic rocks from the Nevado de Toluca 
area of Mexico display features such as 
low concentrations of TiQ> which are 



characteristic of orogenic igneous 
rocks. However, they differ from typi- 
cal island arc rocks in having generally 
higher concentrations of Si0 2 and 
Na,0 and higher Mg/Mg + Fe 2+ ratios. 

2. The Mexican volcanics have rela- 
tively low 87 Sr/ 86 Sr ratios, ranging 
from 0.7032 to 0.7045. There are signif- 
icant differences in 87 Sr/ 86 Sr ratios be- 
tween spatially and temporally related 
rocks. Miocene to Recent rocks from 
the Nevado de Toluca volcano itself 
have an apparent age of 425 m.y. 

3. The low 87 Sr/ 86 Sr ratios and the 
higher Mg/Mg + Fe 2+ ratios in at least 
some of the volcanics suggest an ulti- 
mate origin within the mantle. Dif- 
ferences between continental margin 
and island arc volcanic rocks may re- 
flect lithospheric thickening beneath 
older continental regions. 

References 

Bloomfield, K., and S. Valastro, Late Pleis- 
tocene eruptive history of Nevado de To- 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



213 



luca volcano, Central Mexico, Geol. Soc. 

Am. Bull., 85, 901-906, 1974. 
Boettcher, A. L., Volcanism and orogenic 

belts — the origin of andesites, Tectono- 

physics, 17, 223-240, 1973. 
Cummings, D., and G. I. Schiller, Isopach 

map of the earth's crust, Earth Sci. Rev., 

7, 97-125, 1971. 
Fudali, R. F., Oxygen fugacities of basaltic 

and andesitic magmas, Geochim. Cos- 

mochim. Acta, 29, 1063-1075, 1965. 
Green, D. H., and A. E. Ringwood, The 

genesis of basaltic magmas, Contrib. 

Mineral. Petrol., 15, 103-190, 1967. 
Jakes, P., and J. Gill, Rare earth elements 

and the island arc tholeiitic series, Earth 

Planet, Sci. Lett., 9, 17-28, 1970. 
James, D. E., C. Brooks, and A. Cuyu- 

bamba, Strontium isotopic composition 

and K, Rb, Sr geochemistry of Mesozoic 

volcanic rocks of the Central Andes, in 

Carnegie Inst. Wash. Year Book 73, pp. 

970-983, 1974a. 

James, D. E., C. Brooks, and A. Cuyu- 
bamba, Andean Cenozoic volcanism: 
magma genesis in the light of Sr isotopic 
composition and trace element 
geochemistry, in Carnegie Inst. Wash. 
Year Book 73, pp. 983-997, 19746. 

Nicholls, I. A., A direct fusion method of 
preparing silicate rock glasses for energy 
dispersive electron microprobe analysis, 
Chem. Geol, 14, 151-157, 1974a. 

Nicholls, I. A., Liquids in equilibrium with 
peridotitic mineral assemblages at high 
water pressures, Contrib. Mineral. Petrol. 
45, 289-316, 19746. 

Nicholls, I. A., and D. J. Whitford, Primary 
magmas associated with Quaternary vol- 
canism in the western Sunda arc, In- 
donesia, in Volcanism in Australasia, pp. 
77-90, R. W. Johnson, ed., N. Y., Elsevier 
Publ. Co., 1976. 

Noble, D. C, H. R. Bowman, A. J. Hebert, 
M. L. Silberman, C. E. Heropoulos, B. P. 
Fabbi, and C. E. Hedge, Chemical and 
isotopic constraints on the origin of low- 
silica latite and andesite from the Andes 
of central Peru, Geology, 3, 501-504, 
1975. 

Norrish, K., and B. W. Chappell, X-ray 
fluorescence spectrography, in Physical 
Methods in Determinative Mineralogy, pp. 
161-214, J. Zussman, ed., N." Y., 
Academic Press, 1967. 

Roeder, P. L., and R. F. Emslie, Olivine liq- 
uid equilibrium, Contrib. Mineral. Pet- 
rol, 29, 275-289, 1970. 



Sacks, I. S. and H. Okada, A comparison of 
the anelasticity structure beneath west- 
ern South America and Japan, Phys. 
Earth Planet, Inter., 9, 211-219, 1974. 

Whitford, D. J., Strontium isotopic studies 
of the volcanic rocks of the Sunda arc, In- 
donesia, and their petrogenetic implica- 
tions, Geochim. Cosmochim. Acta, 39, 
1287-1302, 1975. 

York, D., Least squares fitting of a straight 
line, Can. J. Phys., 44, 1079-1086, 1966. 



Preliminary Rb/Sr Data on the 

Minimum Age of the Central 

Andean Precambrian Basement 

Complex 

D. E. James and C. Brooks 

Introduction 

The Andean volcanic arc obscures 
both figuratively and literally the Pre- 
cambrian basement of that part of 
South America. It is generally 
accepted, however, that the orogenic 
belt in Peru is underlain at least in 
part by the westward extension of the 
Brazilian shield (e.g., Stewart, et al., 
1974; Isaacson, 1975), although this 
has not been established for adjacent 
northern Chile (Gonzales-Bonorino 
and Aguirre, 1970). The basement 
complex (see Fig. 39), consisting 
primarily of granite gneisses and mig- 
matites, is best exposed to the west of 
and on the westernmost flank of the 
Cenozoic volcanic belt (Cobbing and 
Pitcher, 1972; Isaacson, 1975). There 
imple evidence to suggest that much of 
the central Andean volcanic arc is 
underlain by ancient sialic crustal 
remnants, although the gneiss massif 
cannot constitute the whole of the 
lower crust as Cobbing and Pitcher 
speculate — if for no other reason than 
that the crust was probably less than 
half its present thickness 100 m.y. ago 
(James, 1971). 

In studies reported in earlier Year 
Books we have concerned ourselves 
with the Peruvian coastal gneiss com- 
plex only enough to show that it cannot 
have contributed in any significant 
way to the contamination of the 



214 



76° 



12° - 



1 4° 



16' 



18° - 



74 c 



72 c 



CARNEGIE INSTITUTION 

70° 68° 



Lima 




Precambnan Basement (schematic) 



I2 C 



I4 C 



16° 



20 c 



78° 



76° 



74° 



72° 



70° 



Fig. 39. Map giving schematic outline of region in which the coastal Precambrian gneiss complex 
crops out in southern Peru. Sample localities are shown for the two samples analyzed in this study. 



Mesozoic and Cenozoic volcanic rocks. 
This neglect, however, is not to deny 
the considerable interest we have in 
knowing the age and composition of the 
basement complex. Its age is of particu- 
lar importance because it bears upon 
the question of the age of the litho- 
spheric mantle that underlies that part 
of the South American continent. We 
have proposed (James et al., 1976) that 
the anomalous isotopic character of the 
late Cenozoic magmas of the central 
Andes is inherited from ancient sub- 
continental lithosphere with a compo- 
sition significantly different from that 
of other parts of the mantle. The only 
practical means available for placing 
age constraints on mantle rocks of the 
continental lithosphere is to determine 
the history of the oldest rocks exposed 
at the surface. 

The Rb-Sr measurements given 
below are ultimately to be used in con- 
junction with zircon age measurements 
not yet completed. We consider the 
Rb-Sr ages to be minimum or 



metamorphic ages. Ages were obtained 
by cutting slabs about 1 cm thick and 
15 cm across from the samples perpen- 
dicular to the banding and measuring 
Rb and Sr concentrations (by x-ray 
fluorescence) and 87 Sr/ 8,; Sr ratios in 
each individual band. 

Results 

The results of the analyses are plot- 
ted in Fig. 40. Sample PE 210 is from 
the Charcani gneiss exposed in the Rio 
Chili River valley near Arequipa, 
Peru, which represents one of the most 
easterly exposures of the Peru coastal 
gneiss complex. Its Rb-Sr age of 
1012 ±52 m.y. is significantly greater 
than the K/Ar ages of about 650 m.y. 
obtained by Stewart et al. (1974) for 
two gneiss samples from the same gen- 
eral region of Arequipa. It is remark- 
able that PE 210 still retains a 1 b.y. 
age despite the fact that the outcrop 
from which it was collected is tightly 
sandwiched between opposing flanks of 
Misti and Chachani, two of the largest 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



215 



0.780r 



0.770 



0.760- 



0.750- 



«o° 0.740 



<x> 0.730 



0.720 



0.710 



1 1 1 1 


1 1 1 


1 1 1 


- 






^^-"o 






*^ 










o PE21CXCharcani gneiss) 
• PE111 


- 


0.7195 

1 1 1 1 


1 1 1 


1 1 1 1 





0.10 



0.30 



0.50 



0.70 



0.90 



1. 10 



Rb/Sr 

Fig. 40. Plot of K7 Sr/ Nli Sr vs. Rb/Sr for gneiss slab sections. Age and errors calculated using York 
regression. Solid dots are for PE 111, open circles for PE 210. 



andesitic strato-volcanoes of southern 
Peru. This Rb-Sr age further pushes 
back the minimum age of the central 
Andean basement complex. 

Sample PE 111 is from a coastal out- 
crop south of Pisco on the Paracas 
Peninsula. The outcrop is cut by veins 
and dikes of basaltic material, indicat- 
ing that the gneiss was intimately in- 
volved with Mesozoic or Cenozoic 
magmatism. The scatter in the Rb-Sr 
data may be due in part to the effects of 
this later magmatism. We calculate an 
age of 671 ±138 m.y. from the best-fit 
line through the data but place little 
reliance on this result. The basis of our 
skepticism is found in the considerable 
data scatter and in the progressive 
change of initial K7 Sr/ K,i Sr with position 
in the slab (Fig. 41). The points on the 
isochron diagram that tend to lie below 
the best-fit line come from one end of 
the slab; points that lie above the line 
come from the other end of the slab, 
irrespective of Rb/Sr ratio. This corre- 
lation could, for example, be caused by 
mixing of the gneiss material with in- 
jected material that is isotopically dis- 



tinct. Such an interpretation is 
suggested by prominent veining found 
in the outcrop and in the sample itself. 
Whatever the explanation, the slab is 
not in isotopic equilibrium for a given 
age from one end to the other. Because 
of this, the Rb-Sr age measurements on 
PE 111 are necessarily suspect, and 
hence the difference in isotopic age be- 
tween PE 111 and PE 210 is not neces- 
sarily real. 



References 

Cobbing, E. J., and W. S. Pitcher, Plate tec- 
tonics and the Peruvian Andes, Nature, 
240, No. 99, 51-53, 1972. 

Gonzales-Bonorino, F., and L. Aguirre, 
Metamorphic facies series of the crystal- 
line basement of Chile, Geol. 
Rundschau, 59, 979-994, 1970. 

Isaacson, P. E., Evidence for a western ex- 
tracontinental land source during the 
Devonian period in the central Andes, 
Geol. Soc. Am. Bull., 86, 39-46, 1975. 

James, D. E., Plate tectonic model for the 
evolution of the central Andes, Geol. Soc. 
Am. Bull., 82, 3325-3346, 1971. 



216 



CARNEGIE INSTITUTION 



0.7280 



O 



tr> 

CD 

^ 0.7270 

CO 



0.7260 



1 1 1 

E ^ 

o c 


1 
I / o" 

o -"^ ». 

H 




°A 


Slab 
PEIII f\ key 


- 


1 1 1 


1 





5 10 

Position normal to banding (in cm across slab) 



15 



Fig. 41. Plot of initial "Sr/^Sr vs. position in the slab. Slab cross section is shown in inset. Letters 
key layers analyzed to the K7 Sr/ K,; Sr initial ratios. Abscissa is in centimeters from center of layer A 
along a line normal to the layering (see inset). 



James, D. E., C. Brooks, and A. 
Cuyubamba, Andean Cenozoic vol- 
canism: Magma genesis in the light of 
strontium isotopic composition and 
trace-element geochemistry, Geol. Soc. 
Am. Bull., 87, 592-600, 1976. 

Stewart, J. W., J. F. Evernden, and N. J. 
Snelling, Age determinations from An- 
dean Peru: A reconnaissance survey, 
Geol. Soc. Am. Bull., 85, 1107-1116, 
1974. 



Volcanoes, Q, and Seismicity in 
Western South America 

I. S. Sacks, J. A. Snoke, and A. T. Linde 



Introduction 

Subduction zones are regions where 
oceanic plates, generated at mid-ocean 
ridges, plunge into the asthenosphere 
under an adjacent oceanic or continen- 
tal plate. This interaction generates a 



characteristic pattern of seismic and 
volcanic activity as well as a clearly 
recognizable anelasticity structure. We 
have studied the distribution of these 
features along the extensive subduc- 
tion zone of western South America, 
using their expression in the Japan re- 
gion as a reference for normal distribu- 
tion. Regions of anomalous anelasticity 
structure are also found to have diffuse 
seismicity and an absence of volcanoes. 
The study of the anelasticity struc- 
ture of various parts of the earth has 
been our major focus for a number of 
years (see Year Book 72, pp. 226-233; 
Year Book 74, pp. 256-266). In particu- 
lar, from broadband seismographs in- 
stalled in South America and in Japan, 
we have learned that there is a thick, 
low absorption (high-Q) keel beneath 
the South American continent. This 
has been confirmed by a technique 
using converted shear to compressional 
energy, which is described in the fol- 
lowing paper. From the study in the 
Japan region (Sacks and Okada, 1974), 
we learned, inter alia, that the subdue- 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



217 



tion of a cool oceanic lithosphere can 
reduce the amount of partial melt in 
the adjacent asthenosphere. This 
causes relatively rapid thickening of 
the continental lithosphere and re- 
duces the thermal gradient in the 
downgoing lithosphere. A possible 
manifestation of these effects in the 
subduction zone of western South 
America is described in this report. 

Anelasticity . The absorptive part of 
seismic energy propagation may be 
used to calculate an anelasticity struc- 
ture of a region. The effect of an ab- 
sorptive medium on seismic waves is to 
attenuate the amplitudes at higher 
frequencies more than at lower. Seis- 
mographs with a wide frequency range 
(Sacks, 1966) are used and allow accu- 
rate absorption (1/Q) determinations to 
be made. Records from local seismo- 
graphs, though of smaller bandwidth, 
are used to expand the coverage. Spec- 
tral ratio techniques (e.g., Sacks, 1968) 
are used throughout to avoid depend- 
ence on the source spectra of the 



earthquakes used for the study. 
Anelasticity is often used as a sensitive 
indicator of the lithosphere (high-Q, or 
low absorption) and the asthenosphere 
(low-Q, or high absorption). 

Anelasticity studies of the upper 
mantle have been made for regions be- 
neath Japan (Katsumata, 1960; Utsu, 
1966; Sacks and Okada, 1974), Fiji 
(Isacks et al., 1968; Barazangi and 
Isacks, 1971), South America (Sacks, 
1969, Sacks and Okada, 1974), and 
New Zealand (Mooney, 1970). The 
anelasticity structure of a typical sub- 
duction zone consists of a high-Q (Q p ~ 
1000) lithosphere overlying a low-Q 
(Q p — 200-500) asthenosphere, which in 
turn overlies the dipping subducted 
oceanic lithosphere with Q tJ of 1000 
near the surface and 3000 or more at 
depth. The lithosphere beneath Japan 
is about 70 km thick, whereas that be- 
neath South America is in excess of 300 
km. The study leading to these conclu- 
sions is fully described in Sacks and 
Okada (1974), and the gross anelastic- 
ity structures are shown in Fig. 42. 



MAT 



200 400 600 



T7~ 



»2i S 2C6V1 1 OOQ3a*U3T31I 
21V'-537S?f— 



cuz 



1400 KM 

r-J O -4" 



200 400V^ 600 800 - 10OO 1200 T400 > 



'°°<V&;. £ ;■ , 




1000 



Fig. 42. The Q,,-structure beneath Japan (left) and South America (right). Dots indicate areas of 
high Q (1000-3000); slant lines, intermediate Q- values (300-500); and cross-hatching, low Q- values 
(50-100). The small numbers in the dotted region indicate earthquake hypocenters. The region above 
the dipping seismic plane is of intermediate Q in Japan, whereas in South America it has high Q. The 
high-Q slab in Japan is continuous down to the deepest earthquakes; in South America there is a 
substantial decrease in Q below 350 km even though there are earthquakes 600 km deep. In northern 
Chile and southern Peru, a distinct low-Q region, marked C, overlies the dipping plane. 



218 



CARNEGIE INSTITUTION 



In some regions, such as northern 
Chile and southern Peru, the gross 
anelasticity structure is similar to that 
beneath Japan except for the much 
thicker lithosphere beneath South 
America. In these regions, a low-Q zone 
was found between the high-Q subduc- 
ting lithosphere and the high-Q conti- 
nental lithosphere. Intermediate depth 
(200-300 km) earthquakes occurring in 
the subducting slab were found to have 
low-Q paths to the broadband seismo- 
graph at Toconce, which is 210 km 
from the coast in northern Chile. How- 
ever, paths to the same seismograph 
originating at, or penetrating to, the 
same depth but staying wholly within 
the continental lithosphere have high 
Q. Fig. 43 shows these seismic ray 
paths. Paths to a coastal seismograph 
at Antofagasta have high Q from the 
same intermediate depth subduction 
zone earthquakes which had low Q at 
Toconce. Therefore, there must be a 
low-Q region between the subducting 
lithosphere and the continental litho- 
sphere. It is difficult to determine the 
thickness of this low-Q zone; it is less 
than 100 km but probably not less than 
40 km. This zone has a Q„ of about 400, 
which is in the range of asthenosphere 
values found elsewhere, e.g., Japan. 
The low-Q zone can be traced from 



200 400 600 




Fig. 43. A seismicity section 50 km wide, 
normal to the trench in northern Chile. The 
numbers indicate earthquake locations (accord- 
ing to CGS) and the arrows show the position of 
two seismographs whose records were compared. 
TCC (Toconce) arrivals from 200-250 km events 
show asthenosphere-type Q- values. 



Puno in southern Peru to Peldehue in 
central Chile. Seismograms from these 
stations show the same absorption dis- 
tribution as those from Toconce in 
northern Chile. 

Beneath central Peru, which has 
similar seismic station coverage, we 
have not been able to find an equiva- 
lent low-Q zone. All paths from 
earthquakes at depths less than 300 
km to the broadband seismograph at 
Cusco (340 km from the coast) have 
high Q. This seems to be true also for 
the Huancayo seismograph farther to 
the northwest. These paths are consid- 
ered in greater detail in Sacks (1969). 
The anelasticity structure described 
above is in agreement with the areal 
distribution of absorption given by 
Barazangi et al. (1975). 

In summary, south of Cusco (i.e., 
southern Peru and northern Chile) 
there is a low-Q zone between the sub- 
ducting lithosphere and the continen- 
tal lithosphere; Q values in this zone 
are similar to those found in the as- 
thenosphere. In the region north of 
Cusco, no low-Q zone between the 
high-Q subducting and continental 
lithospheres has been found, presuma- 
bly because such a zone is either very 
thin or nonexistent. 

Seismicity. The seismicity of subduc- 
tion regions is characterized by a dip- 
ping seismic plane some tens of 
kilometers thick, which is thought to 
delineate the upper surface of the sub- 
ducting lithospheric plate. There are 
no earthquakes in the asthenosphere 
on either side of this subducting plate. 
Earthquakes do occur in the continen- 
tal lithosphere above the subduction. 
The seismicity is not uniform along the 
subducting plate; regions of low or zero 
seismic release are found. In some re- 
gions, such as New Hebrides, the 
deeper part of the subducted slab ap- 
pears to be detached (Barazangi et al., 
1973). This is suggested to be the case 
beneath South America as well (Sacks 
and Okada, 1974; Snoke et al, 1974, 
and others). However, Isacks and 
Barazangi (1973) interpret the sub- 
ducting plate to be continuous, though 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



219 



having zero seismicity, in the depth 
range 350-500 km. 

For regions in which the subduction 
is not clearly delineated by the seismic- 
ity, an alternative method developed 
by Okada (1974) may be used to detect 
the upper surface of the descending 
plate. This technique relies on the effi- 
cient conversion of near vertical shear 
waves, ScS, into compressional waves 
at an inclined interface. Figure 44 
shows the ray paths giving rise to the 
phase ScSp. The time difference be- 
tween ScSp and ScS gives a locus of 
possible conversion points in the earth, 
indicated by the shaded line in Fig. 45, 
which also shows an example of the 
phase. The amplitude ratio ScSp/ScS is 
used to determine the depth, indicated 
by a circle, at which the conversion of 
shear-to-compressional energy takes 
place (Snoke and Sacks, 1975; Okada, 
1976). This method has been confirmed 
in areas where the subducting plate is 
clearly delineated by earthquakes, e.g., 
Japan and southern Peru (Okada, 
1974). 

At Peldehue in central Chile, clear 
ScSp arrivals originate from the 



deeper part (150 km) of the dipping 
seismic plane. The interpretation of the 
Arequipa (southern Peru) records indi- 
cates an interface that coincides well 
with that delineated by seismicity. 
Farther north, however, this is no 
longer the case. The Nana (central 
Peru) seismograms show clear ScSp 
arrivals (Fig. 45), which also indicate 
the probable conversion point. This 
conversion point lies on an extension of 
the shallow seismicity (near the 
trench) which seems to disappear 
below 100-km depth. This suggests 
that the subduction has a dip angle 
similar to the other regions discussed 
(southern Peru to central Chile) but is 
of low seismicity. 

There is considerable seismic activ- 
ity in what we have interpreted to be 
the continental lithosphere. This is in- 
terpreted by some, e.g., Stauder (1975) 
and Isacks and Molnar (1971), to be 
subducting oceanic lithosphere with a 
dip of about 15°. The ScSp observations 
and the trend of shallow seismicity 
persuade us that this is not the case. 

Volcanism. Volcanoes generally 
occur in subduction regions at some 



SOu Th 



4M£ R/( 




Station 



• Hypocenter 
*♦ Earthquakes 



CORE 



Fig. 44. A schematic model showing the paths of the ScSp phase (solid line) and the ScS phase 
(dotted line). The shear energy reflected from the earth's core, ScS, is efficiently converted to com- 
pressional energy at the dipping interface. 



220 



DEPTH 



100 



200 



300 
KM 



200 NNA 
_l I_ 



400 



600 

I 



CARNEGIE INSTITUTION 

800 1000 KM 



1 
1 1 1 11 

2 1112122 1 

24351443398462534321 2 

12132235C3331 
> 112 2444513 21 V 

1 1444622111 1 



14A41 1 

1 1 31 1 1 

1 1111 1222141444 1 

12 2 1221 4 412247AT71321111 
111 2 221 1421 1 

i 11 12 11 

11 11 



1 1311 1 
21 111 

1-J1 



s] 1 1 1 11 

v 3 \ 1 1 1 11311 1 1 

1~K 1 11\3 \1 11 111111 1 

11 *V 1 \ \ 1 1 113 2 11 1 

TV \ \1 11 1 1111 21 1 

1 V 1 y 1 V 1 11 222 21 1 

-„ 1 \ \ \ 1 111 12 12311 

Sty \ 1N 1 31434 31 



\$ 



\ \ 



% 



32311 
1 
31 



X \ \ \ \ 

20 sec 15 10 5 



NNA 




1965 Mar 5 SANTIAGO DEL 

m = 5.6 H = 555 KM ESTERO 

A = 19.6° AzB = 141.2° 



ScSp 



ScS 



10 



20 sec 

I 



Fig. 45. Vertical seismicity cross section and traced seismograms of ScS and ScSp arrivals at 
NNA (central Peru) with a circle representing the preferred conversion region. Isacks and Molnar's 
( 1971) gently dipping slab model is shown in the inset at the lower left. Their seismicity cross section 
is taken over the same region as the one above it, but they include only M > 5 (USCGS, 1961-1967). 



fairly well defined position relative to 
the dipping seismic plane. In both 
South America and Japan (and other 
regions as well), the earthquakes in the 
dipping seismic plane are at a depth of 
about 150 km beneath the line of vol- 
canoes. Figure 46 shows the recent vol- 
canism in South America as well as 
seismic sections through Chile and 
Peru. In central Chile, there is a well- 
defined dipping seismic plane down to 
about 150 km, above which the vol- 
canoes occur. In northern Chile, there 
is a similar situation, i.e., volcanoes 
150 km above the earthquake plane, 
except that the seismic plane extends 
down to about 300 km. In southern 
Peru, volcanoes occur about 150 km 
above the dipping seismicity, which ex- 
tends down to 250 km. 



In central Peru, however, there are 
no recent volcanoes. The northern 
margin of volcanic activity coincides 
with the disappearance of a clear low-Q 
zone between the subducting and con- 
tinental lithospheres as well as a 
poorer delineation of the subducting 
plate by the seismicity. 

Discussion 

For the interaction between the sub- 
ducting and continental lithospheres, 
three independent observations 
suggest that there is a difference be- 
tween the southern Peru- Chile region 
and that of central and northern Peru 
(see Fig. 47). 

In the southern regions, there is a 
well-developed low-Q zone, presumably 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



221 





100 


T C 
1 1 


200 KM 


tDj ^%: =•;' 


• X.'.kT ' 


200 


* 


<DM 


300 


- 















200KM 




1 


c; 


n 




I 


1 


A 














• 














100 


- 






o 


Number of 


200 


- 






• • 


earthquakes 

• 1-2 

• 3-4 


300 








• 


o > 5 



T C 

J L 



6 

A 



200 KM 






o 

100- 
200- 
300- 



Fig. 46. Recent volcanism and seismicity of western South America. The circles on the map 
indicate extinct volcanoes; the triangles, recent volcanism (reproduced from Fig. V.ll, p. 105, in The 
Earth's Crust and Mantle by F. A. Vening Meinesz, 1964). The three seismicity sections of CGS 
hypocenters on the right side of the figure are taken normal to the trench and are of equal width. T 
and C stand for trench and coast, respectively. The asterisk indicates the conversion point for the 
ScSp phase recorded by the NNA (Nana) seismograph. The seismograph stations mentioned are 
indicated. 



indicating temperatures high enough 
to cause partial melting, between the 
subducting plate and the thick (greater 
than 300 km) continental keel. Most of 
the seismicity in these regions (see 
lower two seismicity sections in Fig. 
46) is confined to the subducting plate, 
although there is also considerable ac- 
tivity in the continental block. Vol- 
canoes occur, and the seismic plane is 
about 150 km beneath them, as is the 
case in other regions such as Japan. 

In central Peru, however, the low-Q 
zone (sliver of asthenosphere) is small 
or absent, and the subducting slab is 
not characterized by seismicity below 
about 100 km. There is no recent vol- 
canism in this region (see Fig. 46). 
Note that there are earthquakes at 



depths of 150 km, but these occur in- 
land far from the subducting slab (as 
determined in the studies discussed 
above) and are considered to be in the 
continental lithosphere. 

It appears that the contact of hot and 
therefore weak material (characterized 
by low values of Q) with the subducting 
lithosphere provides the boundary con- 
ditions that give rise to a well-defined 
seismic plane and the generation of 
magma. 

We conjecture further that in the 
case of an interaction of a subduction 
zone with a thick continental litho- 
sphere, the cooling effect of the down- 
going oceanic lithosphere in the as- 
thenosphere between it and the conti- 
nental lithosphere can eventually 



222 



CARNEGIE INSTITUTION 



-> '') 




Fig. 47. Schematic drawing indicating the key features in the Chile-southern Peru region (left 
diagram) and the central Peru region (right diagram). The shaded zone indicates asthenosphere. 
Where this exists, the earthquakes (dots) are mainly confined to the subducting slab, and volcanoes 
occur above the 150-175 km deep earthquakes. Where asthenosphere is absent, as in central Peru, 
volcanoes do not occur and the seismicity is more dispersed. 



freeze out that asthenosphere. There is 
then direct contact between the sub- 
ducting lithosphere and the continen- 
tal lithosphere which inhibits vol- 
canism and causes diffuse or low seis- 
micity. Of the 58°-long subduction zone 
south of the equator in western South 
America, 25° is already devoid of active 
volcanism. 

Acknowledgments 

This research was supported in part 
by National Science Foundation Grant 
No. DES 72-01295A02. 

References 

Barazangi, M., and B. Isacks, Lateral vari- 
ations of seismic-wave attenuation in the 
upper mantle above the inclined 
earthquake zone of the Tonga island arc: 
Deep anomaly in the upper mantle, J. 
Geophys. Res., 76, 8493-8516, 1971. 

Barazangi, M., B. Isacks, J. Oliver, J. 
Dubois, and G. Pascal, Descent of litho- 
sphere beneath New Hebrides, Tonga-Fiji 
and New Zealand: Evidence for detached 
slabs, Nature, 242, 98-101, 1973. 

Barazangi, M., W. Pennington, and B. 
Isacks, Global study of seismic wave at- 
tenuation in the upper mantle behind is- 
land arcs using „P waves, J. Geophys. 
Res., 80, 1079-1092, 1975. 

Isacks, B., and M. Barazangi, High- 
frequency shear waves guided by a con- 



tinuous lithosphere descending beneath 
South America, Geophys. J. R. Astron. 
Soc.,33, 129-139, 1973. 

Isacks, B., and P. Molnar, Distribution of 
stresses in the descending lithosphere 
from a global survey of focal-mechanism 
solutions of mantle earthquakes, Rev. 
Geophys. Space Phys., 9, 103-174, 1971. 

Isacks, B., J. Oliver, and L. R. Sykes, Seis- 
mology and the new global tectonics, J. 
Geophys. Res., 73, 5855-5899, 1968. 

Katsumata, M., The effect of a seismic zone 
upon the transmission of seismic waves, 
Kenshinjiho (Q. J. Seismol.), 25, 89-95 
(in Japanese), 1960. 

Mooney, H. M., Upper mantle inhomo- 
geneity beneath New Zealand: Seismic 
evidence, J. Geophys. Res., 75, 285-309, 
1970. 

Okada, H., Geophysical implications of the 
phase ScSp on the dipping lithosphere 
underthrusting western South America, 
Carnegie Inst. Wash. Year Book 73, pp. 
1032-1039, 1974. 

Okada, H., Ph.D. thesis, Hokkaido Univer- 
sity, Sapporo, Japan, in preparation, 
1976. 

Sacks, I. S., A broad-band large dynamic 
range seismograph, in Geophys. Monogr. 
10, Am. Geophys. Union, pp. 543-553, 
1966. 

Sacks, I. S., Q for P-waves in the mantle, 
Carnegie Inst. Wash. Year Book 66, pp. 
28-29, 1968. 

Sacks, I. S., Distribution of absorption of 
shear waves in South America and its 
tectonic significance, Carnegie Inst. 
Wash. Year Book 67, pp. 339-344, 1969. 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



223 



Sacks, I. S., and H. Okada, A comparison of 
the anelasticity structure beneath west- 
ern South America and Japan, Phys. 
Earth Planet. Inter., 9, 211-219, 1974. 

Snoke, J. A., and I. S. Sacks, Determination 
of the subducting lithosphere boundary 
by use of converted phases, Carnegie Inst. 
Wash. Year Book 74, pp. 266-273, 1975. 

Snoke, J. A., I. S. Sacks, and H. Okada, A 
model not requiring continuous litho- 
sphere for anomalous high-frequency 
arrivals from deep-focus South American 
earthquakes, Phys. Earth Planet. Inter., 
9, 199-206, 1974. 

Stauder, W., Subduction of the Nazca plate 
under Peru as evidenced by focal mecha- 
nisms and by seismicity, J. Geophys. Res., 
80, 1053-1064, 1975. 

Utsu, T., Regional differences in absorption 
of seismic waves in the upper mantle as 
inferred from abnormal distributions of 
seismic intensities, J. Fac. Sci. Hokkaido 
Univ., Ser. 7 (Geophys.), 2, 359-374, 
1966. 

Vening Meinesz, F. A., The Earth's Crust 
and Mantle (Developments in Solid Earth 
Geophysics, Vol. 1), Am. Elsevier, New 
York, 1964. 



Determination of the 

Lithosphere- Asthenosphere 

Boundary Using Converted 

Waves 

/. S. Sacks and J. A. Snoke 

Introduction 

There is considerable evidence for a 
thick lithosphere under South 
America. A study of surface-wave dis- 
persion across the shield area by Alex- 
ander (1972) showed an absence of a 
low-velocity zone down to at least 200 
km, which implies that the lithosphere 
extends to at least that depth. Sacks 
and Okada (1974) found that there is a 
Q reversal at about 350 km depth 
under the continent and that the Q at a 
given depth above 350 km is approxi- 
mately the same as in the downgoing 
plate. They concluded that the litho- 
sphere extended to about 350 km de- 
pth. Also, the seismicity is absent be- 
tween depths of about 350 km and 500 
km, which is consistent with the theory 



SEISMOGRAPH 




EARTHQUAKE 



Fig. 48. Ray paths of direct S, direct P, 
and sp. 



that this depth range contains weak 
(asthenospheric) material incapable of 
supporting the stress concentrations 
necessary for earthquakes. 

In this report we describe a tech- 
nique to determine the depth of the 
lithospheric-asthenospheric boundary 
by utilizing the fact that the astheno- 
sphere probably has a partial melt, so 
that there is a significant rigidity con- 
trast at the boundary, leading to rela- 
tively efficient conversion of incident 
shear waves to transmitted compres- 
sional waves (Fig. 48; see also Sacks, 
1968 and Jordan and Frazer, 1975). 

Data 

Arrivals intermediate between P and 
S were observed at the Carnegie 
broadband stations (Sacks, 1966) CUZ 
in Peru and TCC in northern Chile, 
and at stations employing mechanical, 
horizontal seismographs at An- 
tofagasta (ANT) in northern Chile, and 
in Argentina at La Plata (LPA), Zonda 
(ZON), La Quiaca (LQA) and Jujuy 
(JUJ). The data are summarized in 
Table 13, and station locations and the 
epicentral regions of analyzed 
earthquakes are shown in Fig. 49. 
Examples of the arrivals are shown in 
Figs. 50 and 51. The arrivals were ob- 
served at epicentral distances of 4° to 
13° from events in the depth range 
526 -650 km. The locations of epicen- 
ters and seismographs are such that it 
is impossible that the intermediate 
arrivals result from conversions at in- 
terfaces related to subduction. 



224 



CARNEGIE INSTITUTION 





TABLE 13. 


List of Observed Arrivals between P and S . Locations from 








USCGS/NOAA. 57 1 


= r arrivaI - 


T„- 






Date 




Depth 




Back 


Dist. 




8T 


(d-m-y) 


Mag. 


(km) 


Station 


azim. 


(deg.) 


Comp. 


(s) 


6-7-59, „ 


6.75 


600 


LQA 


137 


6.06 


NS 
EW 


24 
24 


6-7-59 (2) 


6.9 


600 


JUJ 


125 


4.13 


NS 
EW 


22 
22-26 








LQA 


140 


5.76 


NS 
EW 


25 

22 








ZON 


53 


8.04 


NS 


21.5 


28-12-59 




650 


LPA 


327 


8.13 


NS 
EW 


30 


19-8-61 


7.0 


649 


ANT 


357 


12.85 


NS 


42.5 


29-9-62 


6.5 


575 


ZON 


46 


6.33 


EW 

NS 


20.30 


8-12-62 


5.9 


620 


ZON 


40 


7.36 


EW 

NS 


28 
15 


9-12-64 


5.9 


586 


ZON 


51 


6.24 


EW 


29 


5-3-65 


5.5 


573 


ZON 


47 


6.52 


EW 

NS 


19,29 


13-5-65 


5.1 


589 


ZON 


21 


7.25 


NS 


24.5 


30-7-65 


4.5 


526 


TCC 


98 


4.17 


EW 


15 








ZON 


28 


9.78 


NS 


14.5 


20-12-66 


5.7 


586 


ZON 


44 


7.24 


EW 

NS 


24.5 


9-9-67 


5.9 


578 


ZON 


53 


6.18 


NS 


24 


12-1-72 


5.9 


580 


CUZ 





6.67 


UD 

NS 
EW 


23 
24 




Fig. 49. Location map showing seismograph 
stations and the epicentral regions (hatched 
zones) of analyzed earthquakes. The arrival at 
CUZ was for a northern (Peru-Brazil) 
earthquake; all observations at other stations 
were for southern earthquakes. 



Figure 52 has tracings of the 
waveforms for the intermediate arrival 
(labeled sp) and directs for an event as 
recorded at TCC. The similarity of the 
two waveforms suggests that the phase 
was radiated as a shear wave. The in- 
termediate arrival has relatively more 
high frequencies than does direct S 
(Fig. 50), from which we infer that the 
phase traverses the low-Q zone below 
350 km (Sacks and Okada, 1974), with 
a steeper path through that region 
than does direct S. The amplitude of 
the arrival is largest on the vertical 
component of the seismogram (Fig. 50) 
but is in all cases much smaller than 
direct S at long periods. 

These arrivals were not observable 
on WWSSN seismograms because of 
the high magnification and limited 
dynamic range and frequency response 
of these seismographs. 

Theory and Interpretation 

The only model we have found con- 
sistent with the data is that the inter- 



DEPARTMENT OF TERRESTRIAL MAGNETISM 

, 1 MINUTE 

V, 1 SEC 



225 



V, 2 SEC 



E-W, 2 SEC 



-S, 2 SEC 



||^|^.m>/a>vw^^^^ 



P sp S. 









CUZCO 12 JAN 1972 H = 580 



A= 6.1 



5.9 



Fig. 50. Three-component, filtered P, sp, and S arrivals for an earthquake as recorded by the 
Carnegie broadband seismograph CUZ. The upper trace is the vertical component filtered at 1-s 
period. The other traces are vertical, E-W (transverse horizontal), and N-S (radial horizontal) fil- 
tered at 2-s periods. The sp arrival is strongest on the vertical and weakest on the transverse, as 
predicted. The shorter periods ( =£ s) are attenuated in the low-Q region below the lithosphere for S and 
sp. 

mediate arrivals are sp phases for tinuity, in which case the arrival is ob- 

which the S-to-P conversion occurs at served as an immediate precursor to 

an approximately horizontal interface directs. 

at about 400 km depth (Fig. 48). Previ- The variation of the displacement 

ous studies (e.g., Jordan and Fraser, amplitude ratio at sp to S with epicen- 

1975) of sp or Sp have considered only tral distance is shown in Fig. 53 (verti- 

S-to-P conversion at the Moho discon- cal component) and Fig. 54 (horizontal 



a 



b 



|l| Mil J, i 'II- i ' I'M III ,' ' 



P sp S 



P sp S 



LPA 27 DEC 1959 
h = 650km 
A=8.I3° NS 



LQA 6 JULY 1959, 
h=600 km m = 6 8 
A = 6.06° E W 



Hi 1 1 ' 
I |I)|.J ,|| j| , , 

/If 1:1 'i ; i 



P sp 



LQA 6 JULY 1959 
h=600km m = 6.9 
A = 5.76° E W 



(2) 




Interruption 
of energy 

supply 



ZON 20 DEC 1966 
h= 586 km m= 5.7 
A =7. 24° EW 



Fig. 51. Tracings ofP, sp, and S arrivals for earthquakes as recorded by mechanical seismographs 
at ZON, LQA, and LPA. (See Fig. 50 for station locations.) Note that the clear sp arrival on record d 
has the same polarity as S. 



226 



CARNEGIE INSTITUTION 



TCC E-W 30 JULY 1965 A = 4.I7 



/ ^V VN 



Sp (X5) 




Fig. 52. Tracings of filtered (at 2.5-s period) 
sp and S arrivals for an earthquake as recorded 
by the Carnegie broadband station TCC. The 
waveforms are very similar, and the polarity is 
the same. 



radial component). (The sp amplitude 
should be zero on the horizontal trans- 
verse component.) 

The ratios are calculated using ray 
theory and include the effects of 
geometrical spreading, S-to-S and 
S-to-P conversion at the interface (for 
enhancements of both 5% and 10% of 
the velocities above the interface), re- 
lative attenuation below the interface, 
and the free-surface conversion of 
amplitudes into ground displacement. 
For this example of a focal depth of 580 
km and a conversion depth of 400 km, 
there is a cutoff at about 12°. The rela- 
tive amplitude is larger on the vertical 
(Fig. 53e) than on the radial (Fig. 54e) 
but is always much less than unity. 
(Including earthquake radiation- 
pattern effects may change the 
amplitude ratios somewhat.) 



RELATIVE AMPLITUDE OF sp TO S 

VERTICAL DISPLACEMENT 

FOCUS 580km CONVERSION 400km 



RELATIVE AMPLITUDE OF sp TO S 

RADIAL DISPLACEMENT 

FOCUS 580 km, CONVERSION 400 km 




DISTANCE (degrees) 

Fig. 53. (a) The transmission coefficient forP 
transmitted with an incidents vs. epicentral dis- 
tance for sp. (b) and (c) vertical displacement 
amplitude for sp and S, respectively, based on 
geometrical spreading, (d) Relative attenuation 
of S to sp below the conversion depth at 400 km 
assuming Q s = 50 and a period of 2.5 s. (e) The 
relative amplitude of sp to S for vertical dis- 
placement. The focus is at 580 km, and 5% and 
10% enhancements for V im _IV i{m+ are included. 



DISTANCE (degrees) 

Fig. 54. Same as in Fig. 53 but showing the 
relative radial displacement amplitudes instead 
of the vertical. 

The amplitude ratios would be ap- 
proximately the same if one assumed 
instead a 5% or 10% decrease in the ve- 
locities above the interface, and for our 
data set, one cannot determine the di- 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



227 



rection of the velocity change at the 
interface on the basis of travel time 
alone (Fig. 56). It is important 
geophysically to know which it is: A 
decrease above the interface could 
imply that the usual 400-km discon- 
tinuity is a sharp discontinuity rather 
than the more usually modeled gradual 
(over 60-km) velocity increase with 
depth, while an increase above the 
interface would lead to the interpreta- 
tion that the interface is the boundary 
between the continental lithosphere 
and the underlying asthenosphere. 

The polarity of the outgoing P wave 
for an S-to-P refraction depends on the 
sign of the velocity change at the con- 
version interface. The sign of the veloc- 
ity change can be determined by com- 
paring the waveforms, or even the di- 
rections of first-motion, of the sp and 
the direct S on radial-component seis- 
mograms. Enhanced velocities above 
the interface result in iivphase 
waveforms and hence in first-motions 
in the same direction; enhanced veloci- 
ties below the interface result in oppo- 
site polarity waveforms and hence in 
first-motions in opposite directions. 
The first-motions and waveforms from 
Figs. 52 and 5 Id are in phase, indicat- 
ing that the interface is a velocity re- 
versal which we interpret to be the 
lithosphere-asthenosphere boundary. 

From the plot 8T = T sp -T P vs. epi- 
central distance (Fig. 55) we determine 
that a one-second increase in ST results 
from an increase in focal depth of about 
12 km at short epicentral distances, or 
about 7 km for larger epicentral dis- 
tances. Keeping the focal depth fixed 
and decreasing (increasing) the con- 
version depth gives similar results for 
increases (decreases) in ST. 

Figure 56 has curves of 8T vs. epi- 
central distance for different velocity- 
depth models. Compared with the nor- 
mal J-B model, a model with a sharp 
increase-with-depth of the velocities 
instead of the normal 400-km discon- 
tinuity gives essentially identical re- 
sults. A model with velocities above the 
interface enhanced by 5% (10%) gives 
similar 8Ts by up to 3 (6) seconds. For 



50r 



40- 



30- 



20- 



10 



5 10 15 

DISTANCE (degrees) 

Fig. 55. The calculated difference in sp andP 
arrival times is plotted vs. epicentral distance for 
a conversion depth at 400 km and focal depths at 
526, 580, and 650 km. The long-distance cutoff is 
indicated by a dashed line. The J-B model is 
used. 





1 


1 
FOCAL 


DEPTH km 




CONVERSION 


DEPTH IS 400 km 


Je>50 


- 






A - 








I 

/lu 

'c/> 
.q580_ 

° 














/3 






/ // 


526 


^ 


1 


1 





40 



30- 



uj 20 



PERCENT 
ENHANCEMENT 



CONVERSION DEPTH =400 km 
FOCAL DEPTH = 580 km 




DISTANCE (degrees) 

Fig. 56. Differential travel times as in Fig. 55 
for a focal depth of 580 km and a conversion 
depth of 400 km for different velocity-depth mod- 
els. Indicated are the percentages of the velocity 
contrasts for the velocity above the interface rel- 
ative to those below. 



228 



CARNEGIE INSTITUTION 



our data these differences are not sig- 
nificant. 

Figure 57 is a plot of the observed 8T 
minus the calculated 8T (using the 
normal J-B model) vs. epicentral dis- 
tance. It should be noted that the data 
include events for focal depths of 526- 
650 km and for epicentral distances of 
4°-13° and are at a large range of 
azimuths. The observations suggest a 
probable conversion depth of 400 ±30 
km. 

An indication of the thickness of the 
transition zone can be obtained from 
the high-frequency content of the arri- 
vals. From Fig. 50 we see that the arri- 
val is clearly recorded at periods as 
small as 2 seconds, so that to a 10-km 
wavelength the discontinuity is sharp. 
Hence the transition zone must be only 
a few kilometers thick. 

Concluding Remarks 

Anelasticity and surface-wave 
studies, the seismicity and the sp 
studies provide conclusive evidence 
that there is a thick lithosphere under 
South America. Thesp arrivals provide 
a high-resolution technique to deter- 
mine the lithosphere-asthenosphere 
boundary. Our results show that the 



bottom of the lithosphere is approxi- 
mately horizontal under a substantial 
part of the continent; the depth of the 
boundary is within a few tens of 
kilometers of 400 km; and the transi- 
tion zone is at most a few kilometers 
thick with a velocity contrast of 5%- 
10%. 

Preliminary investigations of other 
geographical regions indicate that sp 
and Sp arrivals exist and that conver- 
sion depths correlate with other evi- 
dence for the lithosphere-astheno- 
sphere boundary in those regions. 

Acknowledgments 

This research was supported in part 
by National Science Foundation Grant 
No. DES 72-01295A02. 



References 

Alexander, S. S., Crust-mantle structure of 
shields and their role in global tectonics, 
Trans. Am. Geophys. Union, 53, 1043, 
1972. 

Jordan, T. H., and L. N. Frazer, Crustal and 
upper mantle structure from Sp phases, 
J. Geophys. Res., 80, 1504-1518, 1975. 

Sacks, I. S., A broad-band large dynamic 
range seismograph, in Geophys. Monogr. 





1 1 1 1 




i 


1 


1 1 1 1 1 

INTERFACE DEPTH 


8 
6 


O 


- 


- 


- 


___ _- Z= 350 km 


4 


o 
cm 










FOCAL DEPTH 
a < 570 km 




? 










O 570-610 km 






o 
o o 


8 








A >6IOkm 











A_ Z=400km 


2 


o 
o 




& 






4 










D 


6 
8 


i i i i 


■ — 


o 
1 


1 


~~ — — Z = 450 km 
1 1 1 1 1 



3 4 5 6 7 8 9 10 II 12 13 14 

DISTANCE (degrees) 

Fig. 57. Plot of observed 87" minus calculated 8T vs. distance for a conversion depth of 400 km. 
Also included are lines corresponding to 8T obse , , ved = 8T calcuIated for conversion depths of 350 and 450 
km. An interface depth of about 400 km is preferred. The normal J-B model was used to obtain the 
calculated 8T. 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



229 



10, Wash., D. C, Trans. Am. Geophys. 
Union, pp. 543-553, 1966. 

Sacks, I. S., Converted waves from the 
interface at a depth of 410 km. Carnegie 
Inst. Wash. Year Book 66, pp. 32-35, 
1968. 

Sacks, I. S., and H. Okada, A comparison of 
the anelasticity structure beneath west- 
ern South America and Japan, Phys. 
Earth Planet, Inter., 9, 211-219, 1974. 

Shallow Seismicity in 
Subduction Zones 

/. S. Sacks, A. Rodriguez B.*, 
J. A. Snoke and A. T. Linde 

Introduction 

The study described here is one of the 
results of long-term collaboration be- 
tween this department and the 
Characato Observatory of the Univer- 
sity of San Agustin in Arequipa. 

The seismicity of subduction regions 
is characterized by a dipping seismic 
plane, some tens of kilometers thick, 
which is thought to delineate the upper 
surface of the subducting lithospheric 
plate. There are no earthquakes in the 
asthenosphere on either side of this 
subducting plate. Earthquakes do 
occur in the continental lithosphere 
above the subduction. The seismicity is 
not uniform along the subducting 
plate; regions of low or zero seismic re- 
lease are found. 

In this report we describe and inter- 
pret the shallow seismicity under 
southern Peru using high-resolution 
data from a net operated by the Uni- 
versity of San Agustin in 1965 (James 
et al., 1969). The hypocenters (Figs. 
59,60) delineate a 30° dipping Benioff 
zone but also indicate intraplate activ- 
ity. Of particular interest is the exis- 
tence of an aseismic wedge between the 
shallower earthquakes in the subduct- 
ing lithosphere and those in the conti- 
nental lithosphere. We show below 
that this aseismicity can be explained 
quite simply on the basis of the elastic 
forces involved in the tectonics. 

* Characato Geophysical Observatory, San 
Agustin University, Arequipa, Peru. 



Data 

During 1965 the Characato Obser- 
vatory of the University of San Agustin 
operated a net of nine high-sensitivity 
seismographs in southern Peru. The lo- 
cations of these stations, as well as 
some others used for this study, are 
shown in Fig. 58. 

The method used to locate the 
earthquakes recorded by the Arequipa 
net is discussed in detail by James et al. 
(1969). Stable hypocenters for the local 
earthquakes were determined by an it- 
erative method utilizing both theP and 
the S arrival times. In this method the 
origin time is determined indepen- 
dently from the spatial location pa- 
rameters at stations at which S is ob- 
served, and a least-squares iteration 
procedure is applied to only the three 
spatial parameters. 

Figures 59 and 60 show vertical dis- 
tributions of shallow (<100 km) 
earthquakes located by the Arequipa 
net. For comparison, Fig. 61 is a verti- 
cal seismicity cross section for the same 
geographical area as in Fig. 60, but 
using earthquake hypocenters located 



■^A 


' TRP 


V 








') 


BRAZIL 






PERU 


t - — 




%NNA 


HUA 


\ 




/O <g^. 


CSP 

• 


cus j 




■y %j 








O ?& 


ICA 


V 




s ■%>>. 


• 






/O <%&. 










Ik CRV 0NG 


SHP A o 






^%s • » / 


r\ - 


IV : 


^&" 


ATlS^st 


.ARE PUN^< 






SGPks, 


/ — 






°e« ^ 


Hr~~ AYE ''' *" 






v„ 








u 





Fig. 58. Location map of seismograph sta- 
tions used in this study. The stations in the 
Arequipa net are CUS, PUN, CRV, ONG, SHP, 
ATI, SGP, AYE, and ARE. Also used, but not 
shown, were ANT (Antofagasta, Chile) and LPB 
(La Paz, Bolivia). 



230 



CARNEGIE INSTITUTION 



100 

DEPTH 
(KM) 

200- 



T 



100 



DISTANCE (KM) 

c 



200 



300 



1911311 I I 

1245412 I I I 

I I 122 I 21 I 

I 2 1 223 I 



24 8433 ill I 

I II62ET52 

I 3< r 634l I I 2 I I I 

I 22154451421 III 
M I 1212 313 13 



400 




1965 SEISMICITY (AREQUIPA NET) 

Fig. 59. Vertical cross section of the 1965 seismicity above 100 km located by the Arequipa net for 
events inside the rectangle on the map. C and T stand for coast and trench, respectively. Of particular 
interest is the aseismic wedge just below the C. 



DISTANCE (KM) 



o? 


T 


ioo C 


200 


300 




40 


u 




i 

i 


i iii i i 

1 12 1 
II 12 21 111 11151 
14 1 111 1212 
1 221 21 1 1 1 1 1 1 III 
1 2 3 1 2 1 22 1 1 2 II 
1 II 1 1 1 41 121 1 


i 




100 


- 




1 1 123253132 5 1 

2 2 111 58«9 

1 22 24BB72I 1 1 
1 133 133 23 1 ! 1 1 
II 388332 1 1 1 1 


1 i 


- 


DEPTH 
(KM) 


- 








- 


200 












- 


















V '* . 
















NNA V 

15° '■■•tj 












mmi!- ■ 














^iat- 














S 
















o 

o> in 

30° S |v 


WwT 


t o 













1965 SEISMICITY (AREQUIPA NET) 
Fig. 60. Same as Fig. 59 for an adjacent region. 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



231 



by USCGS/NOAA for the period 
1964-1974. 

In addition, bathymetric data (Getts, 
1975), as shown in Fig. 61, indicate a 
curvature in the ocean floor on both 
sides of the trench. 

The Aseismic Wedge 

The seismicity cross sections in Figs. 
59 and 60 indicate that between the 
trench and the west there is an aseis- 
mic wedge of about 80 km in lateral 
extent and about 50 km in maximum 
depth. This feature can also be seen on 
the cross section using USCGS/NOAA 
locations (Fig. 61), but because of the 
smaller data set and less reliable loca- 
tions one could not propose the exis- 
tence of this wedge on the basis of the 
USCGS/NOAA data alone. 

A model that is consistent with the 
observations is illustrated in Fig. 63. It 
is proposed that the hatched area 
labeled "low stress zone" in Fig. 63C 
corresponds to the aseismic wedge. The 
relative plate motion leads to drag at 



the upper boundary of the downgoing 
plate and compressive stress in the 
oceanic and continental plates far from 
the subduction (Fig. 63a). (The com- 
pressive stresses in these regions are 
consistent with the focal mechanisms 
found by Stauder, 1975; Mendiguren, 
1966). Counteracting the compression 
near the subduction zone is tension 
caused by the downwarping of the con- 
tinental lithosphere due to the drag of 
subduction (Fig. 63b). The curvature of 
the interacting plates at the trench can 
be seen from the bathymetry (Fig. 62). 
The tension can be explained by 
beam-bending theory (e.g., Sokolnikoff, 
1956), according to which, near the 
upper surface of a (convexly) bent plate 
there is a tension inversely propor- 
tional to the radius of curvature. A 
superposition of the stresses indicated 
in Figs. 63a, 63b results in a low-stress 
zone (hatched regions in Fig. 63c). 
Earthquakes occur in regions of high or 
rapidly varying stress, so that the 
lower-stress zones in Fig. 63c are also 
seismically quiet regions. That this is a 



DISTANCE (KM) 
S> |! 100 C 200 



300 



400 



1001- 

DEPTH 
(KM) 

2001- 



1 
1 


1 
1 
111 II 

1 2 1 
3 11 1 

1 1 

1 
1 


1 

1 
1 


1 


t 

1 

1 




1 

2 
2 

1 


- 












- 






0° 




■{fizz 












15° 




NNAV 














Tfljjkj?. ; 
























i 






bttij&sjf? - 










o 

0) 
30° 


s 


in 




o 













USCGS/NOAA 1964-1974 

Fig. 61. Vertical cross section of the seismicity down to 100 km as located by USCGS/NOAA from 
1964 to 1974. The events are for the same area as Fig. 60. Even though the data are for a period eleven 
times as long as the Arequipa net data, there are only one-fourth the number of events. 



232 



CARNEGIE INSTITUTION 






1 | l | 1 

SEA LEVEL 


1 


1 1 I 1 
CONTINENT >, 


















• 










• 




1 






• 






BATHYMETRY 




• 
• 






OFF 




• 




2 










DEPTH 


ATICO, SOUTHERN PERU 




• 
• 




(KM) 






• 




3 






• 
• 
• 
• 




4 


• 
• 
• 




• 
• 
• 




5 


— • 
• 
• 
• 
• 




• 
• 
• 
• 
• 




6 


— • 

• 




• 
• 
• 




7 
ft 


i 1 i 1 i 


• 

• 

1 


1 1 i 1 





500 



400 



300 200 
DISTANCE FROM COAST (KM) 



100 



Fig. 62. The bathymetry off Atico, southern Peru. The vertical scale is 50 times the horizontal 
scale. The curvature of both the oceanic plate (left of trench) and the continental plate is apparent. 



COMPRESSION 






Fig. 63. Our model for explaining the aseismic wedge, (a) Stresses due to relative plate motion 
inferred from fault-plane solutions and frictional drag, (b) Tension due to downwarping of the plates 
at the trench due to the drag of subduction. (c) The superpositions of the stresses from a and b result in 
low-stress (hatched) regions as shown. 









DEPARTMENT OF TERRESTRIAL MAGNETISM 



233 



global phenomenon can be seen from 
high-precision seismicity studies such 
as were made in the Aleutians (Eng- 
dahl and Sleep, 1976), which also show 
a shallow aseismic wedge. 

Shallow earthquakes are therefore 
not expected between the trench and a 
region fairly well inland. In most sub- 
duction regions, the coast (C in Figs. 
59, 60) is earthquake free. 

Acknowledgments 

This research was supported in part 
by National Science Foundation Grant 
No. DES 72-01295A02. 

References 




ILLUMINATED 
REGION, LARGE 
AMPLITUDE 




ILLUMINATED 
REGION, SMALL 
AMPLITUDE 



Engdahl, R. E., and N. H. Sleep, Seismicity 
and stress beneath the central Aleutian 
Arc, Trans. Am Geophys. Union, 57, 329, 
1976. 

Getts, T., Preliminary contour map, Hawaii 
Institute of Geophysics, unpublished, 
1975. 

James, D. E., I. S. Sacks, E. Lazo, L. and P. 
Aparicio G., On locating earthquakes 
using small networks, Bull. Seis. Soc. 
Am., 59, 1201-1212, 1969. 

Mendiguren, J. A., Focal mechanism and 
pressure axis direction in Central 
America and South America, USEE 
Bull., 3, 1-20, 1966. 

Sokolnikoff, I. S., Mathematical Theory of 
Elasticity, McGraw-Hill, New York, 1956. 

Stauder, W., Subduction of the Nazca Plate 
under Peru as evidenced by focal mecha- 
nisms and by seismicity, J. Geophys. Res., 
80, 1053-1064, 1975. 



A Model for Laterally 

Heterogeneous Velocity 

Structure at the Base of the 

Mantle 

J. A. Snoke and I. S. Sacks 



Introduction 

Sacks and Beach (1974) showed that 
the core phase PKPAB (Fig. 64), which 
has near-grazing incidence at the 
mantle-core boundary, has an 



Fig. 64. Ray paths for the phasePKPAB lead- 
ing to large-amplitude (a) and small -amplitude 
(b) arrivals due to scattering in the lower mantle 
near the core-departure crossing. 

amplitude anomaly that correlates 
with the regions of the core-mantle 
boundary crossings. Regions of high 
and normal transmission were iden- 
tified, and a correlation was found be- 
tween these regions and the isostatic 
anomalies in the earth's gravity field 
as determined from satellite orbits. We 
have used a ray-theory approach to 
model regions at the base of the mantle 
to determine the characteristics of the 
velocity structure that gives rise to 
large-amplitude PKPAB arrivals. 

It was suggested by Sacks and Beach 
that there are two types of regions at 
the base of the mantle: regions with 
small convection cells (A in Fig. 65) 
and regions that are involved in 
large-scale (whole mantle) convection 
(B in Fig. 65). In this report we de- 
scribe, for the A-type region, a 
velocity-structure model that leads to 
results consistent with the observed 
amplitude anomalies. 

The Model 
Figure 66 shows the velocity- 
structure model used for the bottom 
150 km of the mantle at either the 
core-entry or core-departure region for 



234 



CARNEGIE INSTITUTION 




nTiDluTr) 




Fig. 65. Models for the lowermost mantle. (A) Nonconvecting whole mantle. If the whole mantle 
does not convect, the lower viscosity region (Sacks, 1972) at its base probably will, the convection cell 
being bounded at the top by static material with a higher viscosity. (B) Whole-mantle convection (or 
plumes). 




MANTLE 



VELOCITY MODEL AT 
BASE OF MANTLE 



V(X,Z)=V (Z) 



He 



I + COS(2tt-^ +</>)] j 



Fig. 66. The velocity model for the lowest 150 km of the mantle used in this study. x u is fixed 
relative to the source. The minimum velocity at a given depth z is V (z), and the maximum is V (z) 
(\+A). 



the core phase PKPAB , the phase that 
has the amplitude anomaly when it 
crosses a type-A region. Figure 67 is a 
comparison of our model and the struc- 
ture inferred by the small-scale convec- 
tion cells. 

Our model superimposes a cosine 
lateral (x) variation on a normal 
velocity-depth model (the B 1 model 
developed by Jordan and Anderson, 
1974). In our model the lateral varia- 
tion is independent of depth in the low- 
ermost 150 km of the mantle. The pa- 
rameter jc„ is kept fixed relative to the 
source, so there are three variables in 
the model (Fig. 66): A is the maximum 
deviation of the velocity from the nor- 



mal value at a given depth (all devia- 
tions are positive); L, the "wavelength" 
of the cosine, is a measure of the lateral 
extent of a cell; and $, the "phase" of 
the cosine, gives the lateral position of 
the cells relative to the source. Only 
two-dimensional structure was consid- 
ered, and only great-circle paths were 
calculated directly. The simulation of 
the third dimension is discussed below. 
For each model run A , L, and $ were 
fixed for either a core-entry or core- 
departure crossing. Then ray paths 
were calculated to include all PKPAB 
arrivals with epicentral distances 
greater than 158°. Relative amplitudes 
were calculated by comparing the 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



235 




MANTLE 



Fig. 67. Comparison of our velocity model and the convection cells it is intended to simulate. The 
velocity variations in the model are independent of the depth. 



change in epicentral distance for a 
given change in departure angle from 
the source (Fig. 64). The two-dimen- 
sional ray-tracing routine is based on a 
method developed by Jacob (1970). 

In a laterally homogeneous medium, 
the ray parameter p = dT/dA is con- 
stant along a ray path, where T is the 
travel time and A the epicentral dis- 
tance between any two points along the 
ray path. Lateral heterogeneities re- 
sult in variations in p. Rays traveling 
to a single point at the earth's surface 
can come from a region at the base of 
the mantle of extent d, which can be 
determined if the variation in p is 
known. We simulate off-great-circle 
paths by which a ray travels from 
earthquake -to seismograph by assum- 
ing that the deviations off the great 
circle are similar to those along it — 
that is, by superimposing, with a suit- 
able weighting, runs of a range in 4> of 
8</> = 360d/L. 

The lateral position of the cells rela- 
tive to the source is probably not de- 
terminable, so we average over $. 
Hence our model is not strictly deter- 
ministic, and the comparison with data 
must be on a probabilistic or statistical 
basis. 

The observed AB amplitudes are de- 
fined relative to the PKBDF amplitude 
for the same earthquake-seismograph 
pair (Fig. 68). The DF branch has ap- 
proximately normal incidence at both 



core-mantle crossings, which makes it 
far less sensitive than the near-grazing 
AB rays to any lateral velocity struc- 
ture at the base of the mantle. Our 
criterion for large AB amplitude is that 
it be 1.5 times the size of the corre- 
sponding DF amplitude. Our compari- 
son with the data will be on the basis of 
the percentage of large amplitudes and 
the ability of the model to predict the 
observed frequency dependence of the 
amplitude anomaly. 

Results 

For each run, the lateral heterogen- 
eities cause multiple arrivals at most 
epicentral distances (Fig. 69). Suffi- 
ciently small steps in departure angle 
were taken to ensure that no aliasing 
occurred. The difference in arrival 
times of the mutliple arrivals is small 
enough (<0.2 s for a 1.5% heterogene- 
ity model) that, for periods considered, 
the arrivals can be added coherently. 
We considered models with L = 40 km, 
150 km, and 600 km; and A = 0.008, 
0.015, and 0.03. The analysis will be 
described in detail for the model we 
found to be most successful: A = 0.015 
and L = 150 km. (The rejected models 
and the criteria for their rejection will 
be discussed later.) So far we have 
analyzed in detail applications of the 
model to only the core-departure re- 
gion. 



236 



DF' AB 

SP 1 



DF 



CARNEGIE INSTITUTION 



CAR 24 MARCH 1970 A = 163° 




AB 



PTO 7 OCTOBER 1966 A = 160° 



-^V\/y\^A/*^^•^^•v^A-^»«'■^'W\/-^A/*~■^^^^ 



LP ,\ 



MAL 7 OCTOBER 1966 A = 164° 



DF ¥ U AB 



Fig. 68. Tracings oiPKPAB and PPBDF arrivals from long-period and short-period seismograms 
illustrating the possible frequency dependences for large A5-amplitude anomalies. For the top pair 
the AB amplitude is large on only the short-period record; for the middle pair it is large on both, and 
for the bottom it is large on only the long-period record. For an explanation in the context of our 
model, see text and Fig. 74. 





l50XI50Km,l.5%, 0=0° 




to 


[V 1234.5 - (A- 159) X 4. \\ 




<L> 

to 






TRAVEL TIME 

O 
ho 


• 


/ 










- 


Q 
UJ 
















S oi 

C .08 










t i-fif 


- 


•0 6 






- 


.0 4 






- 


.02 


1 1 


— 



159.0 



159.5 
EPICENTRAL DISTANCE (degrees) 



160.0 



Fig. 69. Plot of part of the travel-time curve for a model run with L = 150 km, A = 0.015, and (/> = 
0°. The circles are model-derived points, and the lines connecting the circles indicate the continuous 
variation in the input ray parameter/} = clT/dA. Note that the scatter in arrival time is <0.2 sec. 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



237 



Figure 70 contains a summary of the 
model data. Plotted are the large- 
amplitude regions at epicentral dis- 
tances for {/> ranging from 0° to 330° in 
30° steps. For each run (a fixed (f>) one 
sees there are brightly illuminated re- 
gions as well as small-amplitude re- 
gions, as is required by energy conser- 
vation. Some of those bright regions 
are less than 0.1°; others are as large as 
a half degree. For different 0s the pat- 
terns are similar, but the position of 
the bright regions shifts. For these 
runs the standard deviation in the 
change in the ray parameter/? was 0.04 
s/degree, which implies a source region 
at the base of the mantle of extent d ~ 
200 km. Following our technique for 
simulating off-great-circle paths, this 
results in a range in cf> to be superim- 
posed of ±240°. The resulting model 
probability for large amplitudes is 
34%, and this result is insensitive to 
the central value of (/>. 

In the data analysis, event- 
seismograph pairs were selected so 
that the data set included a large area 
at the core-entry crossing for the rays 
for a given core-departure region (Fig. 
71). The data therefore include rays 



with traversal regions of both type A 
and type B on the core-entry side. As- 
suming that the model results for the 
core-entry side will be similar to our 
results for the core-departure side and 
that the two types of regions have ap- 
proximately equal probability, we get 
an additional 11% in expected large- 
amplitude arrivals and thus an overall 
expectation of 45%. The observed 
large-amplitude-arrival probability for 
type A regions is 48% (Fig. 72). 

A model with L = 600 km and A = 
0.015 led to a 7% probability of large 
amplitude arrivals, and a model with L 
= 40 km and A = 0.015 led to a proba- 
bility approximately the same as the 
model withL = 150 km and A = 0.015. 
The probability for L = 600 km can be 
increased by increasing A , but in order 
to get results comparable with the 
other model, A must be large enough to 
cause measurable shifts in the travel 
times, which are not observed. Hence 
we reject L = 600 km. 

The other two models give different 
predictions for the size of the illumi- 
nated (large-amplitude) regions on the 
earth's surface (Fig. 73). The L = 150 
km model is capable of explaining the 



300 

"^ 240 
a> 

% 180 



^Ql 



120- 



60- 



□ 



am 
II 



I I 



□ 






1 




D 


II 


a 




DD 



II 

I 



1 1 



□ 



II 



I 



D 

D I 
I II I 



150 XI50Km, 1.57. 

Large amp. probability 34% 

S.D. A«l.8° 

S.D. dT/dA= .04 sec /degree 



155 



160 

EPICENTRAL DISTANCE (degrees) 



165 



Fig. 70. Results of model runs for A = 0.015 and L = 150 km and different values of (/>. Large- 
amplitude arrival regions on the earth's surface are indicated by rectangles. Regions smaller than 
0.1° are filled rectangles. See the text for the method of determining the large-amplitude-arrival 
probability. 



238 



CARNEGIE INSTITUTION 




Fig. 71. Technique used to resolve the ambiguity of the two core crossings. The dotted lines 
indicate areas at the base of the mantle whose transmission properties are being investigated. L 
indicates low-amplitude transmission, and H indicates high amplitudes. Circles are seismographs; 
asterisks, earthquakes; full lines show mantle paths; and broken lines, paths in the core. The average 
amplitude for theL region sampled is {LL + LH + LL + LH)/4. If the adjacent// region was sampled, 
the average would be (HL + HH + HL + HH)/4, which is clearly larger. 



different types of observations (Fig. 68; 
see also Fig. 74): (1) where the 
amplitude of the AB is large on the 
short-period but not on the long-period 
seismograms (because the extent of the 
illuminated region is small compared 
to the wavelengths for which the long- 
period seismometer is sensitive), and 
(2) where the amplitude is large on 
both short-period and long-period in- 
struments (because the extent of the il- 
luminated region is above the 
threshold for the long-period seis- 
mometer). For the L = 40 km model 
there are no illuminated regions larger 
than 0.1° in length (Fig. 73). We can 
therefore rule out the L = 40 km model 
because it cannot explain the observa- 
tions for which the AB arrivals are 
large on both long-period and short- 
period seismograms (e.g., Fig. 68). 

Model runs for the PKPDF branch 
using the same parameterization as for 



the PKPAB branch produced results 
that were essentially the same as for 
the laterally homogeneous {A = 0) 
case. 

Concluding Remarks 

We have shown, therefore, that in 
some regions at the base of the mantle 
cells of 150 km x 150 km with a 
maximum lateral velocity variation of 
1.5% can explain the observed 
amplitude anomaly of PKPAB arrivals. 
The possibility of convection occurring 
is proportional to the cube of the cell 
size, so it is encouraging for the convec- 
tion model that smaller cells are not 
required to fit the seismic data. The 
limits on the cell size and velocity vari- 
ations obtained from this study impose 
constraints that will be invaluable in 
studies of convection and heat flow at 
the core-mantle boundary. 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



239 




SIZE OF AB ARRIVAL 

Fig. 72. Number versus size of the PKBAB 
amplitude arrival for a type-A (small-scale con- 
vection) region for the core-departure crossing of 
the PKPAB rays. The percentage of large arri- 
vals is 48. 



Acknowledgments 

This research was supported in part 
by National Science Foundation Grant 
No. DES 72-01295A02. 



References 

Jacob, K. H., Three-dimensional seismic 
ray tracing in a laterally heterogeneous 
spherical earth, J. Geophys. Res., 75, 
6675-6689, 1970. 

Jordan, T. H. and D. L. Anderson, Earth 
structure from free oscillations and travel 
times, Geophys. J. R. Astron. Soc, 36, 
411-459, 1974. 

Sacks, I. S., Viscosity and Q, Carnegie Inst. 
Wash. Year Book 71, pp. 227-229, 1972. 

Sacks, I. S. and L. Beach, Lateral 
heterogeneity at the base of the 
mantle — an indication of whole mantle 
convection, Carnegie Inst. Wash. Year 
Book 73, pp. 1020-1032, 1974. 



50 




I 


1.5% Heterogeneity 
Height ofcell = l50Km 




30 




Short period seism. 


Long period seism. 


- 


10 






\ 150 X 150 Km 


- 


5 


- 






- 


3 
1 




1 


40X150 Km \ 





0.001 



0.01 0.1 

ILLUMINATED DISTANCE (degrees) 



1.0 



Fig. 73. Number versus size of the illuminated (large-amplitude) region on the earth's surface for 
L = 150 km and L = 40 km. Also shown are the lower limits of the sensitivity of the WWSSN 
long-period and short-period seismometers for an arrival to be recorded as large. The fact that there 
are observed large amplitudes on long-period seismograms rules out L = 40 km. 



240 



CARNEGIE INSTITUTION 




Fig. 74. Explanation for the observed frequency dependence of the large AB amplitudes (see Fig. 
68 for examples). A large amplitude on only short-period records (A,) occurs for a triplication over an 
epicentral distance range of less than 0.1°. A large amplitude on only the long-period record (A 2 ) 
results from diffractive scattering from the caustic. Such arrivals were not considered in this study. 
Large amplitudes on both short-period and long-period records (A,) result from a triplication over an 
epicentral distance range greater than 0.1° in extent. 



Single Site Phase Velocity 
Determinations 

/. S. Sacks, J. A. Snoke, J. R. Evans, G. C. P. 
King* and R. J. Beavan* 

The lithosphere beneath oceans is 
generally less than 120 km thick. Evi- 
dence is mounting that beneath large, 
old continental areas, such as South 
America, the lithosphere is very much 
thicker. Anelasticity studies by Sacks 
and Okada (Year Book 72, pp. 226-233) 
and the interpretation of converted sp 
phases (Sacks and Snoke, this Year 
Book) indicate that the lithosphere be- 
neath South America must be about 
400 km thick and that it overlies as- 
thenosphere which has much higher 
absorption (lower Q). 

A method to determine subcrustal 
velocity structure utilizes surface-wave 
phase velocities (see Fig. 75). Phase ve- 
locities are traditionally obtained by 
using sets of widely spaced seismome- 
ters which record simultaneously. The 
velocity determination follows then 
from measurements of the travel times 
of different frequencies over known 
distances. For low-frequency (long- 



* Department of Geodesy and Geophysics, 
Cambridge University, Cambridge, England. 



wavelength) waves, the separation of 
the instruments must be large to per- 
mit satisfactory resolution, and even at 
shorter wave lengths, large spacing is 
an advantage as it prevents velocity es- 
timates being contaminated by multi- 
pathing resulting from lateral refrac- 
tion and diffraction. The best results 
for phase velocity using these methods 
are obtained only when two or more 
stations lie close to a greater circle in- 
cluding the seismic epicenter (Brune 
and Dorman, 1963). 

The lithosphere asthenosphere 
boundary is characterized by a velocity 
reversal and beneath large, old conti- 
nents this boundary may be as deep as 
400 km (see Sacks and Snoke, this Year 
Book). Because of the long-period limi- 
tations in the determination of phase 
velocities by the technique described 
above, Alexander (1972) was able to 
show only that there is no velocity re- 
versal under the shield area in South 
America above 200 km. 

We describe a method whereby it is, 
in principle, possible to determine the 
phase velocity of any seismic wave 
from the simultaneous measurement of 
the wave amplitude (or its time deriva- 
tive) and its spatial derivative (strain 
or tilt) at the same site. Mikumo and 



DEPARTMENT OF TERRESTRIAL MAGNETISM 

PHASE VELOCITY DETERMINATION 



241 



TWO STATION METHOD 



SEISMOMETERS 4 AT A AND B 



C(T) 



t B -t. 



SINGLE SITE METHOD 




SEISMOMETER 
AND STRAINMETER 
AT SINGLE SITE 



C(T)- 



DISPLACEMEN T 
STRAIN 



Fig. 75. Sketch illustrating the two methods for determining the phase velocity: the single-site 
method discussed in this report and the traditional two-site method. T, the period, = 2tt/<d. 



Aki (1964) have previously used this 
technique for the combined analysis of 
records from a horizontal seismometer 
and a horizontal strainmeter at a 
single station. For the five earthquakes 
studied they found good agreement be- 
tween measured and predicted phase 
velocities for body waves and in some 
cases for surface waves. 

Rodgers (1968) has shown that for 
long periods (>100 s) horizontal accel- 
eration and tilt cannot easily be sepa- 
rated instrumentally, and recent work 
by King et al. (1976) indicates that 
near-station crustal inhomogeneity 
may give rise to strain-strain and 
strain-tilt interactions. Thus, records 
from long-period horizontal strain- 
meters are heavily contaminated by 
tilt and strain. Records written by ver- 
tical seismometers do not suffer this 
defect. We rederived (Sacks et al., 1976) 
the principle of single-site velocity de- 
termination, taking into account the 
strain-coupling effects noted by King et 
al. and using vertical acceleration. Our 
discussion is presented for Rayleigh 
waves and assumes that the ellipticity 
of particle motion at the surface is in- 
dependent of frequency. (The validity 
of this assumption may be tested if 
there is a site-calibrated horizontal 
seismometer at the same site.) 

In this report we present and discuss 
a method for phase velocity determina- 
tion employing a vertical seismometer 



and a volume strainmeter. The deriva- 
tion for this case and for the more gen- 
eral case of a vertical seismometer and 
a linear strainmeter are to be found in 
Sacks et al. (1976). 

From Sacks et al. (1976) the phase 
velocity c as a function of angular fre- 
quency, co, can be written 



c(co) = YW(co)/co, 



(1) 



where Y is a constant (assuming the 
ellipticity to be independent of fre- 
quency) and 



W(to) = u :i (co)/D(co), 



(2) 



where u :i is the vertical displacement 
and D the dilatation. The ~ indicates 
that the quantity is averaged over 
areas of extent large compared to the 
size of the local heterogeneities but less 
than or equal to the wavelengths of 
interest. If the region^were uniformly 
layered, u A = u :i and D = D so that W 
from Equation 2 would be given in 
measured quantities. However, if there 
are local heterogeneities, King et al. 
(1976) have shown that, to a good ap- 
proximation 



and 



u-> (co) = u : , (co) (3) 



D(d,co)=A(d)D(co), (4) 



242 



CARNEGIE INSTITUTION 



where 6 is the back-azimuth of the 
Rayleigh wave. Hence from Equations 
2-4, Equation 1 can be rewritten in 
terms of the measured quantities u :i 
and/): 



where 



c((o) = YA($)W(e,ai)/(o, (5) 
W(d, o)) = u :i ((o)/D(6, w). (6) 



There is therefore an azimuthally 
dependent factor Aid) in the expression 
for the phase velocity due to the pre- 
sence of local heterogeneities. Figure 
76 is a sketch of log 10 c(^>) vs. co for dif- 
ferent azimuths based on Equations 5 



Log, [EM] 



Fig. 76. Sketch to show the expected rela- 
tionship of log,„[c(w)] versus w for Rayleigh 
waves arriving from three different directions 0, , 
6>, and :i . The curves are shifted vertically rela- 
tive to each other by the presence of local 
heterogeneities, but their shape remains un- 
changed. 



and 6. If the region is uniformly 
layered the curves for different 
azimuths should superimpose. The pre- 
sence of local heterogeneities, on the 
other hand, would result in the curves 
being displaced from one another. 
Hence determining the phase velocity 
for Rayleigh waves from a number of 
different azimuths allows an estimate 
of the local heterogeneity. (An alterna- 
tive technique for determining the site 
effects using tidal observations was de- 
scribed by King et al., 1976.) 

The shape of the curves in Fig. 76 is 
independent of Aid). By normalizing 
these curves at some frequency and av- 
eraging over determinations for a 
number of different azimuths, it should 



be possible to reduce the effects of 
signal-generated noise in the determi- 
nation of the spectral shape of the 
phase velocity. If c(&>) can be deter- 
mined at any frequency o> by other 
methods (such as from crustal refrac- 
tion), Equation 5 can be rewritten as 



c(a>) = c(w o )W(0,w)/W(0,a>„) 



(7) 



Hence the phase velocity can be deter- 
mined for all frequencies without 
knowing the ellipticity or Aid). 

Single-site phase velocity determi- 
nations have two main advantages: 
There is no inherent limitation in the 
maximum period such as is caused by 
finite station spacing using the tradi- 
tional techniques; and phase velocities 
can be averaged for any azimuth, 
which will increase the precision of the 
velocity determinations, particularly 
at longer periods. This technique will 
be particularly useful in studies of the 
structure of continental lithospheres 
with thicknesses up to 400 km. 

The implementation of this tech- 
nique requires seismometers (ac- 
celerometers) and strainmeters with 
high sensitivity at long (>400 s) 
periods. Suitable strainmeters have al- 
ready been developed at the Depart- 
ment of Terrestrial Magnetism (see 
Year Book 68, pp. 448-452; Year Book 
69, pp. 426-430; Year Book 70, pp. 
336-340; Year Book 71, pp. 321-325; 
Year Book 72, p. 326; Year Book 73, 
1058-1060; Year Book 74, pp. 287- 
291). A suitable seismometer is being 
developed. 

Acknowledgments 

This research was supported in part 
by National Science Foundation Grant 
No. DES 72-01295A02. 



References 

Alexander, S. S., Crust-mantle structure of 
shields and their role in global tectonics, 
EOS. Trans. Am Geophys. Union, 53, 
1043, 1972. 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



243 



Brune, J. N., and J. Dorman, Seismic waves 
and earth structure in the Canadian 
Shield, Bull. Seis. Soc. Am., 53 (1), 167- 
209, 1963. 

King, G. C. P., W. Zurn, R. Evans, and D. 
Emter, Site correction for long-period 
seismometers, tiltmeters, and strain- 
meters, Geophys. J. Roy. Astron. Soc, 
44, 405-411, 1976. 

Mikumo, T., and K. Aki, Determination of 
local phase velocity by intercomparison of 
seismograms from strain and pendulum 
instruments, J. Geophys. Res., 69, 721- 
731, 1964. 

Rodgers, P. W., The response of the horizon- 
tal pendulum seismometer to Rayleigh 
and Love waves, tilt and free oscillations 
of the earth, Bull, Seis, Soc. Am., 58, 
1364-1406, 1968. 

Sacks, I. S., R. Evans, G. King, J. Beavan, 
and J. A. Snoke, Single site phase veloc- 
ity measurement, to appear in Geophys. 
J. Roy. Astron. Soc., 1976. 



Maximum Entropy Spectral 
Analysis 

C. Seber, D. E. James, 
J. A. Snoke, and A. T. Linde 

Introduction 

Spectral analysis of seismic waves is 
used to investigate a wide range of 
problems in seismology. This is be- 
cause a great deal of information con- 
tained in a seismogram is not readily 
observable in the time domain but can 
be observed in the frequency domain. 
Frequency domain information for 
both body waves and surface waves had 
been used by us previously to study 
earthquake mechanisms and earth 
structure (Year Book 69, pp. 447-464, 
and Year Book 70, pp. 343-344). For 
body waves generated by earthquakes, 
we calculated spectral amplitudes as 
functions of frequency to investigate 
the nature of rupture processes in the 
earth (e.g., fault length, speed of rup- 
ture propagation along the fault, fault 
offset); for dispersed surface waves, 
period was plotted vs. time to obtain 
dispersion curves that yielded informa- 
tion on the velocity structure of the 



earth's crust and the upper mantle be- 
neath the central Andes. 

In nearly all previous work, both by 
us and by others, frequency domain in- 
formation was obtained by Fourier 
analysis. The advantages of the 
Fourier technique are many, including 
the fact that it is completely under- 
stood theoretically and has a fast com- 
puter algorithm. Because of these obvi- 
ous advantages, most workers have 
been content to ignore, or at least tol- 
erate, the manifest disadvantages of 
the technique, notably that it gives 
poor resolution of spectral peaks, espe- 
cially at long periods, and that it 
makes arbitrary and predetermined 
assumptions about the behavior of the 
signal outside the interval in which it 
is being analyzed. To understand these 
limitations better, we must first recog- 
nize that any block of data is by neces- 
sity only a finite subset of an infinite 
data set. Therefore, any scheme of 
spectral analysis necessarily results 
only in a "spectral estimate." To keep 
the error of this spectral estimate 
within tolerable limits using Fourier 
analysis, it is usually necessary that 
the length of data be many times 
longer than the greatest period of 
interest. Even when this is possible, 
however — and it commonly is not — 
Fourier analysis often cannot yield 
satisfactory results because it assumes 
that the signal outside the data block is 
either zero or a periodic duplication of 
the data within the block to be 
analyzed. These inherent weaknesses 
in Fourier techniques led Burg and 
others in the late 1960s to discard such 
convenient devices and to seek a more 
flexible approach to spectral analysis. 
(For a more complete and particularly 
lucid summary of the rationale for dis- 
carding Fourier techniques and apply- 
ing the maximum entropy methods in- 
stead, see McGee 1969.) 

Maximum entropy spectral analysis 
as formulated by Burg (1967, 1968) re- 
sults from applying entropy consid- 
erations to spectral determinations so 
that information is carried from the 
time domain (e.g., seismogram) to the 



244 



CARNEGIE INSTITUTION 



frequency domain with a minimum of 
distortion. The method is particularly 
powerful in that it avoids the Fourier 
assumptions about the nature of the 
signal outside the analyzed region; in- 
stead, no a priori assumption is made, 
and an estimate of the signal outside 
the known interval is based only upon 
information (entropy) extracted from 
the data themselves. (Entropy is used 
in the sense of statistical entropy as de- 
fined by Shannon and Weaver [1949] 
and may be considered to be equivalent 
to information. Since we wish the 
spectral estimate to exhibit as much in- 
formation as is fully consistent with 
the data and no more, we state that 
[McGee, 1969]: "The most reasonable 
spectral estimate for any set of data is 
that estimate which exhibits 
maximum entropy and yet fully agrees 
with the available data."). The 
maximum entropy method (MEM) has 
been shown to give improved spectral 
resolution compared to Fourier 
analysis (FA) for many kinds of sig- 
nals. 

The theoretical basis for MEM is 
given by Burg (1967, 1968), McGee 
(1969), Claerbout (1976), Seber (1976), 
and many others. A recent review of 
the technique is given by Ulrych and 
Bishop (1975), who also evaluate the 
optimum filter length criterion of 
Akaike (1969, 1970). Our computa- 
tional methods are based on algorithms 
given by Ulrych and Bishop. 

Most previously published work on 
MEM has involved simple synthetic 
signals such as sine waves or superim- 
posed sine waves with various amounts 
of added random noise. Results from 
these studies have shown that: (1) 
MEM produces highly peaked spectra 
compared to FA for short record seg- 
ments (one half to two cycles) of con- 
tinuous sine waves, giving frequencies 
that are correct to within a few percent. 
The addition of noise to the signals 
does not significantly degrade the re- 
sult. The largest single effect on calcu- 
lated frequency appears to be the start- 
ing phase of the input signal (Chen and 



Stegen, 1974). (2) For superimposed 
sinusoids of different frequencies, 
MEM resolves the frequencies far bet- 
ter than FA (Fig. 77). If the sinusoids 
have equal amplitude, the power 
spectra should have equal areas under 
the curves. MEM appears to err in this 
regard by as much as a factor of 2. The 
theoretical reason for this is not 
known, and as a consequence 
amplitude instability has been a major 
reason MEM has not been more widely 
used for spectral analysis. We note, 
however, that FA produces even 
greater errors for these simple signals. 
It is the purpose of this study to test 
MEM on synthetic seismic signals to 
evaluate the applicability of the 
method to problems in seismology. We 
have analyzed both body waves and 
dispersed surface waves and obtained 
results that show sufficient promise to 
warrant further investigation. 



Synthetic Seismic Signals 

Body waves. Difficulties in the 
Fourier frequency analysis of body 
waves often arise because the signal of 
interest may be affected by either ear- 
lier or later arriving phases. The prob- 
lem is to determine the spectral con- 
tent of the complete signal, uncon- 
taminated by other arrivals. Because 
MEM works well on short pieces of in- 
formation relative to Fourier tech- 
niques, it might prove a more effective 
tool for extracting frequency informa- 
tion from signals suffering interference 
from earlier or later arrivals. 
Moreover, the prediction potential of 
MEM might allow one to predict fre- 
quency content of the interfering sig- 
nal so that it could be subtracted from 
the signal of interest. Another tech- 
nique that might be considered (Ulrych 
et al. , 1973) is using MEM to extend the 
given short signal to a length where 
Fourier analysis can be used success- 
fully. This seems to have been done 
with considerable success. Where 
phase information is of interest this 
would be especially useful, since phase 






DEPARTMENT OF TERRESTRIAL MAGNETISM 

1.0 



245 



Ld 

O 
Q_ 

Q 
LU 
N 



o 



0.8 



0.6 



0.Q 





00 40 80 120 160 200 24.0 
TIME (DATA INTERVALS) 



^ 



-- . I--. 



0.2 



0.3 



0.4 



0.5 



FREQUENCY (CYCLES/DATA INTERVAL 



The Fourier power spectrum is given 
by 

PM = n -^-—{l-e-"[2a+b)cos 

bco +2b(o sin bw] + e~ 2b [(1 + b) 2 
+ b 2 or} 



Fig. 77. Burg power spectrum and periodogram of signal shown in inset. Input signal consists of 
20 points at 1-s intervals of a time series made up of three superimposed equiamplitude sine waves 
with frequencies of 0.04, 0.08, and 0.12 Hz. Adapted from Ulrychet al., 1973. 

information is available in the Fourier 
transform but is lost in MEM. 

Simple radiated pulse. Brune (1970) 
gives an expression for the radiated 
pulse for a simple earthquake model. 
From many studies using real data, it 
has been shown that the shape of the 
displacement amplitude in the fre- 
quency domain for an earthquake is 
reasonably well approximated by 
Brune's empirically derived model. 
This model has simple analytical ex- 
pressions in both the frequency and 
time domains and provides a 
straightforward comparison of the re- 
lative accuracy of FA and MEM for 
signals of the kind encountered in 
seismology. 

Brune's model is defined by a func- 
tion, f(t), for a finite time interval such 
that fit) =te~'H(t) where 



H(t) = 



t <0 

1 t>0 



where b is the length of the signal in 
the time domain. No truncation corre- 
sponds to the limit 6^> °°. 

Figure 78 shows a Brune's model 
pulse with various points of truncation. 
The maximum entropy spectrum 
agrees very closely with the theoretical 
spectrum (see Fig. 79). Truncating the 
signal on the left (reducing the number 
of zeros) had no significant effect on the 
spectrum. With just one zero value pre- 
ceding the signal onset, MEM was still 
able to detect the discontinuity and 
give a good spectrum. 



246 



CARNEGIE INSTITUTION 



The effects of successive truncations 
on the right side of the signal are 
shown in Fig. 78. For a -1 to 4 second 
record, MEM was significantly better 
than Fourier analysis. A signal trun- 
cated at point 61 (see Fig. 78) gives a 
MEM spectrum that compares very fa- 
vorably with theory. Similarly, signals 
from points 15 to 31 (5 s) and from 
points 15 to 25 (3 s) also give very good 
spectra, although they are not as accu- 
rate at higher frequencies. The 15-21 
(2 s) segment produces a spectrum that 
deviates significantly from the spec- 
trum of the full signal. It was observed 
that the zero frequency amplitude was 
inversely proportional to the length of 
the window. This seems to indicate a 
high degree of amplitude stability. 

The MEM results can be compared to 
those obtained using Fourier analysis. 
At a 5-s truncation, there is a strong 
effect on the spectrum which progres- 



sively worsens with increasing severity 
of truncation (see Fig. 78). 

We conclude that the MEM gives a 
very good estimate of the true spectral 
content even if the pulse is severely 
truncated. This result is encouraging 
and indicates that MEM may be useful 
in estimating the spectral content of 
truncated seismic signals. 

Sine wave convolved with instrument 
response. Linde and Sacks (1971) have 
described the spurious spectral 
amplitudes in the long-period part of 
the spectrum that result from having 
to analyze long-period body-wave arri- 
vals of essentially one cycle. Studying a 
sine wave convolved with instrument 
response, they found that increasingly 
severe truncations resulted in increas- 
ing emphasis of long-period ampli- 
tudes, even though the amplitude of 
the truncated part of the signal was 
small (see Fig. 79). 



Q_ 
< 

O 

LlI 
M 




0.01 



0.001 



FREQUENCY 
Fig. 78. Maximum entropy and Fourier power spectra of Brune's model pulse (see inset) for 
various truncation points indicated in inset. Point intervals over which signal has been analyzed are 
indicated on the figure. MEM andFA denote maximum entropy method and Fourier analysis, respec- 
tively. 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



247 




TRUNCATION POINTS 



60 
I 



>/oOF MAX. 



TIME (SEC) 



I CYCLE OF 20 SEC PERIOD, CONVOLVED 
WITH 30 SEC SEISMOMETER-100 SEC 
GALVANOMETER INSTRUMENT 




/,•/•'" \ \ — 78 SEC 
// / \ \— — 50 SEC 

/ // '—-38 SEC 

INPUT SIGNAL SPECTRUM 
/ (.78.5 SEC) 

I 



200 100 50 



30 20 

PERIOD (SEC) 



EFFECT OF TRUNCATION 
NOISELESS, BOXCAR WINDOW 20SEC SINGLE CYCLE 



Fig. 79. Normalized maximum entropy power spectra (dashed lines) and Fourier amplitude 
spectra (solid lines) showing the effect of truncation in the time domain (upper figure) of a noiseless 
1-cycle 20-s sine wave convolved with a 30-s seismometer, 100-s galvanometer instrument. Successive 
truncation at shorter and shorter times drastically affects the long-period end of the Fourier spectrum 
but results mostly in a shift in the peak frequency for the maximum entropy spectra. The input signal 
spectrum is the theoretical Fourier spectrum. 



When this signal is essentially of in- 
finite length, the area under the curve 
is zero (i.e., the zero frequency term 
will be zero) but truncation results in a 
positive area (i.e., positive zero- 
frequency term). Erroneous low- 
frequency amplitudes are further 
magnified when multiplied by the 
transform of the instrument response, 
which is large at very low frequencies. 

It was hoped that MEM would not 
produce these spurious effects for the 
long-period part of the spectrum. As 



can be seen in Fig. 79, MEM makes a 
good estimate of the frequency of the 
convolved sine wave but does not pro- 
duce zero amplitude at very long 
periods. Hence, multiplication by the 
instrument correction causes an er- 
roneous rise in amplitude at the long- 
period end of the spectrum. We note, 
however, that the amount of spurious 
long-period signal is not due to trunca- 
tion effects but seems to remain 
roughly constant regardless of the 
amount of truncation. We are unable to 



248 



CARNEGIE INSTITUTION 



explain these spurious amplitudes at 
long periods, since for single sine 
waves MEM has no difficulty in recog- 
nizing zero area under the curve. As a 
further test of MEM on more complex 
signals with zero spectral amplitude at 
zero frequency, we also analyzed a 
double Brune pulse. The double pulse 
has the property that its Fourier trans- 
form at zero frequency is zero. The 
function fit) is given by fit) — {te~' — 
a 2 te at \ Hit). It begins discontinu- 
ously in the time domain and dies off 
exponentially as t — » °°. Once again 
MEM gives similar incorrect results in 
the low frequency range, tending to a 
nonzero amplitude plateau rather than 
to zero. 

Surface waves. All modern analytical 
techniques used to obtain dispersion 
curves for surface waves require that 
some form of filtering (in either the 
time or the frequency domain) be 
applied to the record. These techniques 
are generally satisfactory for short 
periods but are incapable of high reso- 
lution at long periods. This is to say 
that as the period of a given wavelet 
increases it becomes more difficult to 
determine its exact "arrival time." 
Some of this difficulty arises from an 
inherent uncertainty of measuring the 



time of arrival to an accuracy much 
greater than the time scale of a single 
cycle, but it seems likely that part of 
the problem lies with the analytical 
techniques. Because MEM can give 
vastly improved results over Fourier 
analysis for short segments of har- 
monic waves, it is appropriate to inves- 
tigate whether or not it can provide 
better time resolution for the arrival of 
long periods over conventional tech- 
niques. 

To test the performance of MEM on 
dispersed waves, we generated a syn- 
thetic signal that was linearly disper- 
sive with a flat amplitude spectrum be- 
tween 0.078 and 0.156 Hz. The spec- 
trum was cosine-tapered over 20 
Fourier harmonics outside that fre- 
quency interval. Various window 
lengths for MEM analysis were used, 
and the results are shown in Fig. 80. 
Over a fairly wide range of frequencies 
MEM gives results close to the theoret- 
ical line in the nontapered frequency 
range, between 0.078-0.156 Hz. The 
results of this analysis are encourag- 
ing, although the long period part of 
the spectrum still presents some prob- 
lems. Further experimentation needs 
to be done using real data, with various 
window lengths. 



140 

120 
100 

50 



1 

WINDOW 


i 

LENGTH ( 


sec) 




I i 




_ ■ 
O 


25 
20 






* " J^ 


- 


• 


1 5 








- e 


10 




e 


^^*! 


_ 


„amp =0 | cosine taper i 

1 m o 






i I 

1 cosine taperj amp = 


- 


i 

i £& 


#T 








- 


<" life 
i 
i 


i 






1 1 


- 



0.05 



O.IO 0.15 

FREQUENCY (HZ) 



020 



Fig. 80. Maximum entropy frequency analysis of synthetic dispersed wave train using a sliding 
window of varying length. Time is taken to be the center of the window. The synthetic signal is of 
constant amplitude over the frequency range 0.078-0.156 Hz, and its amplitude spectrum is cosine 
tapered to zero over the frequency range shown. Frequencies over which spectral amplitudes are zero 
are indicated. The solid sloping line through the points is the theoretical linear dispersion curve. 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



249 



Concluding Remarks 

Maximum entropy analysis of syn- 
thetic signals of geophysical interest 
suggests that this technique may be a 
powerful tool. These preliminary in- 
vestigations indicate that, at least for 
the Brune model and the dispersed 
wave, MEM does not suffer from the 
signal truncation effects in the same 
manner as the Fourier transform. For 
the dispersed wave, MEM correctly ob- 
tains a linear dispersion curve over a 
fairly wide range of frequencies. The 
failure to find the curve at the upper 
and lower ends of the frequency range 
may be due to artificial tapering of the 
data. MEM does not seem to provide 
the desired results at the low frequency 
end of the spectrum of signals such as 
the sine wave convolved with instru- 
ment response or the double Brune 
pulse. It is no t clear why MEM pro- 
duces a nonzero plateau at the long- 
period end of the amplitude spectrum 
yet succeeds on simple sine waves and 
a few more complex signals such as the 
Brune pulse. 

Acknowledgments 

This research was supported in part 
by National Science Foundation Grant 
No. DES 72-01295A02. 

References 

Akaike, H., Fitting autoregressive models 
for prediction, Ann. Inst. Statist. Math., 
21, 243-247, 1969. 

Akaike, H., Statistical predictor identifica- 
tion, Ann. Inst. Statist. Math., 22, 203- 
217, 1970. 



Brune, J.N., Tectonic stress and the spectra 
of seismic shear waves from earthquakes, 
J. Geophys. Res., 75, 4997-5009, 1970, 
erratum, 76, 502, 1971. 

Burg, J. P., Maximum entropy spectral 
analysis, paper presented at the 37th An- 
nual International Meeting, Soc. of 
Explor. Geophys., Oklahoma City, Okla., 
Oct. 31, 1967. 

Burg, J. P., A new analysis technique for 
time series data, paper presented at Ad- 
vanced Study Institute on Signal Process- 
ing, NATO, Enschede, Netherlands, 
1968. 

Chen, W.Y., and G.R. Stegen, Experiments 
with maximum entropy power spectra of 
sinusoids, J. Geophys. Res., 79, 3019- 
3022, 1974. 

Claerbout, J .F ., Fundamentals of Geophysi- 
cal Data Processing, McGraw-Hill, New 
York, 274 pp., 1976. 

Linde, Alan T. and I. Selwyn Sacks, Errors 
in the spectral analysis of long-period 
seismic body waves, J. Geophys. Res., 76, 
3326-3335, 1971. 

McGee, T.M., On Berg's method of spectral 
analysis, unpublished manuscript, Ven- 
ing Meinesz Laboratory, Utrecht, 1969. 

Seber, C, Maximum entropy spectral 
analysis with particular interest in appli- 
cations to seismic waves, Unpublished 
LBA Thesis, Kalamazoo College, 1976. 

Shannon, C.E. and W. Weaver, The Math- 
ematical Theory of Communication, Uni- 
versity of Illinois Press, Urbana, 117 pp., 
1949. 

Ulrych, Tad J., and Thomas N. Bishop, 
Maximum entropy spectral analysis and 
autoregressive decomposition, Rev. 
Geophys. Space Phys., 13, 183-199, 1975. 

Ulrych, T.J., D. E. Smylie, O. G. Jenson, 
and G. K. C. Clarke, Predictive filtering 
and smoothing of short records by using 
maximum entropy, J. Geophys. Res., 78, 
183-200, 1973. 



ATOMIC PHYSICS AND PROPERTIES OF MATERIALS 

L. Brown, A. W. Hofmann, M. Magaritz, and G. H. Pepper 



Diffusion in Silicate Melts 
and Glasses 

Albrecht W. Hofmann and Mordeckai Magaritz 

Introduction 

Our experimental work on cation dif- 
fusion in natural melts was originally 



motivated by the need to know more 
about the length and time scales of 
chemical processes in a partially mol- 
ten upper mantle (see Hofmann, Year 
Book 73, pp. 935-941, 1974; Year Book 
74, pp. 183-189, 1975). For this pur- 
pose, calcium and strontium diffusion 
were investigated in a natural basalt 



250 

(olivine tholeiite). We have continued 
and expanded this work and report 
more complete diffusion data for stron- 
tium and data for barium and cobalt in 
the same basalt matrix. The results 
confirm that at temperatures of 
1300°-1400°C, i.e., somewhat above 
the basalt liquidus at 1 atm. pressure, 
the diffusion coefficients of all divalent 
cations so far investigated lie in the 
range of 10 7 to 10 _,) cm 2 s" 1 . 

In addition, we have started exten- 
sive diffusion studies on glass of grani- 
tic composition. This glass serves as a 
model for melts typical of those occur- 
ring in the earth's crust, and the re- 
sults are of particular importance to 
the interpretation of Rb-Sr age meas- 
urements on partially molten crustal 
rocks, migmatites. We report results 
for Sr and Ba diffusion in this glass. We 
find that diffusive transport distances 
of strontium in granitic melts do not 
exceed a few tens of centimeters in 10" 
yr, so that the range of isotopic equilib- 
ration of Sr in migmatites is expected 
to be small also. 

The measured diffusion coefficients 
for Ba and Sr in both basaltic and 
granitic melts (which have viscosity 
differences of several orders of mag- 
nitude) demonstrate that the mobility 
of these cations is not related in a sim- 
ple way to the viscosity of the liquid as 
would be predicted by the Stokes- 
Einstein relation. Rather, ionic charge 
and ionic radius are shown to have a 
very strong effect on the mobility, es- 
pecially in the lower temperature 
range of the obsidian measurements of 
675°-890°C. Many ionic species such as 
Fe, Co, and Eu can be present in more 
than one oxidation state in the silicate 
melt, and the contrast in diffusivities 
can be used to determine the propor- 
tions, for example, of Co +:! /Co +2 in the 
melt. The determination of oxidation 
state is particularly important for 
europium, whose partition coefficient 
in plagioclase melt systems depends on 
the relative abundance of the 2 + state. 
In this way it is advantageous to use 
kinetic measurements to determine 
equilibrium behavior. Because of its 



CARNEGIE INSTITUTION 

importance to rare earth geochemistry, 
we plan to explore this prospect more 
fully in the future. 

Diffusion in Basalt Melt 

Tracer diffusion of K5 Sr and ' i! Ba was 
determined by the thin-source tech- 
nique explained in last year's report 
(Hofmann, Year Book 74, pp. 183-189, 
1975). To overcome the effects of initial 
convective movement in the region of 
the tracer source, the charges (basalt- 
filled Pt tubes) were generally 2.0-2.5 
cm long, and run durations were sev- 
eral days. The Ba-diffusion and Co- 
diffusion measurements were made in 
a new high-temperature furnace with a 
large (about 5-in diameter) heating 
chamber, MoSi 2 heating elements, and 
thermocouple (Pt i0 Rh,j) controlled tem- 
perature regulation. The sectioning 
method was improved by using a bench 
micrometer capable of measuring 
lengths to about l/xm. 

Figure 81 shows the results of the 
measurements of the residual activity 
of 1:i:i Ba after successive length reduc- 
tions by grinding the basalt rod from 
the "hot" end. The residual activity for 
diffusion from a thin source into a 
semiinfinite medium is given by: 



A resid /A„ = erfcy 
where y = x/2 / Dt 



(1) 



For convenience of data analysis the 
measured activities are converted (us- 
ing Equation 1) to the dimensionless 
parameter y, which is then plotted as a 
function of length removed, x, or as a 
function of x/2 /Mvhere t is the dura- 
tion of the diffusion experiment. The 
analytical data will define a straight 
line if the results conform to the initial 
and boundary conditions of one- 
dimensional Fick's Law diffusion with 
constant diffusion coefficient from a 
thin source into an infinite medium. A 
least-squares regression analysis de- 
termines the best line through the 
data, and the diffusion coefficient is 
given by the square of the slope of the 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



251 



1 1 1 1 1 1 1 I 1 1 1 1 1 1 


- 


1260 ^V, X*~^I350 x / 




>V / J^ / 


_ 


o / / y x ^-1440 


- 


/ • / / / 


- 


//// x/ 


- 




1 / £, A 

// // x^ 


- 


/• // x 


: 


- <iy x / 


_ 


/ // / x 




o«* /« 




r 1 1 1 1 1 1 1 1 1 1 1 1 1 1 


- 



1.8 

1.6 

1.4 - 

1.2 - 

1.0 
0.8 
0.6 - 
0.4 - 
0.2 - 



5 10 15 

I0 4 -x/2nr(cmsec- |/2 ) 

Fig. 81. Linearized plot of residual activity versus x/2 vTfor m Ba diffusion in olivine tholeiite 
basalt: x is the length removed from the cylinder; t, the derivation of the diffusion experiment; and Y 
is the dimensionless parameter calculated from the measured residual activity using Equation 1. 
Each symbol plotted represents a separate diffusion experiment; the numbers are the temperatures in 
degrees Celsius for each experiment. 



line. Inspection of Fig. 81 shows that 
the data do in general define excellent 
straight lines. Significant deviations 
occur only in the region near the origin 
where convection appears to disturb 
the diffusion profiles and where the 
melt frequently forms a convex or con- 
cave meniscus against air so that the 
position for the origin itself is poorly 
defined. Occasionally, significant de- 
viations are also found for large values 
of x/2 JT~ where small errors in the 
background, correction cause signifi- 
cant systematic deviations from the 
straight line. 

Figure 82 is an Arrhenius plot (log 
diffusion coefficient versus the inverse 
absolute temperature) of the new Ba, 
Sr, and Co data. Also shown is the Ar- 
rhenius line for Ca diffusion previously 
reported (Year Book 74, pp. 183-189) 
and three additional measurements of 
Sr diffusion determined by the 
diffusion-couple method reported in 
Year Book 73, pp. 935-941. In that re- 
port the error for the two lower- 
temperature points had been estimated 
to be within a factor of 2, and that of 
the highest-temperature measurement 
had been judged less reliable because of 



deformation of the diffusion couple dur- 
ing the run. These error estimates ap- 
pear to have been realistic; the Ar- 
rhenius line is drawn on the basis of 
the new thin-source data alone; and the 
activation energy of 44 kcal/mol super- 
sedes the previously given value of 114 
kcal/mol. The extreme error in the 
original estimate of the activation en- 
ergy is largely a result of the error in- 
troduced by the above-mentioned 
high-temperature point whose D value 
is too large by a factor of 3. The Ar- 
rhenius lines given in Fig. 82 are de- 
scribed by the equations: D(Ca) = 
0.535 exp(-44.0 x WIRT), D(Sr) = 
0.278 exp( -43.5 x 1 W/RT), and D(Ba) 
= 0.059 exp(-39.4 x 1(P/RT), where R 
is the gas constant and T is the abso- 
lute temperature. 

Figure 82 shows that there is a small 
but significant decrease in D values as 
a function of increasing ionic radius 
(Ca 2+ = 0.99A, Sr 2 * = 1.13A, Ba 2+ = 
1.35A Pauling radii). On the other 
hand, the activation energies for the 
three species (44,44, and 39 kcal/mol) 
are not significantly different from one 
another. Also shown on Fig. 82 are a 
few results for cobalt diffusion. Under 



252 



CARNEGIE INSTITUTION 



1500 



1400 



T PC) 



1300 




6.0 6.2 6.4 

I0 4 /T(°K _I ) 

Fig. 82. Log,,, D versus \IT (Arrhenius plot) for l:ii Ba, K5 Sr, 45 Ca, ''"Co diffusion in olivine tholeiite 
basalt in air at 1 atm. Triangles = Co, circles = Sr, crosses = Ba. The line representing Ca diffusion is 
taken from Hofmann ( 1975). 



the conditions of the experiment, dry 
heating in air, cobalt ions are expected 
to be primarily divalent at tempera- 
tures above 1000°C (Coons et al., 1976), 
and the diffusivities are very close to 
those of calcium. These results conform 
roughly to the "compensation law" pos- 
tulated by Winchell (1969), which 
states that the diffusion coefficient of 
all species is the same at some critical 
temperature. Winchell and Norman 
(1969) found that the critical values 
were D ~ lO^cirfs » and T = 1500°C. 
In detail, our results differ from this 
behavior in that the four Arrhenius 
lines are essentially parallel rather 
than intersecting at a single point. 
Nevertheless, estimates of diffusion 
coefficients using the compensation 
law appear to be much closer to the 
true values than those based on the 
Stokes-Einstein relation as done by 
Donaldson (1975). In the Stokes- 
Einstein relation the diffusion coeffi- 
cient is proportional to the inverse of 
the viscosity of the liquid. As 
Donaldson showed, the estimated dif- 
fusion coefficients are within a factor of 
10 or 20 of the correct value for basalt 
near its liquidus, but this agreement 
appears to be fortuitous, because as 
will be shown below, the Stokes- 
Einstein estimate of the diffusion coef- 



ficient fails by many orders of mag- 
nitude when it is applied to high- 
viscosity silicate liquids such as gran- 
ite melts. 

When these data are compared with 
those given in the companion report by 
Hofmann and Brown, it appears that 
the diffusion coefficients for many ca- 
tions in basalt in the temperature 
range of 1300°C to 1400°C are approx- 
imately logZ) = -6.5±0.9. If the (un- 
known) effects of higher pressure and 
possible volatile content can be ig- 
nored, the characteristic transport dis- 
tance by diffusion in a partially molten 
mantle can be estimated using the ex- 
pression x — JDt, or about 1 km in 10 9 
yr. This very limited chemical trans- 
port in the mantle provides some strin- 
gent restrictions on the choice of physi- 
cally possible models for the chemical 
evolution of the mantle. Some of these 
have been discussed in previous reports 
(Hofmann, Year Book 73, pp. 935-941, 
1974; and Year Book 74, pp. 183-189, 
1975). The following may serve as an 
additional example of the consequences 
.of the slow diffusion transport. 

Kay (1975) proposed a model for the 
upper mantle "involving downward 
zone refining within a thickening 
lithosphere plate." The lithosphere is 
assumed to grow by downward solidifi- 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



253 



cation of a partially molten mantle. 
"LIL(= Zarge ion Zithophile) elements 
and volatiles are largely excluded from 
the crystallizing solid phases and ac- 
cumulate in a downward migrating 
zone at the base of the plate." The re- 
sult is taken to be a zone of LIL 
element-enriched mantle that ulti- 
mately serves as the source material of 
oceanic islands. This model, like other 
zone refining processes, requires chem- 
ical transport and mixing in the molten 
zone. Without such transport the bulk 
composition of the solidifying liquid 
cannot change and no refining takes 
place. In the vicinity of the slowly ad- 
vancing solidification front the melt 
fraction must be very small. This is re- 
quired by the phase relations of ther- 
modynamic systems that contain 
phases of continuously variable compo- 
sition. As the melt fraction decreases 
toward the solidification front, the bulk 
viscosity of the material increases, 
thus inhibiting convection progres- 
sively toward the boundary. In addi- 
tion, the zone refining mechanism re- 
quires that mixing occur on a scale that 
is small relative to thickness of the so- 
lidified material (so that a significant 
enrichment factor can be attained) but 
large enough so that the quantity of 
enriched material is of some geochemi- 
cal significance. Thus, ideally the mix- 
ing zone below a 100-km-thick litho- 
sphere should be on the order of 10 km 
thick. We .must therefore consider a 
zone of increased viscosity (relative to 
the bulk of the partially molten man- 
tle) and restricted size of convection 
cells (relative to "normal" mantle con- 
vection cells expected to have a charac- 
teristic size of 100-1000 km or more). 
Simple dimensional considerations 
show that the velocity in thermally 
driven free convection is proportional 
to pagUAT/fjL (Bird et al., 1960, Ch. 9), 
where p is the density, /x the viscosity, 
a the thermal expansion coefficient, g 
the gravitational acceleration poten- 
tial, L a characteristic length, and AT" 
the temperature difference. Conse- 
quently, if we allow a velocity of 10 
cm/yr for large plate-driving convec- 



tion cells with a minimum cell diame- 
ter of 100 km and if the bulk viscosity 
in the vicinity (i.e., within 10 km) of 
the solidification front increases by a 
minimum of a factor of 10, the 
maximum velocity in a hypothetical 
mixing cell of 10-km diameter would be 
on the order of 10~ 2 cm/yr or less — that 
is, at least an order of magnitude 
slower than the average rate of litho- 
sphere growth (assumed to be 100 
km/10 8 yr or 0.1 cm/yr). In view of this 
it seems reasonable to consider the case 
where chemical transport occurs pre- 
dominantly by diffusion alone. This 
process will produce an LIL element- 
enriched zone in the vicinity of the sol- 
idification front. If the diffusion coeffi- 
cient is 10 _,i cm 2 sec -1 , and the velocity 
of the front is 0.1 cm/yr, a steady-state 
chemical gradient will evolve within 
about 10 5 years, after which the sol- 
idifying lithosphere will have a compo- 
sition identical to that of the original 
partially molten asthenosphere. The 
width of the enrichment zone can be 
roughly estimated by the relation x = 
D/V, where D is the diffusion coeffi- 
cient and V is the velocity of the sol- 
idification front (see Albarede and Bot- 
tinga, 1972). This yields a characteris- 
tic distance of 3 meters. It is unlikely 
that an enrichment zone of such min- 
ute thickness would be of any impor- 
tance to the chemical evolution of the 
mantle, and Kay's model of downward 
zone refining is not expected to be ef- 
fective. On the other hand, some degree 
of reenrichment of the asthenosphere 
as a whole cannot be excluded. 

Diffusion in Obsidian Glass 

An obsidian sample from Valles 
Caldera, New Mexico, was chosen as a 
model for granitic melts. Shaw (1963, 
1974) had previously used this sample 
for measurements of viscosity and 
water diffusion. Cylinders were drilled 
from slabs of this glass, and radioactive 
tracer was loaded on the polished ends 
of the preannealed cylinders. Care was 
taken to avoid contamination of the 
sides of cylinders (1) by loading and 
drying only small drops of tracer solu- 



254 



CARNEGIE INSTITUTION 



tion near the center of cylinder face 
and (2) by annealing the loaded cylin- 
der for about 60 min at 700°C to allow 
diffusion into a thin surface layer of the 
glass and subsequently dissolving the 
excess tracer salt from the surface by 
washing in dilute HC1. After this pre- 
liminary treatment the cylinders were 
annealed at temperatures between 
675° and 950°C for periods of several 
days. The diffusion profiles were then 
determined by the same method of sec- 
tioning and residual-activity counting 
as previously explained for basalt. 

Figure 83A shows two typical activ- 
ity profiles for Sr and Ba at 850°C. Fig- 
ure 83B shows the same data plotted as 
explained above for Fig. 81. Figure 84 
is an Arrhenius plot of all the results 
for Sr and Ba. Judging from the fit of 
the data to the least-squares regression 
line, the precision is considerably bet- 
ter than was achieved in the basalts. 
We ascribe this to the absence of any 
convective movement in the case of the 
obsidian glass. In addition, the activa- 
tion energy (proportional to the slope of 
the Arrhenius line) is more precisely 
defined because of the greater spread of 
measurements along the 1/T axis. The 
Arrhenius equations are: D(Sr) = 0.09 



exp (-43.8 x WVRT) and D(Ba) = 
0.053 exp (-45.8 x 1WRT). 

The results are relevant to the in- 
terpretation of Rb-Sr isochron meas- 
urements of migmatites (rocks of 
roughly granitic composition formed by 
partial melting of crustal rocks). Mig- 
matites are typically inhomogeneous 
in composition and appear in layers of 
centimeters or tens of centimeters 
thickness. Thus it is immediately obvi- 
ous that the high-grade metamorphic 
process that produced the migmatites 
did not cause chemical homogenization 
on a scale greater than a few tens of 
centimeters. On the other hand, 
isotopic homogenization is required 
(and frequently assumed to occur in 
high-grade metamorphism) in order for 
an Rb-Sr isochron to be "reset" by the 
metamorphism. The minimum dis- 
tance of isotopic equilibration can be 
estimated assuming minimum values 
for temperature and duration of the 
metamorphism. Choosing values of t = 
10 (i years for the duration of near-peak 
temperature and 700°C for the temper- 
ature itself, we obtain D = 10"' 's -1 and 
the characteristic transport distance of: 

x = fDi = 20 cm. 



'0.5 









1 








1 1 
Sr o 


I 

A 
















Ba x 




-\\ 














Ge 


- 


l\\ 
















- 


-I \ , 

-1 \ 

-1 
















- 


1 


X 


x x 


— X 


— 







i i 






200 



400 



600 



x (Mm) 



Fig. 83. Diffusion profiles in obsidian for Sr, Ba, and Ge at 850°C. (A) Plot of residual activity 
versus length of cylinder removed. The solid lines are ideal diffusion curves, Equation 1. 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



255 




I0 5 ' x/2fl7(cm 2 sec- l/2 ) 

Fig. 83. (B) Linearized plot of the same data with Y calculated from Equation 1 (see also Fig. 81). 
These results show that diffusion of Sr and Ba can be described by a single constant diffusion 
coefficient, but that the Ge data either require a variable diffusion coefficient, or have more than one 
chemical species of Ge present. 



io- 



10" 



CM 

§ io- 



10" 



900 

I 



T(°C) 
800 
I , 



700 



-r^ — i 1 r 




8 



J L 



J I I L 



10 



J I I L 



io 4 /t (°k _i ) 

Fig. 84. Arrtenius plot for Sr (circles) and Ba (crosses) diffusion in obsidian (in air). 



256 



CARNEGIE INSTITUTION 



Such small transport distances must 
be considered when samples are taken 
for isochron measurements in order to 
determine the age of migmatization. 
For example, it is common practice to 
collect blocks of at least 20 cm diameter 
as individual samples and to space 
such samplings over meters or even 
kilometers. Isochron measurements on 
such suites of whole-rock samples are 
then frequently interpreted as deter- 
minations of the age of metamorphism. 
Although nondiffusive migration of 
aqueous pore fluids or melt may con- 
ceivably have accomplished such 
wide-ranging isotopic homogenization 
of strontium, that interpretation would 
need to be confirmed by small-scale 
sampling. 

The similarity of inferred distance of 
Sr migration and major-element equi- 
librium leads to the rule of thumb that 
centimeter-scale sampling should be 
included for Rb-Sr age determinations 
of metamorphic rocks and that the 
range of major-element equilibrium in 
a rock should be used as a guide to the 
presumed range of isotopic equilibrium 
needed for the age measurements. 

Some caution is in order for the 
geological interpretation of these diffu- 
sion data because the effect of water on 
these diffusivities is unknown. We are 
therefore preparing a new set of exper- 
iments to be conducted under hy- 
drothermal conditions. 

The determination of one of the fun- 
damental properties of natural melts, 
ion mobility, has many applications in 
the earth and planetary sciences as 
well as in material science. These in- 
clude such diverse problems as deter- 
mination of quenching times and spa- 
tial configurations used in experimen- 
tal petrology; interpretation of concen- 
tration gradients in natural terrestrial 
and lunar glasses; problems of crystal- 
lization kinetics, such as the develop- 
ment of oscillatory zoning in plagio- 
clase (Bottingae^a/., 1966); determina- 
tion of the structure of silicate melts; 
and measurement of oxidation state of 
ionic species in melts. We will discuss 



briefly the latter two of these applica- 
tions. 

It is shown on Fig. 83 that the diffu- 
sion profile for Ge in obsidian is not 
compatible with a single constant dif- 
fusion coefficient. Similar results were 
obtained for Co diffusion. Both ele- 
ments can be present in more than one 
oxidation state, and we have attempted 
to deconvolute the Ge profile into two 
components, each diffusing indepen- 
dently of the other. The solid symbols 
shown in Fig. 85 form approximately 
straight lines, which correspond to a 
diffusion coefficient of D = 1.8 x 10~ 8 . 
This was calculated by subtracting 
from the measured profiles (open sym- 
bols) a slow-moving component, assum- 
ing this accounts for 90% of the total 
Ge activity and has a diffusion coeffi- 
cient of D =3.8 x 10" 12 cm 2 s" 1 . Our 
interpretation that the two species in- 
volved are two oxidation states of Ge is 
subject to confirmation by experiment 
under controlled partial pressure of 
oxygen. This offers the possibility of 
measuring the proportions in which 
different oxidation states of the same 
chemical element are present in a melt. 
To our knowledge, other methods for 
determining this are not feasible for 
trace elements. 

Figure 86 shows a compilation of our 
diffusion data. Also shown are tracer 
diffusion coefficients for alkalies in ob- 
sidian measured by Jambon and Car- 
ron (1973) on a single diagram. It is 
clear that extrapolation for the obsi- 
dian data to the temperature range of 
the basalt liquidus yields diffusion 
coefficients that differ from the various 
basalt diffusivities by factors of about 
20 for barium and 2 for strontium. In 
contrast, the viscosity of these two 
melts varies by about three to four or- 
ders of magnitude (Murase and McBir- 
ney, 1973). From this it is evident that 
the Stokes-Einstein relation (which re- 
lates diffusivities to viscosity) is not 
applicable to liquids of this kind. Fur- 
thermore, the results are compatible 
with structure models that consider ca- 
tion species such as Ca 2 , Sr 2 ^ and Ba 2+ 



DEPARTMENT OF TERRESTRIAL MAGNETISM 
2.0 



257 



Y 



1.5 



1.0 



0.5 



i 1 1 r 



1 r 



i r 



1 r 



1 r 



<P 



10 



15 



x/2\IT (cm sec - " 2 ) 



Fig. 85. Diffusion profiles for Ge in obsidian at 850°C. Open circles and open diamonds denote the 
results of the linearization procedure for two separate experiments as used in Figs. 81 and 83; the 
solid circles and solid diamonds were calculated by subtracting 90% of the total activity as the 
slow-moving Ge component (D = 3.8 x 10~ 12 cm 2 s _1 ) in order to isolate the fast moving component 
which yields a straight line Y versus X relationship and a diffusion coefficient of D = 1.8 x 
10- 8 cm 2 s-'. 



1400 1200 1000 
4 i l_, I i , i 



T(°C) 
800 



600 




I0 4 /T(°K"') 



Fig. 86. Compilation of diffusion data in obsidian (at temperatures less than 1000°C) and basalt 
(at temperatures greater than 1250°C). The Arrhenius lines for K, Rb, and Cs are taken from Jambon 
and Carron (1973). The points for Ge are derived by using the deconvolution procedure shown in Fig. 
85 and assuming that the slow and fast species involved are Ge^ + and Ge 2+ , respectively. 



258 



CARNEGIE INSTITUTION 



as single ions that migrate through the 
network of silica tetrahedra at a rate 
primarily dependent on ionic size and 
charge and nearly independent of the 
degree of polymerization of the silica 
tetrahedron. Figure 86 shows, for 
example, that Ba 2+ and K + , which have 
similar ionic radii but different ionic 
charge, differ drastically in their mo- 
bility in obsidian. At 800°C the tracer 
diffusion coefficient for K + is greater by 
a factor of 10 ! than the coefficient for 
Ba 2+ . In addition, it appears that the 
effect of ionic radius differences is more 
important for univalent ions than for 
divalent ions. Preliminary data for tri- 
valent ions (Gd 3+ and Al :i+ , also shown 
on Fig. 86) give diffusion coefficients 
smaller than about 2 x 10~ 12 cm 2 s _1 at 
850°C in obsidian. 

The above model for the structure of 
silicate liquids contrasts with models 
that propose the existence of larger 
complexes or "units of structure related 
to species or subspecies of minerals" 
(Yoder, 1973). These complexes are 
thought to mimic the composition of 
solid phases that would crystallize 
below the liquidus temperature. The 
presently available diffusion data are 
not compatible with such a model be- 
cause the cations are evidently not suf- 
ficiently bound to the silica tetrahedra 
to form stable complexes. 



Acknowledgments 

We are grateful to Dr. H. S. Yoder, 
Jr., for supplying the basalt sample 
and for making the facilities of the 
Geophysical Laboratory available to 
us. We especially appreciate the use of 
a platinum-wound furnace at the 
Geophysical Laboratory. We also 
thank Dr. R. L. Smith, who kindly 
made the sample of obsidian available 
for this study. Financial support was 
given by the National Science Founda- 
tion, Grant No. GA-40250. 

References 

Albarede, F., and Y. Bottinga, Kinetic dis- 
equilibrium in trace element partitioning 



between phenocrysts and host lava, 
Geochim. Cosmochim. Acta, 36, 141-156, 
1972. 

Bird, R. B., W. E. Stewart, and E. N. Light- 
foot, Transport Phenomena, John Wiley 
& Sons, New York, 1960. 

Bottinga, Y., A. Kudo, and D. Weill, Some 
observations on oscillatory zoning and 
crystallization of magmatic plagioclase, 
Am. Mineral, 51, 792-806, 1966. 

Coons, W. E., J. R. Holloway, and A. Nav- 
rotsky, Co 2+ as a chemical analogue for 
Fe 2+ in high temperature experiments in 
basaltic systems, Earth Planet. Sci. Lett., 
30, 303-308, 1976. 

Donaldson, C. H., Calculated diffusion coef- 
ficients and the growth rate of olivine in a 
basalt magma, Lithos, 8, 163-174, 1975. 

Hofmann, A. W., Strontium diffusion in a 
basalt melt and implications for Sr 
isotope geochemistry and geochronology, 
in Carnegie Inst. Wash. Year Book 73, pp. 
935-941, 1974. 

Hofmann, A. W., Diffusion of Ca and Sr in a 
basalt melt, in Carnegie Inst. Wash. Year 
Book 74, pp. 183-189, 1975. 

Jambon, A., and J. P. Carron, Etude ex- 
perimentale de la diffusion des elements 
alcalins K, Rb, Cs dans une obsidienne 
granitique, C.R. Acad. Sci. D, 276, 
3069-3072, 1973. 

Kay, R., Chemical zonation of the oceanic 
mantle (Abstract), EOS Trans. Am. 
Geophys. Union 56, 1077, 1975. 

Murase, T., and A. R. McBirney, Properties 
of some common igneous rocks and their 
melts at high temperatures, Geol. Soc. 
Am. Bull., 84, 3563-3592, 1973. 

Shaw, H. R., Obsidian-FLO viscosities at 
1000 and 2000 bars in the temperature 
range 700° to 900°C, J. Geophys. Res., 68, 
6337-6343, 1963. 

Shaw, H. R. Diffusion of H,0 in granitic 
liquids: Part I, Experimental data; Part 
II, Mass transfer in magma chambers, in 
Geochemical Transport and Kinetics, A. 
W. Hofmann, B. J. Giletti, H. S. Yoder, 
and R. A. Yund, eds., Carnegie Inst. 
Wash.,Publ. 634, pp. 139-170, 1974. 

Winchell, P., The compensation law for dif- 
fusion in silicates, High Temp. Sci., I, 
200-215, 1969. 

Winchell, P., and T. H. Norman, A study of 
the diffusion of radioactive nuclides in 
molten silicates at high temperatures, in 
High Temperature Technology, 3rd 
Intern. Symp., Asilomar 1967, pp. 479- 
492, 1969. 

Yoder, H. S., Jr., Contemporaneous basaltic 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



259 



and rhyolitic magmas, Am. Mineral., 58, 
153-171, 1973. 



Diffusion Measurements Using 
Fast Deuterons for in situ 
Production of Radioactive 
Tracers 

A. W. Hofmann, and L. Brown 

In connection with our program to 
measure diffusion in silicate melts we 
have carried out preliminary experi- 
ments to determine the feasibility of 
creating a sharply defined zone of 
radioactive tracers within a sample of 
quenched basalt glass. Such a tech- 
nique has at least two potential advan- 
tages over the more conventional tech- 
nique of applying a thin layer of tracer 
to the surface of the sample (see com- 
panion report by Hofmann and 
Magaritz): (1) Relatively volatile ele- 
ments such as sodium can be easily 
contained within the sample; and (2) 
convective movement of the melt dur- 
ing initial heating of the sample may 
be less severe in the center than at the 
surface of the sample. 

Sodium is quite volatile at tempera- 
tures of 1300° to 1400°C, and much of 
an externally applied sodium tracer 
would be expected to be lost by surface 
evaporation. In addition, the boundary 
conditions for the diffusion experi- 
ments would be uncertain and temper- 
ature dependent. This problem is elim- 
inated if the sodium tracer is at the 
center of the sample and if the diffusion 
experiment is terminated before a sig- 
nificant amount of the tracer reaches 
the surface. 

Convective mixing was found to 
cause significant errors in short-term 
diffusion experiments (less than one 
day) and it was believed that this con- 
vection may be confined to a relatively 
narrow zone near the open end of the 
platinum capillary tube that contains 
the melt (see Hofmann, Year Book 74, 
pp. 183-189, 1975). If this is correct, 
the problem of convective interference 



could be circumvented by introducing 
the tracer in the center of the Pt tube 
and thus avoiding the near-surface 
convection. In addition, it is very dif- 
ficult to prove conclusively that trans- 
port occurred primarily by volume dif- 
fusion rather than by some other pro- 
cess, perhaps resembling eddy diffu- 
sion, unless differential transport be- 
tween different chemical species can be 
demonstrated. 

Experimental Method 

The basalt (olivine tholeiite, 1921 
eruption of Kilauea) was contained as 
quenched glass in 2-mm Pt tubes. 
These were exposed to a collimated 
d beam of 40 MeV energy from the 
Lawrence Berkeley 88-in cyclotron 
which had passed through a 0.0025-cm 
Al foil and through 2 in of air before 
reaching the samples. A slotted 0.178- 
cm-thick brass plate was used for col- 
limation. The width of the collimation 
slit was 1 mm in some experiments and 
0.5 mm in others. The exposure varied 
between 5 and 40 /i.A-min per sample. 
In order to determine the initial distri- 
bution of activity within the sample, 
one basalt-filled tube was sectioned 
after irradiation, and the activity was 
measured using a 3 x 3 in Nal well 
detector with the sample placed in the 
well. Figure 87 shows this initial pro- 
file of the 0.847-MeV gamma peak, 
which is produced by 5, 'Co within the 
basalt. Also shown is the profile for the 
0.099-MeV gamma peak combined 
with lower-energy Pt-x-ray peaks pro- 
duced by I9,i Au and 195 Au within the Pt 
tube. The gold isotopes are produced by 
deuteron interactions with platinum. 
The activity profiles in Fig. 87 show 
that the collimation is excellent and 
that the activity within the irradiated 
zone is uniform. The nonhorizontal 
slope on the right side is due to absorp- 
tion effects during counting. The small 
tail beyond the 1-mm zone amounts to 
less than 2% of the total activity and is 
considered negligible for the purpose of 
these experiments. 

A gamma-ray energy spectrum 
taken with the Nal detector showed 



260 



CARNEGIE INSTITUTION 



I I 

+ 0.847 MeV 
o < 0.20 MeV 



|~lmm-«) 




+-+- 



0.2 



0.4 



0.6 



0. 



x (cm) 
Fig. 87. Profile of residual activity (arbitrary 
scale) versus length (x) of cylinder after deuteron 
irradiation and before diffusion anneal. A con- 
ventional, well-type Nal detector was used for 
this profile only. The low-energy profile contains 
0.099 MeV gamma and lower-energy Ptr-x rays 
and shows the effects of absorption by the sam- 
ple: The increase in apparent activity occurs as 
absorbing material near the "hot" zone is re- 
moved by successive sectioning. The absorption 
effect is much less pronounced in the 0.847-MeV 
profile for 5H Co. The actual width of the irradia- 
tion window is shown by the dashed vertical 
lines. 

prominent peaks at 0.1, 0.35, 0.51, and 
0.85 MeV, and a broadened peak at 
about 1.25 MeV. The only energies that 
were apparently associated with a 
single isotope were due to 196 Au and 
5,i Co. Fortunately, Dr. J. Rowe of the 
U.S. Geological Survey kindly made a 
Ge(Li) detector available for this study. 
This permitted us to obtain a high- 
resolution spectrum in which the fol- 
lowing species were easily resolved: 
22 Na, 46 Sc, 48 V, 54 Mn, 56 Co, 57 Co, ,95 Au, 
,9,s Au. The only overlap of spectral lines 
occurred between the 0.835-MeV and 
0.847-MeV peaks of 54 Mn and 56 Co. 

Five annealing experiments were 
made at a temperature of 1300°C with 
run durations varying between 17 min 
and 15.5 hr, using the equipment and 



technique described in the companion 
report by Hofmann and Magaritz (this 
Year Book). The results of the 15.5-hr 
run are shown in Fig. 88. This clearly 
demonstrates the differential transport 
between the different chemical species. 
Two features argue strongly that this 
transport is primarily caused by vol- 
ume diffusion rather than some other 
process such as convection: (1) the dif- 
ference in transport distances between 
species, such as Na and V, and (2) the 
fit of the profiles to theoretical 
volume-diffusion profiles drawn as 
solid lines for Na {D = 2.45 x 
lO^citfs- 1 ), Co and Mn (D = 4.58 x 
10 7 cm 2 s ■) and V (D = 5.03 x 
10" 8 cm 2 s~ 1 ). Only Sc does not fit such a 
diffusion profile. This may mean that 
there are two Sc (chemical) species pre- 
sent, perhaps with different oxidation 
states and with two different diffusion 
coefficients. These results show that 
any contribution by convective trans- 
port must have been smaller than the 
smallest total transport, that of van- 
adium. It has been our experience, 
however, that a small but significant 
convective contribution to the total 
transport is not necessarily detected as 
a significant deviation from the ideal 
diffusion profile. For example, if an in- 
finitely thin source were spread out to 
a width of 1 mm by convection at the 
beginning of the annealing experi- 
ment, the resulting profiles after sev- 
eral hours of diffusion annealing would 
be essentially indistinguishable from 
profiles due to pure volume diffusion. 
Such convective contributions do, how- 
ever, show up quite well when short- 
duration and long-duration anneals 
are compared (see Hofmann, Year Book 
74, pp. 183-189, 1975). We believe 
therefore that isothermal time studies 
are indispensable in demonstrating the 
validity of any experimental technique 
concerned with diffusion. 

The results of a series of five experi- 
ments at 1300°C for Na and Co trans- 
port are shown in Fig. 89, where the 
apparent diffusion coefficients are plot- 
ted against run duration. The total 
range of apparent D value is less than a 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



261 




« 0.5 



x (cm) 

Fig. 88. Diffusion profiles (residual activity versus sample length) for five deuteron-activated 
species: 22 Na (1.275 MeV), >' ; Co (0.847 and 1.238 Mev), 54 Mn (0.835 MeV), 4,i Sc (0.889 and 1.121 MeV), 
and J8 V (0.983 and 1.312 MeV). The temperature was 1300°C, and the run duration was 15.5 hr. The 
solid curves are calculated diffusion profiles for D = 2.45 x 10~ H cm 2 s~'( 22 Na), D = 4.58 x 
10- 7 cm 2 s-'( 5,i Coand 54 Mn),and£) = 5.03 X 10 K cm 2 s-'( 48 V). 



factor of 3 for Co and less than 1.5 for 
Na. In no case do the measured profiles 
deviate significantly from calculated 
volume diffusion profiles. The results 
indicate that some convective trans- 
port does occur in some of the experi- 
ments. However, the effect is much less 
pronounced than that found in a study 
of Ca diffusion reported last year 
(Hofmann, Year Book 74, pp. 183-189, 
1975). In that study a thin source of 
45 Ca was applied to the open surface of 
the melt at the end of the Pt tube. Also 
a Pt furnace with a vertical muffle tube 
was used in the Ca study, whereas we 
now use a furnace with a completely 
enclosed heating chamber, MoSi 2 - 
heating elements, and SCR-type tem- 
perature control. 

We believe that convective move- 
ment occurs during heating-up time 
(which takes several minutes) and that 



its magnitude depends on small dif- 
ferences in the heating schedule not 
yet under our control. In particular, we 
were surprised to find the apparent dif- 
fusion coefficients derived from the 
shortest (17-min) run to be identical 
within analytical error to the values 
derived from the longest run (15.5 hr). 
This indicates that there was a negli- 
gible amount of convective movement 
during this short run. This result and 
the comparatively small total range of 
apparent D values lead us to the esti- 
mate that the results obtained from the 
longest run (shown in Fig. 88) are cor- 
rect within 20% for Na and 50% for Co 
and Mn. We are also encouraged by the 
excellent agreement obtained for Co 
diffusion at 1300°C between the con- 
ventional technique using a surficial 
thin source (D ((1 = 4.7 x 10~ 7 cm 2 s _1 ; see 
companion report by Hofmann and 



262 



CARNEGIE INSTITUTION 



Q 



- 


] 1 


1 1 


1 


- 


o 


o 






- 


"O 






o 


" 


+ 








_ 


- 


+ 


+ 


+ 


- 


g- 










- 


' 1 1 


1 1 


1 


- 



6XICT 



t (sec 



Fig. 89. Results of isothermal diffusion study for tholeiite basalt at 1300°C under atmospheric 
conditions. D is the apparent diffusion coefficient for Na (circles) and Co (crosses), and t is the run 
duration. 



Magaritz) and the result of this study, 
O co = 4.6 x lO^cnr-'s" 1 . 

Further improvement in accuracy 
should be easily obtainable by simply 
increasing the run duration. This was 
avoided in the current experiments in 
order to prevent the Na profile from 
reaching the ends of the cylinders (al- 
though this did occur inadvertently in 
one experiment for which no D value is 
plotted for Na; it also occurred margi- 
nally in the experiment shown in Fig. 
88). We conclude that this is an excel- 
lent technique to study diffusion of cer- 
tain chemical species, including the 
volatile species Na, and we expect that 
it will be applicable also to diffusion 
studies at high pressure for which the 
more conventional techniques are not 
suitable. 

The results also show that the mag- 
nitude of the diffusion coefficient of ca- 
tions is strongly dependent on ionic 
charge: Na (D = 2.5 x lO^cmV) is 
monovalent, Mn and Co (D = 4.6 x 
10" 7 ) are divalent; Sc is generally be- 
lieved to be trivalent (although the 



shape of the diffusion profile might be 
taken to indicate the presence of some 
divalent Sc); and V (5.0 x 10" 8 ) is at 
least trivalent. This effect is even more 
pronounced at lower temperatures (see 
the companion report by Hofmann and 
Magaritz). We believe that the appar- 
ently very simple relationship between 
ionic charge and diffusivity will prove 
to be an extremely useful tool to de- 
termine the oxidation state of transi- 
tion elements in natural melts. 



Acknowledgments 

We are grateful to Dr. H. E. Conzett 
and Ms. Ruth Mary Larimer of Law- 
rence Berkeley Laboratory for col- 
laborating with us and carrying out the 
irradiations and to Dr. Jack J. Rowe of 
the U.S. Geological Survey in Reston, 
Va., for making available a Ge(Li) de- 
tector. We are also grateful to the Na- 
tional Science Foundation for provid- 
ing financial support through Grant 
No. GA-40250. 






DEPARTMENT OF TERRESTRIAL MAGNETISM 



263 



Observation of X Radiation 
Characteristic of a United Atom 

L. Brown and G. H. Pepper 



Introduction 

During the past year the Van de 
Graaff accelerator was used exclu- 
sively for the investigation of processes 
present in the collision of heavy ions at 
energies of a few MeV. Crucial to these 
studies, as well as serving as an incen- 
tive for them, was a newly purchased 
lithium-drifted silicon detector capable 
of resolving 800-eV spectral lines. This 
instrument, which is decidedly 
superior to the one we previously used 
through loan, has allowed us to 
examine regions of the x-ray spectrum 
that have disclosed two phenomena: 
One is the clearest manifestation yet 
observed of the formation of a united 
atom; the other is a selective transfer of 
an electron vacancy from a K shell to a 
single L subshell, a form of transfer not 
heretofore recognized. 

When considering the collision of 
heavy ions wherein the electron shells 
of the two atoms interpenetrate, it is in 
some cases useful to consider them as 
having formed a united atom, i.e., an 
electronic system that has the nuclei of 
both target and projectile briefly form- 
ing a common center of attraction. This 
view, with its attendant idea of the 
formation of states in the united atom 
by way of the transformation from 
separated-atom levels through 
quasimolecular states, has difficulties, 
some of which were treated in last 
year's Annual Report, but it has cer- 
tain elements of truth. When heavy 
ions collide, a continuum of x rays is 
observed (Macdonald et al., 1973) that 
can result from neither nucleus- 
nucleus nor electron-nucleus brems- 
strahlung. This continuum is generally 
interpreted as resulting from a transi- 
tion of an electron to a vacancy in a 
molecular state formed during the col- 
lision. Since such states have energies 
that vary with time, these transitions 



obviously form continua. However, 
transitions to orbitals terminating on a 
given united atom shell should have a 
limit given by the binding energy of 
that shell. This has not been observed 
for the K band. Instead, an 
exponential-shaped continuum is seen 
to extend well beyond the united atom 
limits. 

The energies attainable with our ac- 
celerator do not allow an excitation of 
the K band sufficiently intense for 
study but do allow excitation of the L 
and M bands. These have been ob- 
served by others (Mokler et al., 1975) 
and are more suggestive of a united 
atom than the K band, but these previ- 
ous observations suffer from the inter- 
ference of characteristic lines with the 
continuum and from few data showing 
the dependence of the continua on 
atomic number and projectile velocity. 
We have found a projectile-target com- 
bination that allows these two de- 
ficiencies to be removed and that shows 
continua, both of L and M bands, with 
easily recognized limits, which move 
exactly as predicted when the atomic 
number of the target is changed. Ob- 
servations of the projectile velocity de- 
pendence of the continua show two ob- 
vious features: (1) Near the united- 
atom limit the cross section increases 
rapidly near the threshold and then 
levels off at high energies; (2) the con- 
tinuum extends beyond the united- 
atom limit and its width increases with 
projectile velocity. 

The lifetimes of united atoms could 
be estimated by dividing the radius of 
the shell in question by the projectile 
velocity. These lifetimes then give an 
upper limit to the uncertainties in the 
energies of the states through simple 
application of the Heisenberg uncer- 
tainty principle. It has been clear from 
the beginning that the observed exten- 
sion of the spectrum beyond the 
united-atom limit follows from this, 
but previous attempts to explain 
K-band spectra theoretically (Macek 
and Briggs, 1974; Miiller, 1976) have 
been in vain. Two circumstances com- 
bine to make interpretation difficult: 



264 



CARNEGIE INSTITUTION 



(1) No united- atom limit has been ob- 
served for the K band; (2) the observed 
broadening increases as the square 
root of the projectile velocity, whereas 
a simple approach to Heisenberg 
broadening would seem to require a 
broadening proportional to the veloc- 
ity. 

We observe a clear limit for L and M 
bands, as shown in Figs. 90 and 91. Its 
dependence on atomic number is 




Photon Energy 



Fig. 90. The Z dependence of united-atom 
limits. The top three spectra show continua re- 
sulting from vacancies in the target L shell (2p 
and 2s) that transform through molecular orbi- 
tals into united-atom L shells; transformations 
are shown morphologically at the upper right. 
The energy of the 2s united-atom subshell is 
shown by vertical arrows for sodium (Z = 11) 
projectiles onto targets of titanium, iron, and zinc 
(Z = 22, 26, and 30, respectively). The spectra are 
normalized at the united-atom limit. The lower 
two spectra show the same phenomenon for 
M-shell limits produced by sodium projectiles on 
targets of palladium and tin (Z = 46 and 50) but 
use a different manner of presentation. The pal- 
ladium spectrum has been shifted 285 eV to the 
right, the difference in the 3s energies of the 
united atoms Z = 57 and 61. This shows the iden- 
tical shape of the two continua beyond the 
united-atom limit. This manner of presentation 
could have been used with equal effect for the 
L-limit data. 




'10-2 



10-2- 



Photon Energy (keV) 

Fig. 91. The velocity dependence of the 
broadening beyond the united-atom limit. The 
continuum extends beyond the united-atom limit 
because that energy must be uncertain by an 
amount AE = fy/At, where At is the lifetime of the 
state, a lifetime dependent on the projectile ve- 
locity. Spectra for three projectile energies are 
shown with smooth curves, which are halves of 
Gaussians that have been corrected for thick 
target effects, fitted to the portion beyond the 
united-atom limit of Na + Ni (11 + 28). The 
smooth curves are reproduced at the top to bring 
out the velocity dependence of the data. If the 
effects of detector resolution are removed, the 
broadening is found to be proportional to the 
square root of the projectile velocity. 



exactly as predicted. The broadening 
beyond the united-atom limit is very 
nearly half of a Gaussian, the flank of 
which drops more than two orders of 
magnitude; this appearance of the 
united-atom limit was expected when 
the molecular-orbital continuum was 
discovered, but determined searches 
failed to reveal anything but the expo- 
nentially shaped continuum. We 
undertook a study of the energy de- 
pendence of the broadening and found 
that it was proportional to the square 
root of the projectile velocity. Although 
in agreement with the K-band studies, 
this was somewhat disappointing be- 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



265 



cause we had hoped that this simple 
manifestation would have the equally 
simple velocity dependence, i.e., a 
broadening proportional to velocity. A 
curious aspect of the matter is that the 
only theories now under serious con- 
sideration predict a square-root de- 
pendence but do so on the basis of as- 
sumptions that seem to us questiona- 
ble. Since our data are the best verifi- 
cation of this dependence, our pre- 
judices to the contrary notwithstand- 
ing, we are seeking to remove the weak 
elements of these theories or to find an 
acceptable alternate set of assump- 
tions. 



Vacancy Sharing in Heavy-Ion 
Collisions 

The most striking phenomenon of 
heavy-ion collisions is the large proba- 
bility for producing electron holes in 
the less tightly bound of two inter- 
penetrating shells that have nearly 
equal binding energies, e.g., K shells of 
two atoms with Z, ~ Z 2 . Conventional 
theory predicts a small probability, if 
applied strictly. The novel approach 
proposed here last year has found little 
acceptance because it predicts only the 
relative Z dependence (quite satisfac- 
torily) and utilizes a mechanism not 
observed elsewhere. Whatever the de- 
tails of its operation, molecular excita- 
tion depends in some way on the trans- 
formation of atomic into molecular 
wave functions, and its complete eluci- 
dation is the central problem of this 
field of research. 

If one sets aside the mysterious 
manner in which vacancies appear in 
the more weakly bound shell and con- 
siders instead the method by which 
vacancies appear in the more strongly 
bound shell, then one finds things in a 
more satisfactory condition. Meyerhof 
(1973) demonstrated that, given the 
vacancy in a light K shell, he could cal- 
culate the probability of its transfer to 
the heavy K shell. The mechanism for 
this is radial coupling, which links 
molecular states of the same parity. 



We set about to see if this process 
also coupled K and L shells and noted 
an unusual circumstance best illus- 
trated with the raw data. In Fig. 92a is 
shown the spectrum of praseodymium 
produced by bombardment with 2-MeV 
Li + ; it is a normal example of Coulomb 
excitation, showing all the L lines that 
the detector is capable of resolving 
with intensities easily understood 
through conventional theory. In Fig. 
92b is shown the spectrum of 
praseodymium when bombarded with 
2-MeV K + ; the K lines of potassium are 
the most prominent feature and result 
from the molecular excitation dis- 
cussed previously. L lines from 
praseodymium are also shown but are 
in marked contrast to the spectrum of 
Fig. 92a, as the total intensity is much 
less and the intensity ratio is entirely 
different; indeed only the L a and L^ 2 
lines are observed. 

Reference to x-ray tables quickly dis- 
closes that these lines are transitions 
to vacancies in the 2p 3/2 subshell and 
that transitions to the other L sub- 
shells, the 2p 1/2 and 2s 1/2 , are absent 
within the observational limits im- 
posed by the continuum radiation. The 
transfer of vacancies from the K shell 
of potassium, if the process is a trans- 
fer, is obviously selective. A study of 
the velocity dependence of the ratio of 
potassium K lines to praseodymium L 
lines shows values that lie along an ex- 
ponential in 1/v, which present in- 
terpretations would indicate as charge 
exchange. 

Our tentative interpretation of the 
process can be understood by reference 
to Fig. 93a, which shows the manner by 
which the atomic energy levels trans- 
form through molecular orbitals into 
the energy levels of a united atom for 
the collision of K + Pr. One notices 
that the potassium K shell transforms 
by a 3dcr orbital which has the initial 
vacancy and the three praseodymium L 
subshells by a 2p77, 2pcr, and 2scr. Ra- 
dial coupling proceeds between states 
of the same molecular parity, here be- 
tween the 3dcr and the two orbitals 2pcr 
and 2scr. Rotational coupling between 



266 



■2 10 



-6 



I0" 8 - 



i r~i i i i — i — r 



Pr-L 


a 


•03. 


.CvJ 


1 t~) 



CVJ 
K 

/ 



I I I 



I V I 



CARNEGIE INSTITUTION 
1 I I 1 1 1 



K-K 
o 



Pr-L 
a 

'■■ CM 
01 



5 5 

E Ph (keV) 

Fig. 92. The spectra of praseodymium L lines produced by (a) direct excitation with 2-MeV Li + and 
(b) vacancy sharing with 2-MeV K + . The identity of various praseodymium L lines capable of being 
resolved by the detector are indicated for both spectra with the location of the potassium K lines also 
shown for spectrum (b). Unmarked peaks are the result of target contaminants and a photon escape 
peak, an instrumental effect. The L lines of praseodymium in spectrum (b) are thought to result 
primarily from the transfer of vacancies from the potassium K shell to the praseodymium L III 
subshell (2p 3/2 ) through rotational coupling. 




-Ne + Ne 



Fig. 93. Correlation diagrams for the collision of (a) K + Pr and (6) Ne + Ne. In both diagrams the 
morphology of the transformation of separated-atom through molecular levels to united-atom energy 
states is shown. The vertical axis is proportional to the logarithm of the energy; the horizontal axis 
shows the internuclear distance schematically. The energy scales for (a) and (b) are different. 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



267 



orbitals of opposite parity is also 
known to occur, but it has been ob- 
served only in collisions in which the 
exchanging orbitals attained nearly 
the same energy at small radii of sep- 
aration, an example of which is shown 
in Fig. 93b. This shows the correlation 
of orbitals in the collision of neon and 
neon, a collision in which the transfer 
of a vacancy from the 2p shell of an 
incident ion is transferred by the 2pcr 
to the 2p7r orbital. The theory of this 
transfer (Fastrup, 1976) agrees well 
with experiment. Except at very low 
velocities, the probability of transfer is 
nearly unity because the two orbitals 
have nearly the same energy at small 
values of internuclear radius. 

Because of the different probability 
observed for 3dcr — » 2p7r compared with 
3dcr — > 2po- and 2sct, we think that it 
must be a new manifestation of rota- 
tional coupling — a case of rotational 
coupling at large rather than small 
internuclear radius. 

References 

Fastrup, B., Experimental studies of 
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collisions, in The Physics of Electronic 
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Macdonald, J. R., M. D. Brown, and T. 
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Macek, J. H., and J. S. Briggs, Collision 
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Meyerhof, W. E., K- vacancy sharing in 
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Mokler, P. H., S. Hagmann, P. Armbruster, 
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Miiller, B., Radiative processes in transient 
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ington Press, Seattle, 1976. 



268 



CARNEGIE INSTITUTION 



BIBLIOGRAPHY 



Publications listed below can be obtained at no 
charge from the Department of Terrestrial Mag- 
netism, 5241 Broad Branch Rd., N.W., Washing- 
ton, D.C. 20015. When ordering, please give re- 
print number(s). No reprints are available for ti- 
tles marked with asterisks. 

Aldrich, L. T., see Ocola, L. C. 

Arculus, R. J., see Shimizu, N. 

Assousa, G. E., see Brown, L., and 

Warner, J. W. 

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*4416 Hart, S. R., Basement rock synthesis: 

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Gardner, G. M. Light, J. L. Duggan 
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Meyer, R. P., see Ocola, L. C. 

4420 Mirabel, I. F., and K. C. Turner, An 
anomalous velocity neutral hydro- 



4421 



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4425 



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4429 



gen structure near the galactic cen- 
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Nelson, J. W., see McDaniel, F. D. 

Ocola, L. C, L. T. Aldrich, J. F. Get- 
trust, R. P. Meyer, and J. E. 
Ramirez, S. J., Project Narino I: 
Crustal structure under southern 
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from seismic refraction data, Bull. 
Seis. Soc. Am., 65, 1681-1695, 1975. 

Pepper, G. H., and L. Brown, The 
15 N(p,a) 12 C reaction with polarized 
protons from 0.34 to 1.21 MeW,Nucl. 
Phys.,A260, 163-171, 1976. 

Pepper, G. H., see McDaniel, F. D. 

Peterson, C. J., see Rubin, V. C. 

Ramirez, J. E., S. J., see Ocola, L. C. 

Rohrer, U., and L. Brown, The 
9 Be(p,n) 9 B reaction with polarized 
protons from 2.4 to 2.9 MeV, Nucl. 
Phys.,A261, 141-148, 1976. 

Rubin, V. C, W. K. Ford, Jr., and C J. 
Peterson, Evidence for contraction 
in the nuclear ring of the barred spi- 
ral galaxy NGC 3351, Astrophys. J., 
199, 39-48, 1975. 

Rubin, V. C, N. Thonnard, and W. K. 
Ford, Jr., Observations of NGC 
6764, a barred Seyfert galaxy, As- 
trophys. J., 199, 31-38, 1975. 

Rubin, V. C, see Warner, J. W. 

Shimizu, N., Geochemistry of ul- 
tramafic inclusions from Salt Lake 
Crater, Hawaii, and from Southern 
African kimberlites, in Physics and 
Chemistry of the Earth, Vol. 9, pp. 
655-669, L. H. Ahrens, J. B. Daw- 
son, A. R. Duncan, and A. J. Erlank, 
eds., New York, Pergamon Press, 
1975. 

Shimizu, N., and R. J. Arculus, Rare 
earth element concentrations in a 
suite of basanitoids and alkali 
olivine basalts from Grenada, Les- 
ser Antilles, Contrib. Mineral. Pet- 
rol, 50, 231-240,1975. 

Shimizu, N., and I. Kushiro, The parti- 
tioning of rare earth elements be- 
tween garnet and liquid at high 
pressures: Preliminary experi- 
ments, Geophys. Res. Lett., 2, 413- 
416,1975. 

Snoke, J. A., Archambeau's elas- 
todynamical source-model solution 
and low-frequency spectral peaks in 
the far-field displacement 
amplitude, Geophys. J. R. Astron. 
Soc, 44, 27-44, 1976. 

Thonnard, N., see Brown, L., and Ru- 
bin, V. C 

Turner, K. G, see Mirabel, I. F. 

Van Rinsvelt, H. A., see Brown, L., and 
McDaniel, F. D. 



DEPARTMENT OF TERRESTRIAL MAGNETISM 



269 



*4430 Wallace, D. G., Prediction of the sec- 
ondary and tertiary structure of 
plastocyanin, Biophys. Chem., 4, 
123-130, 1976. 

*4431 Wallace, D. G., The sequence- 
immunology correlation among 
higher plant plastocyanins, Im- 
munochemistry , in press. 

*4432 Wallace, D. G., and D. Boulter, Im- 
munological comparisons of higher 
plant plastocyanins, Phytochemis- 
try, 15, 137-141, 1976. 

4433 Warner, J. W., G. E. Assousa, and 

others, Radio spectra of OH471 and 
OQ112, Nature, 249, 743-746, 1974. 

4434 Warner, J. W., and V. C. Rubin, Physi- 

cal conditions and structure in NGC 
7293, "The Helix," Astrophys. J., 
198, 593-603, 1975. 

4435 Wetherill, G. W., Late heavy bom- 

bardment of the moon and terrest- 
rial planets, Proc. 6th Lunar Sci. 
Conf.,2, 1539-1561, 1975. 

4436 Wetherill, G. W., Radiometric chronol- 

ogy of the early solar system, in An- 
nual Review of Nuclear Science, Vol. 
25, pp. 283-328, E. Segre, J. R. 
Grover, and H. P. Noyes, eds., An- 
nual Reviews, Inc., Palo Alto, 
California, 1975. 

*4437 Yeats, R. S., and S. R. Hart, Introduc- 
tion and principal results, Leg 34, 
Deep Sea Drilling Project, in Initial 
Reports of the Deep Sea Drilling 
Project, Vol. 34, 814 pp., T. L. Val- 
lier, ed., Washington, D.C., U.S. 
Government Printing Office, 1976. 

Zander, A. R., see McDaniel, F. D. 



Abstracts and Minor 
Publications 

Ballard, G. S.,see Evans, J. R. 

Beavan, R. J., see Evans, J. R., and King, G. C. P. 

Bilham, R. G.,see Evans, J. R. 

Brooks, C, D. James, and S. R. Hart, Mantle 
isochrons and their implications for the 
isotopic composition of subcontinental litho- 
sphere (abstr.), EOS Trans. Am. Geophys. 
Union, 57, 351, 1976. 

Evans, J. R., G. S. Ballard, R. J. Beavan, R. G. 
Bilham, G. John, and G. C. P. King, A tidal 
strain survey of Great Britain (abstr.), EOS 
Trans. Am. Geophys. Union, 57, 233, 1976. 

Evans, J. R., see King, G. C. P. 

Ford, W. K., Jr., see Peterson, C. J., and Rubin, 
V.C. 

Graham, J. A., see Rubin, V.C. 

Hart, S. R., see Brooks, C. 

James, D., see Brooks, C. 

John, G., see Evans, J. R. 

King, G. C. P., I. S. Sacks, J. R. Evans, R. J. 
Beavan, and A. Snoke, Single site phase veloc- 



ity measurement (abstr.), EOS Trans. Am. 
Geophys. Union, 57, 286, 1976. 

King, G. C. P., see Evans, J. R. 

Linde, A. T., I. S. Sacks, and J. A. Snoke, A nu- 
merical model study of multiple rupture 
earthquakes (abstr.), EOS Trans. Am. 
Geophys. Union, 57, 154, 1976. 

Linde, A. T., see Sacks, I. S. 

Nicholls, I. A., see Whitford, D. J. 

Pepper, G. H. and L. Brown, An observation of 
the united-atom limit for the molecular orbi- 
tal continuum (abstr.), Bull. Am. Phys. Soc, 
21, 649, 1976. 

Peterson, C. J., Rotation of the nuclear region of 
M31 (abstr.), Bull. Am. Astron. Soc, 7, 254, 
1975. 

Peterson, C. J., V. C. Rubin, and W. K. Ford, Jr., 
The peculiar galaxy NGC 1275 (abstr.), Bull. 
Am. Astron. Soc, 8, 297, 1976. 

Peterson, C. J., V. C. Rubin, W. K. Ford, Jr., and 
N. Thonnard, Motions of the stars and excited 
gas in the barred spiral galaxy NGC 3351 
(abstr.), Bull. Am. Astron. Soc, 7, 538, 1975. 

Peterson, C. J., see Rubin, V. C. 

Roberts, M. S., see Rubin, V. C. 

Rodriguez B., A., see Sacks, I. S. 

Rubin, V. C, and W. K. Ford, Jr., The motion of 
our galaxy and the local group of galaxies 
(abstr.), Bull. Am. Astron. Soc, 7, 253, 1975. 

Rubin, V. C, C. J. Peterson, and W. K. Ford, Jr., 
The rotation curve of the E7/S0 galaxy NGC 
3115 (abstr.), Bull. Am. Astron. Soc, 8, 297, 
1976. 

Rubin, V. C, N. Thonnard, W. K. Ford, Jr., J. A. 
Graham, and M. S. Roberts, A galaxy of coeffi- 
cients: A,„ A-,, A„,A LC ,D,„ andLX ( abstr. ),Bull. 
Am. Astron. Soc, 7, 531, 1975. 

Rubin, V. C, see Peterson, C. J. 

Sacks, I. S., and A. Snoke, Heterogeneous veloc- 
ity structure at the base of the mantle (abstr.), 
Trans. Am. Geophys. Union, 57, 284, 1976. 

Sacks, I. S., J. A. Snoke, A. T. Linde, and A. Rod- 
riguez B., Shallow seismicity in subduction 
zones (abstr.), 47th Annual Meeting of the 
Eastern Section of the Seismological Society of 
America held in St. Louis, Missouri, 
November 6-7, 1975. 

Sacks, I. S., see King, G. C. P., Linde, A. T., and 
Snoke, J. A. 

Snoke, J. A., Implications of moving boundaries 
in elastodynamics: Comments on "the pulse 
shapes and spectra of elastic waves generated 
when a cavity expands in an initial shear 
field" by Robert Burridge, J. Geophys. Res., 
81, 1035-1036, 1976. 

Snoke, J. A., and I. S. Sacks, Determination of 
the lithosphere-asthenosphere boundary 
using converted waves (abstr.), EOS Trans. 
Am. Geophys. Union, 57, 283, 1976. 

Snoke, J. A., see King, G. C. P., Linde, A. T., and 
Sacks, I. S. 

Thonnard, N., The 21-cm structure of WLM and 
its surroundings (abstr.), Bull. Am. Astron. 
Soc, 7, 550, 1975. 

Thonnard, N., see Peterson, C. J., and Rubin, V. 



270 CARNEGIE INSTITUTION 

Wetherill, G. W., Total number of earth-crossing Whitford, D. J., and I. A. Nicholls, Potassium 

Apollo objects with radii larger than 500 me- variation in quaternary lavas across the 

ters (abstr.), EOS Trans. Am. Geophys. Union, Sunda arc in Java and Bali (abstr.), EOS, 

57, 275, 1976. Trans. Am. Geophys. Union, 57, 347, 1976. 



PERSONNEL 

Staff Members 

L. Thomas Aldrich David E. James 

George E. Assousa Alan T. Linde 

Louis Brown Nancy R. Rice 4 

Dean B. Cowie 1 Richard B. Roberts 5 

Scott E. Forbush, Emeritus Vera C. Rubin 

W. Kent Ford, Jr. I. Selwyn Sacks 

Stanley R. Hart 2 Norbert Thonnard 

Albrecht W. Hofmann Kenneth C. Turner 

Bill H. Hoyer :) George W. Wetherill, Director 

Distinguished Service Member of Carnegie Institution 
Merle A. Tuve 



Research Associates (Staff) 

Tom I. Bonner 11 J. Arthur Snoke 

Mordeckai Magaritz 7 Kiyoshi Suyehiro 8 



Research Associates (Nonresident) 

Mateo Casaverde, Instituto Geofisco del Hiromu Okada, Hokkaido University, Sap- 
Peru, Lima, Peru poro, Japan 9 

Leonidas Ocola, Instituto Geofisico del Anibal Rodriguez B., Universidad Nacional 

Peru, Lima, Peru de San Agustin, Arequipa, Peru 

Shigeji Suyehiro, Japan Meteorological 
Agency, Tokyo, Japan 10 



Carnegie Fellows (Postdoctoral) 

John R. Evans" R. Sundar Rajan 12 

George H. Pepper Stuart J. Weidenschilling 1 ' 

Charles J. Peterson David J. Whitford 14 



■Institut Pasteur, Paris, France, from Jan. 20, K From April 1, 1976. 

1975. 9 Through Dec. 31, 1975. 

^Resigned Aug. 31, 1975. 10 Through Dec. 31, 1975. 

'Resigned June 30, 1975. "From Jan. 1, 1976. 

'Resigned Feb. 15, 1976. l2 From Sept. 24, 1975. 

"'Retired Dec. 31, 1975. ' 'From Feb. 1, 1976. 

"Resigned Feb. 15, 1976. 14 From Dec. 1, 1975. 
7 FromFeb. 1,1976. 



DEPARTMENT OF TERRESTRIAL MAGNETISM 

Carnegie Fellows (Predoctoral) 



271 



John R. Bannister, University of Alberta, Arturo Cuyubamba, McGill University, 
Edmonton, Canada Montreal, Canada 15 

Visiting Investigators 

Claude Allegre, University of Paris, Paris, Montreal, Montreal, Canada 17 

France 1 " John R. Cleary, Australian National Uni- 

Christopher Brooks, Universite de versity, Canberra, Australia 18 



Charles L. Bennett 1 a 
Gregory S. DeWitt 2 " 



Trainee Fellows 

Antonio Flores 21 
Michael Schneider- 
Christine M. Seber 2 ' 



Collaborators 



E 



E 



C. Aguirre B., Universidad Mayor de San 
Andres, La Paz, Bolivia 
Arnal, Institute Argentino de Radioas- 
tronomia, Villa Elisa, Argentina 
G. Arnold, Ohio State University, Co- 
lumbus, Ohio 

Bajaja, Institute Argentino de Radioas- 
tronomia, Villa Elisa, Argentina 

B. Balick, University of Washington, Seat- 
tle, Washington 

R. J. Beavan, Lamont Doherty Geological 
Observatory, Palisades, New York 

E. Berg, University of Hawaii, Honolulu, 
Hawaii 

K. Bloomfield, Institute of Geological Sci- 
ences, London, England 

M. Bracamonte, Trujillo, Peru 

B. F. Burke, Massachusetts Institute of 
Technology, Cambridge, Massachusetts 

R. Cabre, S. J., Observatorio San Calixto, 
La Paz, Bolivia 

G. N. Cohen, Institut Pasteur, Paris, 
France 

R. Colomb, Instituto Argentino de Radioas- 
tronomia, Villa Elisa, Argentina 

H. E. Conzett, Lawrence Berkeley Labora- 
tory, Berkeley, California 

G. L. Davis, Geophysical Laboratory, Car- 
negie Institution of Washington, Wash- 
ington, D. C. 



S. del Pozo, Instituto Geofisico Boliviano, 

La Paz, Bolivia 
J. W. Erkes, State University of New York, 

Albany, New York 
A. J. Erlank, University of Capetown, Ron- 

debosch, South Africa 

D. W. Evertson, University of Texas at 
Austin, Texas 

E. Filloy, Instituto Argentino de Radioas- 
tronomia, Villa Elisa, Argentina 

C. Garavito, Planetario Destrital, Bogota, 

Colombia 
L. Gelinas, Genie Mineral, Ecole Polytech- 

nique, Montreal, Quebec, Canada 
A. A. Giesecke, Instituto Geofisico del Peru, 

Lima, Peru 
M. Gil, Instituto Argentino de Radioas- 

tronomia, Villa Elisa, Argentina 
Nicol Girardin, Institut de Physique du 

Globe, Paris, France 
M. Gordon, Instituto Argentino de Radioas- 

tronomia, Villa Elisa, Argentina 
J. A. Graham, Cerro Tololo Interamerican 

Observatory, La Serena, Chile 
C. Heiles, University of California, Berke- 
ley, California 

C. E. Helsley, University of Texas, Dallas, 
Texas 

D. Huaco, Instituto Geofisico del Peru, 
Lima, Peru 



'•'Through Dec. 31,1975. 
"'From June 15, 1976. 
''Through Dec. 31, 1975. 
IK From Sept. 15 to Dec. 31, 1975. 
'"May 15- Aug. 15, 1975, and June 1-July 31, 
1976. 



2 "From June 1, 1976. 
-'From May 1 to Aug. 31, 1975. 
22 From Sept. 29, 1975 to Mar. 3, 1976. 
2! From Jan. 1 to Mar. 31, 1976. 



272 



CARNEGIE INSTITUTION 



G. C. P. King, University of Cambridge, 
Cambridge, England 

C. K. Kumar, Howard University, Wash- 
ington, D. C. 

I. Kushiro, Geophysical Laboratory, Car- 
negie Institution of Washington, Wash- 
ington, D. C. 

Ruth M. Larimer, Lawrence Berkeley Lab- 
oratory, Berkeley, California 

R. P. Meyer, University of Wisconsin, 
Madison, Wisconsin 

I. Mirabel, Instituto Argentino de Radioas- 
tronomia, Villa Elisa, Argentina 

R. Morras, Instituto Argentino de Radioas- 
tronomia, Villa Elisa, Argentina 

Y. Motoya, Kamikineusu Seismological 
Observatory of Sapporo University, 
Hokkaido, Japan 

G. Olafsson, Akureyri, Iceland 

J. Oort, Sterrewacht, Leiden, The Nether- 
lands 

R. W. Page, Bureau of Mineral Resources, 
Canberra, Australia 

E. Pimental, Cuzco University, Cuzco, Peru 

W. Poppel, Instituto Argentino de Radioas- 
tronomia, Villa Elisa, Argentina 

G. Poupeau, Centre des Faibles Radioacti- 
vites, Gif-sur-Yvette, France 

J. E. Ramirez, S. J., Instituto Geofisico de 
los Andes Colombianos, Bogota, Colom- 
bia 

M. S. Roberts, National Radio Astronomy 
Observatory, Charlottesville, Virginia 



R. Rodriguez, Observatorio San Calixto, La 
Paz, Boliva 

J. Rowe, U.S. Geological Survey, Reston, 
Virginia 

R. H. Sanders, National Radio Astronomy 
Observatory, Charlottesville, Virginia 

J. G. Schilling, Narragansett Marine Labo- 
ratory, University of Rhode Island, 
Kingston, Rhode Island 

D. W. Schwartzman, Howard University, 
Washington, D. C. 

R. G. Seyler, Ohio State University, Co- 
lumbus, Ohio 

H. Sigtrygsson, Reykjavik, Iceland 

D. Simoni, Arequipa, Peru 

R. Stefansson, Reykjavik, Iceland 

L. Tamayo, Universidad Nacional de San 

Agustin, Arequipa, Peru 
J. L. Telleria, Planetario Destrital, Bogota, 

Colombia 
P. Truffa-Bachi, Institut Pasteur, Paris, 

France 
F. Volponi, Universidad Nacional de Cuyo, 

San Juan, Argentina 
R. E. White, Steward Observatory, Univer- 
sity of Arizona, Tucson, Arizona 
J. G. Williams, Jet Propulsion Laboratory, 

Pasadena, California 
Y. Yamagishi, Matsushiro Seismological 

Observatory of Japan, Meteorological 

Agency, Matsushiro, Japan 

E. Zinner, McDonnel Center for Space Sci- 
ences, Washington University, St. Louis, 
Missouri 



Supporting Staff 



Caroline A. Busch, Typist, Fiscal 

Assistant 24 
R. Paul Busch, Caretaker (temporary) 
Louis S. Carter, Instrument Maker 
Gloria J. Counts, Clerk-Typist- 5 
Dorothy B. Dillin, Librarian 
John B. Doak, Electronics Research 

Specialist 
William N. Dove, Office Manager 
Everett T. Ecklund, Design Engineer 
Mark Feigenson, Laboratory Assistant 

(temporary) 
Esther Gordon, Research Assistant 
Bennie Harris, Caretaker 
E. Kathleen Hill, Secretary 2 ' 1 



Janet C. Hunt, Secretary 27 

Judith H. James, Typist 28 

Willis Kilgore, Jr., Caretaker 

Charles A. Little, Electronics Research 

Specialist 
Niels M. Pedersen, Fiscal Officer 
Glenn R. Poe, Electronics Research 

Specialist 
Elliot M. Quade, Assistant Maintenance 

Foreman 
Carl M. Rinehart, Instrument Maker 
Michael Seemann, Design Engineer — 

Mechanical 
E. Lynn Smith, Typist, Fiscal Assistant 2 " 
James E. Spicer, Maintenance Foreman 
Milton T. Taylor, Instrument Maker 



-'From January 12, 1976. 
-"'From January 26, 1976. 
-"Through February 15, 1976. 



"From May 6, 1976. 
"Through January 15, 1976. 
29 Through September 30, 1975. 



Hale Observatories 

Operated by Carnegie Institution of Washington 
and California Institute of Technology 

Pasadena, California 



Horace W. Babcock 
Director 

J. Beverley Oke 
Associate Director 



OBSERVATORY COMMITTEE 

Horace W. Babcock, Chairman 

J. Beverley Oke, Vice-Chairman 

Jerome Kristian 

Guido Munch 

Allan Sandage 

Wallace L. W. Sargent 

Maarten Schmidt 
Arthur H. Vaughan, Jr. 



Carnegie Institution of Washington Year Book 75, 1975-1976 



Contents 



Introduction 277 

Observing Conditions 278 

Physics of the Sun 279 

Solar magnetic fields 279 

Large-scale velocity fields of the sun . . . 280 

Big Bear Solar Observatory 280 

Coronal holes 280 

Surge of 1971 281 

X rays from solar flares 281 

The lifetime and evolution of fibrils .... 281 

Macrospicules and microflares 282 

Differential rotation 282 

Solar System Studies 283 

Jupiter 283 

Io 283 

Satellites and asteroids 284 

Stellar Spectroscopy 284 

White dwarfs 284 

Studies of proper-motion stars 287 

Rotation of B-type stars 288 

P- 3 He stars 288 

Braking of magnetic Ap stars 288 

3 He stars 288 

Circumstellar envelopes 289 

Stellar spectroscopy at 1 /i,: R Aquarii . . 289 

Stellar Chromospheres 289 

Absolute luminosities 289 

H and K fluxes in main-sequence stars . 289 

Infrared and Millimeter-Band Photometry 290 

Extragalactic objects 290 

Galactic center 290 

Nova Cygni 1975 290 

1-mm photometry of compact 
extragalactic objects 291 

Globular Clusters 291 

Dynamical models 291 

Interstellar Matter and Gaseous Nebulae . 292 
High-latitude reflection nebulosities 

illuminated by stars of the 

galactic plane 292 

Interstellar matter 294 

Herbig-Haro object in the Orion Nebula 294 

The H II region W3 295 

Infrared properties of OH masers 295 

Dark clouds 295 

Mapping of thermal emission from 

molecular clouds 296 

NGC 7538 296 

Pulsars 297 

Search for the Hulse-Taylor binary 
pulsar 297 



X-Ray Sources 297 

Supernovae 298 

The Galaxy 299 

Metal abundances in globular clusters . . 299 

Galaxies 299 

New redshifts for bright galaxies 299 

Distances to nearby galaxies derived 

from brightest stars 299 

Supergiant cD galaxies 300 

Markarian galaxies 300 

Seyfert galaxies 301 

Infrared observations 301 

Nuclear regions of M31 302 

Observational search for the 

"missing mass" 302 

Redshifts, magnitudes, and colors 

of a sample of galaxies 302 

Chemical history of galaxies 303 

Large Magellanic Cloud 303 

Population synthesis 304 

Variable stars in the dwarf 

spheroidal galaxies 304 

Image processing of galaxy photographs 305 
Ejection from the spiral galaxy 

NGC 1097 305 

Neighborhoods of galaxies 306 

M87 and its globular clusters 308 

Clusters of Galaxies 308 

Redshifts and magnitudes 308 

The Hubble diagram 309 

Photometry and redshifts of first 

brightest cluster members 310 

Brightest cluster galaxies 310 

The dynamics of galaxies and 

clusters of galaxies 311 

Equality of redshifts of E and SO galaxies 

in the Virgo cluster 311 

A new nearby group 311 

X-ray clusters 312 

Intergalactic hydrogen clouds 312 

Radio Sources 312 

3CR identifications 312 

Perseus A 312 

Quasars and Quasi-Stellar Objects 312 

Spectroscopic observations 312 

Quasar surveys 313 

Search for high-redshift QSOs 313 

Absorption lines 313 

Optical continuum 315 

Emission lines 316 

Observational Cosmology 316 

The Hubble constant and 

deceleration parameter 316 



Theoretical Studies 317 

Gravitational radiation from pressureless 

nonaxisymmetric stellar collapse 317 

Gravitational radiation from axisymmetric 

stellar collapse with pressure 318 

Angular momentum of galaxies 

and tidal interaction 318 

Instrumentation 318 

SIT acquisition and guiding 

system (SITEX) 318 

Remote operation of Cassegrain 

ring rotation 319 

SIT-Vidicon digital spectrographs 319 

Photon-Counting Image-Intensifier 

System 319 

Charge-coupled devices 320 

Photometry of stellar H and K lines .... 320 

Infrared detection systems 321 

Astroelectronics Laboratory 321 



Guest Investigators 322 

Las Campanas Observatory 334 

2.5-meter du Pont Telescope 334 

Cassegrain camera 335 

Cassegrain spectrograph 335 

Television system 335 

Infrared photometer 336 

Coude spectrograph 336 

Astronomers' Lodge 336 



Bibliography 336 

Staff and Organization 341 

Research Fellows and Students 341 



INTRODUCTION 



New data on very faint sources far 
out in the Universe are beginning to 
accumulate at impressive rates. This 
situation, long worked for by observers, 
results from years of effort on the part 
of many individuals and groups to per- 
fect digital or linear photoelectric de- 
vices capable of precise photometry on 
large numbers of picture elements 
within an optical image — in other 
words, to count incoming photons in 
the picture elements and to present the 
resulting large amount of information 
in a useful way. Such devices are 
equally important in the focal plane of 
a telescope or at the output end of a 
spectrograph that is attached to the 
telescope. But the photographic plate, 
especially when employing the most 
modern emulsions of high information 
content and optimum color sensitivity, 
continues to be invaluable, particu- 
larly for wide-angle recording. 

Two groups at the Hale Obser- 
vatories, Drs. Allan Sandage and 
Jerome Kristian, using the photo- 
graphic method, and Drs. James E. 
Gunn and J. B. Oke, using an image 
tube, are reporting the discovery of im- 
pressive numbers of new faint clusters 
of galaxies. Such clusters are of impor- 
tance because it is known that the 
brightest galaxy in a cluster (for rea- 
sons not yet understood) has a standard 
luminosity that is very nearly the same 
for any cluster. Thus the relative dis- 
tances to such clusters can be derived, 
and if the redshifts of a number of the 
galaxies in each cluster can be meas- 
ured with the spectrograph, new points 
in the Hubble diagram of redshift ver- 
sus distance can be plotted. 

Not only are dozens of distant clus- 
ters of galaxies being found, but the 
production of spectra and the meas- 
urement of redshifts of galaxies in 
them is proceeding at a record rate. For 
this work, Westphal, Kristian, and 
Sandage are using an SIT (silicon in- 
tensifier target) Vidicon on a prism 



spectrograph at the 5-meter telescope, 
while Gunn and Oke are using the 
SIT- Vidicon with a grating spectro- 
graph. Both of these SIT systems have 
been derived, with various technical 
improvements, from the system de- 
scribed in 1971 by Dr. Thomas B. 
McCord and Westphal (Year Book 71, 
p. 705). Most of the new redshifts being 
measured are greater than z = 0.2, and 
a substantial fraction have z > 0.4. 
These individual galaxies are of small 
angular extent, and most are much 
fainter than the uniform light of the 
sky (airglow and zodiacal light) that 
overlies them. Spectroscopy involves 
subtracting electronically the spec- 
trum of the sky from that of the object 
plus sky. The principal limitation now 
is not measuring the spectra but find- 
ing faint candidates and positioning 
them on the slit. Observing depends 
upon low-level closed-circuit TV sys- 
tems for these operations, which for- 
merly involved the much more difficult 
technique of making blind offsets from 
nearby stars. 

A significant new finding by San- 
dage further illuminates our under- 
standing of the structure of the Galaxy. 
A photographic survey of the north and 
south galactic polar caps, in progress 
for several years, has not only resulted 
in discovery of many new remote clus- 
ters of galaxies, but has revealed, in 
our own Galaxy, faintly luminous 
nebulous regions, often in irregular 
connective patterns that extend over 
distances of degrees (see Fig. 4). The 
patterns display much local structure. 
Sandage's filter photography shows 
them to be reflection nebulae consist- 
ing of dust. The local surface bright- 
ness of the dust clouds that scatter il- 
luminating radiation depends upon 
their optical thickness. Sandage shows 
that the observations are in good 
agreement with the hypothesis that 
the dust clouds are diffusely illumi- 
nated by general light of stars in the 



277 



278 



CARNEGIE INSTITUTION 



galactic plane. The surface brightness 
of the denser reflection nebulae is pre- 
dicted to be about 25.5 magnitudes per 
square arc second. The brightness of 
the clouds viewed as reflection nebulae 
is directly related to their extinction of 
the light of distant galaxies observed 
through them; this extinction can be as 
high as Am ~ 0.3 even in the direction 
of the pole of the Galaxy. Some parts of 
the north polar cap are free of detecta- 
ble nebulosities to a level of 26 mag per 
square arc sec. Studies of the distribu- 
tion and surface brightness of irregular 
reflection nebulosities in high galactic 
latitudes should produce detailed 
knowledge of the extinction, leading to 
more precise photometric corrections 
for the magnitudes of remote galaxies. 
This year marks the completion of 
basic construction of the 2.5-meter 
Irenee du Pont Telescope of the Las 
Campanas Observatory in Chile. This 
project was initiated in December 1970 
by a gift to the Carnegie Institution 
from Mr. and Mrs. Crawford H. 
Greenewalt. Design of the instrument 
by the staff of the Hale Observatories 
was begun soon thereafter, following a 
resolution by the Trustees that the In- 
stitution should augment the funds 
available to permit construction of a 
telescope of the 100-inch class. The 
telescope was constructed in the 
United States, shipped to Chile, and 
erected at the Observatory under the 
direction of Mr. Bruce H. Rule with the 



collaboration of Las Campanas person- 
nel. In May 1976, optics were installed 
and preliminary test photographs ob- 
tained. Staff members believe that the 
care taken in achieving near-perfection 
of the Cassegrain optical system of the 
telescope should permit the instrument 
to match the excellent quality of the 
natural observing conditions that pre- 
vail at the site. 

The lengthy process of installing and 
adjusting the electronic control system 
and of perfecting all supplementary 
systems required for routine operation 
of the telescope is under way. Comple- 
tion of these activities, together with 
the requirements for adjusting and 
testing the first series of auxiliary in- 
struments for the telescope, is expected 
to require several additional months. 
Construction of the large coude spec- 
trograph is a separate task that is ex- 
pected to require three to four years. 

It can now be made known that Miss 
Henrietta H. Swope was the donor of a 
very substantial gift to the Institution 
in 1967. This gift permitted develop- 
ment of the Las Campanas Observa- 
tory site, with its road and water sys- 
tem, as well as purchase of the 1-meter 
telescope that was installed in 1971. In 
accord with the wish of the donor, this 
gift remained anonymous for a number 
of years. It is evident that Miss Swope's 
generosity was of primary importance 
in the establishment of the new obser- 
vatory. 



OBSERVING CONDITIONS 



The 2.5-meter Hooker Telescope at 
Mount Wilson was used for observa- 
tions on 276 complete nights and 41 
partial nights for a total of 2117 ob- 
serving hours. Rainfall for the year 
was 551 mm, and total snowfall was 
1066 mm. 

The 5-meter Hale Telescope at 
Palomar Mountain was used for a total 
of 3489.8 hours, of which 3085.1 were 
nighttime hours, as shown in Table 1. 



The difference represents twilight time 
that was used for infrared or planetary 
observations not requiring a dark sky. 
Total precipitation at Palomar was 596 
mm, with 1168 mm of snow. The 
maximum temperature was 35.0°C in 
July; minima were -7.8°C in January 
and March. 

Public visitors at the Palomar Ob- 
servatory numbered 133,990 for the 
year. 



HALE OBSERVATORIES 



279 



TABLE 1. 5-Meter Observations 









Zero 


Total 


Hours of 




Complete 


Partial 


Observation 


Hours 


Nighttime 


Month 


Nights 


Nights 


Nights 


Worked 


Observing 


July 


25 


5 


1 


261.5 


239.6 


August 


29 


1 


1 


303.3 


255.9 


September 


29 


1 




305.9 


301.3 


October 


26 


4 


1 


345.4 


294.0 


November 


28 


2 


1 


354.3 


307.6 


December 


25 


3 


3 


326.5 


289.8 


January 


28 


2 


1 


382.3 


329.4 


February 


16 


4 


9 


238.1 


196.9 


March 


22 


4 


5 


278.8 


247.2 


April 


19 


6 


5 


214.3 


199.7 


May 


25 


3 


3 


261.2 


212.2 


June 


27 
299 


35 


3 
33 


218.2 


211.5 


Totals 


3489.8 


3085.1 



PHYSICS OF THE SUN 



Synoptic observations of the sun con- 
tinue on Mount Wilson under the 
supervision of Howard. Between June 
1, 1975, and May 31, 1976, the follow- 
ing observations were obtained: 

Direct photographs 334 

Ha spectroheliograms, 

9-meter focus 621 

K2 spectroheliograms, 

9-meter focus 618 

Full-disk magnetograms 341 

Integrated-light magnetic-field 

measurements 317 

Sunspot drawings 330 

Sunspot magnetic-field 

measurements 327 

Solar observations were made on 

341 days. 

The daily magnetic-field plots are 
published monthly in the National 
Oceanic and Atmospheric Administra- 
tion Bulletin Solar Geophysical Data. 
The magnetic synoptic charts are pub- 
lished in the Quarterly Bulletin on 
Solar Activity of the International As- 
tronomical Union. Partial support for 
these observational programs comes 
from the National Aeronautics and 
Space Administration and the Office of 
Naval Research. 



Solar Magnetic Fields 

A study by Howard of magnetic-field 
measurements made on the same day 
with two different apertures shows a 
daily zero-level error in the setting of 
the Mount Wilson magnetograph of 
about 0.35 gauss (standard deviation). 
From the same observations, the scale 
(calibration) errors were found to be 
about 10%. On the size scale of the 
apertures used in this analysis 
( — 10,000 km), the polarity mixing of 
magnetic elements varied from one 
portion of the period covered by these 
observations to the other. During the 
period of lower flux values, the 
polarities are more mixed. It is this 
mixing of polarities which, at least to 
some extent, is responsible for the 
lower flux measurements. Thus the dif- 
ference in the measured total flux be- 
tween activity maximum and mini- 
mum, which is only a factor of 2, is at 
least partially caused by a mixing of 
positive and negative elements within 
the aperture. 

Howard, in collaboration with Z. 
Svestka, A. S. Krieger, and R. C. Chase 
of American Science and Engineering, 
Inc., has studied transequatorial co- 



280 



CARNEGIE INSTITUTION 



ronal loops seen in soft x rays from the 
A.S.&E. experiment on the Skylab 
mission. The loop system was probably 
born through reconnection within a 
few days after the birth of the second 
region. The loops could be seen in soft x 
rays for 2V2 days. Transient "sharpen- 
ing" of the loops and a striking 
brightening of the whole loop system 
for about six hours seem to have been 
caused by changes in the magnetic- 
field configuration in one of the active 
regions. The electron temperature in 
the loops, equal to 2.1 x 10 (; K in the 
quiet phase, increased to 3.1 x 10" K 
during the brightening. At the same 
time, the loops became twisted while 
rising in the corona. 

A study of the magnetic sector struc- 
ture of the sun has been carried out by 
L. Svalgaard, J. M. Wilcox, and P. H. 
Scherrer at Stanford and by Howard. 
Using interplanetary magnetic-field 
data, 454 sector boundaries were iden- 
tified, and the corresponding solar 
magnetic field configurations were 
mapped. The sector boundaries are 
clearly visible in the photospheric 
magnetic field as north-south polarity 
demarcation lines, extending to about 
35° latitude on both sides of the 
equator. 

Large-Scale Velocity Fields of the Sun 

The study of the accumulated Mount 
Wilson solar velocity data continues by 
Howard and H. Yoshimura of the Uni- 
versity of Tokyo. Evidence has been 
found in several instances for a large- 
scale boundary in the rotational veloc- 
ity separating high and low velocity 
rates. The years preceding the solar ac- 
tivity minimum are characterized by 
an increase in the equatorial rotation 
rate, a striking decrease in the latitude 
gradient of the rotation rate at low 
latitudes (<40°), and a corresponding 
increase in the latitude gradient at 
high latitudes. The rotation of the 
polar latitudes remains unchanged. 
The rotation rate of the equatorial 
latitudes is now (May 1976) nearly up 



to the sunspot rate (2.9 /jl rad s _1 ). This 
is the first time that such rotation 
changes have been observed, and it is 
the first time that such sensitive 
measurements have been available for 
this phase of the solar cycle, which is 
now very close to minimum. 

Big Bear Solar Observatory 

The 65-cm vacuum reflector was fi- 
nally brought into proper alignment, 
and the coude axis was aligned as well. 
Excellent photographs were obtained 
with a limiting resolution of 0.3", but it 
was found that only rarely did the at- 
mosphere permit such good results. 
Drs. Alan Patterson and Kenneth 
Marsh are now resident at Big Bear to 
conduct the observational programs. 

Coronal Holes 

Zirin took part in the Skylab Coronal 
Hole Workshop, which met on several 
occasions in Boulder, Colorado, to com- 
bine the efforts of a number of people 
from different subdisciplines toward 
understanding the phenomena of co- 
ronal holes. Zirin devoted most of his 
efforts to trying to detect the presence 
of coronal holes in ordinary Ha and 
K-line filtergrams. The results of this 
study were somewhat indeterminate; 
coronal holes clearly coincide with ex- 
tensive areas where the chromosphere 
is quiet and relatively free of emission. 
It was found that if a hole is known to 
be present, it is fairly easy to draw in 
its boundaries from the Ha and K-line 
pictures. However, given an Ha or 
K-line picture, it is at present impossi- 
ble to determine whether or not there 
is a coronal hole in it. One may thus 
conclude that the presence of a coronal 
hole has little effect on the underlying 
chromosphere, and that the chromo- 
spheric phenomena we see do not de- 
pend on conduction downward from the 
corona. (See also the report of William 
Adams on solar rotation near coronal 
holes.) 



HALE OBSERVATORIES 



281 



Surge of 1971 

Zirin completed work on the analysis 
of a peculiar solar surge observed in 
1971. This surge ejected a great mass of 
material that flew across the surface of 
the sun and formed a filament several 
hundred thousand kilometers away. 
The filament lasted for about 20 min- 
utes, whereupon the material rose and 
returned to its origin. The significance 
of this most unusual event is not clear. 
It was possible to explain the radio 
emission from the surge with a simple 
thermal model at 200,000 °K. In addi- 
tion, there is a hotter component of 
about 10% of the material at a temper- 
ature of several million degrees. 

X Rays from Solar Flares 

Zirin has made a study of nonther- 
mal x rays from solar flares, comparing 
Big Bear data with data obtained by a 
number of different spacecraft. In 
every case (about 15 flares), the hard 
x-ray flux (greater than 20 keV) meas- 
ures the time derivative of Ha inten- 
sity. However, after the third hard 
x-ray burst ends, the Ha intensity may 
continue near the maximum level if a 
thermal cloud is formed. This cloud is 
then responsible for the soft x-ray 
burst. The energy emitted in Ha is 
often about 10% the energy of the 
nonthermal electrons, which typically 
have a hardness index of about 3. The 
radio spectrum produced by these elec- 
trons was then modeled, using the in- 
ferred density and spectrum from the 
hard x-ray burst and the observed Ha 
area. It is possible to fit the observed 
radio-burst data in almost every case 
by appropriate use of synchrotron 
self-absorption and free-free absorp- 
tion, but unfortunately there are many 
parameters and the results are not 
convincing. Typical values of magnetic 
field of several hundred gauss and den- 
sity of 10 9 electrons in the nonthermal 
plasma are found. It is not possible to 
differentiate between the thin foil and 
thick foil models of hard x-ray bursts. 



It was found that a clear distinction 
existed in Ha between thermal and 
nonthermal x-ray bursts. The non- 
thermal bursts are invariably visible 
in offband Ha (about 0.8 A from line 
center), while the thermal bursts pro- 
duce no emission offband. Thus the line 
width of Ha excited by nonthermal 
particles is broad compared to that in a 
thermal x-ray flare. The profiles in im- 
pulsive x-ray bursts are consistent 
with the possible momentum transfer 
by hard electrons incident on protons. 
It is postulated that the initial energy 
input in a nonthermal flare occurs 
through acceleration of electrons ac- 
cording to a power-law spectrum, the 
entire input occurring in the first mo- 
ments of the flare. 

It was found that x-ray bursts with 
multiple spikes usually were related to 
optical flares with several stages, each 
new Ha brightening or ejection corre- 
sponding to a new spike in the x-ray 
flux. 



The Lifetime and Evolution of Fibrils 

A detailed study has been made of 
the lifetimes and evolution of fibrils in 
McMath 12417, using high-resolution 
filtergrams in Ha and Ca II K made at 
Big Bear Solar Observatory. It was 
found that when viewed near disk cen- 
ter the lifetime of a fibril is a monoton- 
ically increasing function of its 
maximum apparent length. This rela- 
tionship, together with the form of the 
variation of fibril lengths as a function 
of time, suggests that fibrils result 
from material being impulsively in- 
jected into magnetic-field lines at ap- 
proximately 30 km s" 1 and returning 
under gravity. The lifetimes and ap- 
parent lengths of fibrils are then a 
function of the inclination of the field 
lines only. A study of wavelength scans 
through the Ha line confirms that the 
apparent extension and retraction of 
fibrils represent true mass motion. 



282 



CARNEGIE INSTITUTION 



Macrospicules and Microflares 

From careful scrutiny of Big Bear 
full-disk Ha time-lapse movies under 
high magnification, Tang has found a 
class of small flarelike events that 
occur in quiet regions at all latitudes, 
including the polar regions. These 
events are most noticeable at the limb, 
where they typically have the appear- 
ance of erupting loops or surges. They 
are smaller than a supergranule 
(heights, 5-15 x 10 :; km; widths, 1-15 
x 10 :; km), last for 5 to 10 minutes, and 
rise at velocities up to about 100 km 
s~'. In the polar regions, the larger of 
these eruptions are apparently the so- 
called polar-limb surges that have been 
observed in Ha at several observatories 
since at least as long ago as 1957 (G. 
Godoli and F. Mazzucconi, Astrophys. 
J., 147, 1131, 1967). 

Small surgelike events, termed mac- 
rospicules, have been detected and 
studied on the polar limbs in EUV 
spectroheliograms from Skylab (J. D. 
Bohlin et al, Astrophys. J. [Lett.], 197, 
L133, 1975; P. L. Withbroe et al., As- 
trophys. J., 203, 528, 1976). The Ha 
limb events are similar to the EUV 
macrospicules in shape, size, motion, 
and duration. On this basis, Moore and 
Tang (Bull. Amer. Astron. Soc, 7, 423, 
1975) proposed that the Ha limb erup- 
tions were the Ha counterpart of the 
EUV macrospicules and named the Ha 
events Ha macrospicules. 

Another discovery from Skylab was 
the observation of flare events in x-ray 
bright points (L. Golub et al., As- 
trophys. J. [Lett.], 189, L93, 1974). 
X-ray bright points are the x-ray im- 
ages of ephemeral active regions: bipo- 
lar magnetic regions that are smaller 
than supergranules and with lifetimes 
usually less than a day. Since x-ray 
bright points occur at all latitudes and 
are the only observed centers of activ- 
ity in the polar regions, Moore and 
Tang (Bull. Amer. Astron. Soc., 7, 423, 
1975) proposed that macrospicules are 
produced by flares in these ephemeral 
active regions. Because of their very 



small size, these flares are appropri- 
ately termed microflares. 

The above interpretations proposed 
by Moore and Tang were based on indi- 
rect evidence rather than on direct 
identification of Ha events with either 
EUV macrospicules or flares in ephem- 
eral active regions. However, more re- 
cently Moore, in collaboration with J. 
D. Bohlin at the Naval Research Labo- 
ratory and L. Golub at American Sci- 
ence & Engineering, has found several 
direct identifications of Ha macro- 
spicules with EUV macrospicules and 
with flares in x-ray bright points on the 
limb. Thus the identifications proposed 
by Moore and Tang have been con- 
firmed. 

Differential Rotation 

Solar differential rotation meas- 
urements give a wide variety of results, 
depending upon the method of meas- 
urement employed, and it appears that 
the differences are genuine properties 
of the sun, i.e., different rotation trac- 
ers actually rotate at different rates. 
Long-lived filaments have long been 
used as such rotation tracers, but it is 
not at all clear that they are represen- 
tative of filaments as a class (of which 
they compromise only about 5%). A 
study of short-lived filaments was 
therefore begun in the hope that the 
higher quality of the Ha data currently 
available would make it possible to 
overcome the difficulties inherent in 
such an undertaking. The resulting 
measurements produced a rotation 
curve with a differential term approx- 
imately half the size of the correspond- 
ing term in the long-lived filament ro- 
tation curve. 

A variation of the same measure- 
ment technique was then applied to a 
small sample of filaments bounding the 
unipolar magnetic region occupied by 
CHI, a large coronal hole observed by 
Skylab. This coronal hole had shown 
very little differential rotation during 
its several-month life span, and the ro- 



HALE OBSERVATORIES 



283 



tation of the underlying unipolar re- 
gion was consequently of considerable 
interest. Inasmuch as filaments are 
known to form preferentially along 
field reversal lines, they furnish a pos- 
sible tool for measuring the rotation of 



the underlying unipolar region, and 
this provided the motivation for our 
measurements. The resulting rotation 
curve was in remarkably good agree- 
ment with that of the overlying coronal 
hole. 



SOLAR SYSTEM STUDIES 



Jupiter 

The systematic observation of the 
variations in time of the intensity of 
the SI and SO lines of the (3, 0) band of 
H 2 over various regions of the Jupiter 
disk {Year Book 73, p. 131) was con- 
tinued by Munch in collaboration with 
C. Pilcher of the University of Hawaii. 
For the purpose, the coude inter- 
ferometric scanner of the 2.5-meter 
Mount Wilson telescope was used, with 
a spectral resolution of 0.074 A and an 
angular resolution of 2". Similar meas- 
urements were also made on various 
regions of the Saturn disk. 

Io 

With the 150-mm aperture PEPSIOS 
spectrometer of the University of Wis- 
consin installed at the coude focus of 
the 5-meter telescope, Munch, in col- 
laboration with Drs. John Trauger and 
Fred Roesler of the University of Wis- 
consin, has studied the characteristics 
of the sodium emission associated with 
Io. The field optics (f/40) allowed use of 
a maximum entrance aperture 25" in 
diameter for a spectral resolving power 
of 150,000. A series of 60 profiles of the 
D 2 line was obtained when Io was near 
both its eastern and western elonga- 
tions. The intensity-velocity distribu- 
tion of the D 2 emission within 25" of Io 
was studied by line profiles obtained 
with a nested set of concentric annular 
quadrants of various radii. In addition, 
sample D 2 scans were obtained from 
regions 25" in diameter centered on the 
plane of Io's orbit, at distances of 69", 
138", and 207" from Io in the direction 
away from Jupiter, and at a distance of 
69" from Io toward Jupiter. The main 



results of the observations can be 
summarized as follows: 

1. The D 2 emission rate at a distance 
of 2.5" from Io has a value, near eastern 
elongation, of 80 kilorayleigh (KR), 
corresponding to a column density of 2 
x 10 +u cm -2 . The emission rate de- 
creases exponentially from this value, 
with a scale of 3", out to distances from 
Io of around 40". Farther out, the rate 
of decrease in surface brightness ap- 
pears to be slower according to power 
law r~" of the distance r from Io, with n 
= 1.7. 

2. The line profiles for regions cen- 
tered on Io are peaked at the value cor- 
responding to the Io orbital radial ve- 
locity but are asymmetric, with a tail 
extending toward the larger velocities 
without regard to sign. This high- 
velocity tail extends beyond the veloc- 
ity of escape from Jupiter. The degree 
of asymmetry of the profiles appears to 
increase with distance from Io. 

3. At distances ^69" from Io in the 
direction away from Jupiter, peak 
emission occurs at the Keplerian veloc- 
ity around Jupiter; but at r = 69" to- 
ward Jupiter, peak emission corre- 
sponds to the projected Io orbital veloc- 
ity. It thus appears that the density of 
sodium atoms falls off sharply inside 
Io's orbit toward Jupiter. 

4. The brightness distribution of D 2 
line emission is not symmetric with re- 
spect to Io or to the trace of its orbital 
plane. 

The presence of the K I a 7699 line in 
the spectrum of the Io emission region 
was discovered by Munch, Roesler, and 
Trauger on August 28, 1975, also with 
the PEPSIOS spectrometer. Unlike the 
Na D 2 line, the profile of the K I line is 



284 



CARNEGIE INSTITUTION 



symmetrical, with a nearly gaussian 
form of 3.16 km s" 1 dispersion. The 
measured emission rates imply a ratio 
of 20 between the column densities of 
sodium and potassium. The paths of 
line formation for both species are, 
however, different, as indicated by 
their different line shapes. Since the 
lifetime of the neutral K and Na atoms 
is determined by collisional ionization 
with thermal electrons in the Jupiter 
plasmasphere, and the cross section for 
this process is about twice as large for 
K as for Na, for a plasma temperature 
of 100 ev the abundance ratio Na/K 
will be less than the ratio of column 
densities, and depending on the results 
of model calculations under way, it 
may be closer to the value correspond- 
ing to the earth's crust (4:1) rather 
than the solar value (40:1). 

The dependence on time of the shape 
of the Io emission cloud, expected from 
its interaction with the Jupiter mag- 
netospheric plasma, has been studied 
by Munch in a series of spatial inter- 
ferograms. The rings of a 40-mm aper- 
ture Fabry-Perot etalon were projected 
on the plane of the sky around Io by 
means of a collimator and then reim- 
aged with an f/1.4 lens on a magnet- 
ically focused intensifier. The device 
was mounted on the coude spectro- 
graph of the 1.5-meter Mount Wilson 
telescope in order to obtain a field of 
110" diameter. The etalon spacer was 
chosen to make the rings of the D 2 and 
Di lines coincident at a central order of 
interference equal to 7945. Using a 
focal-plane occulting disk to block off 
the light of Io, it was possible to photo- 
graph rings of 35" diameter with an ex- 
posure of one hour. The limiting expo- 
sure, around two hours, was set by the 
amount of scattered continuum light 
from Jupiter. The interferograms ob- 
tained show clearly the strong concen- 



tration of the Na emission toward Io 
and toward a plane close to but not 
coincident with Io's orbit. The emission 
cloud appears to be flattened to a de- 
gree increasing with distance from Io, 
and its form varies with the magnetic 
latitude of Io in the sense that the 
emission lines are weaker toward the 
Jupiter magnetic equator. 

Satellites and Asteroids 

Kowal, working under Sargent's 
general direction, continued his search 
for faint satellites of the outer planets 
and discovered a probable fourteenth 
satellite of Jupiter. The new object was 
photographed with the 1.2-meter 
Schmidt telescope on September 30, 
October 1, and October 2, 1975. Its vi- 
sual magnitude is approximately 21, 
and it is estimated to be between 5 and 
10 km in diameter. Not enough obser- 
vations were obtained to allow the de- 
termination of an orbit for this object, 
but a heliocentric orbit has been ruled 
out. It is hoped that more observations 
can be obtained at the next opposition 
of Jupiter, in November 1976. 

Plates of the outer satellites of Jupi- 
ter were also obtained for the U.S. 
Naval Observatory in a project to de- 
termine better ephemerides of those 
satellites. 

A search was conducted for new 
satellites of Saturn, but none were 
found. 

On December 27, 1975, Kowal dis- 
covered a new Apollo asteroid with the 
46-cm Schmidt telescope. This object, 
designated 1975 YA, was found on a 
film taken for the supernova search 
and was moving across the sky at a 
rate of 21° per day when first sighted. 
The asteroid's orbit has the unusually 
high inclination of 64° and a perihelion 
distance of 0.9 A.U. 



STELLAR SPECTROSCOPY 



White Dwarfs 

A deeper insight into the physical 
processes that separate the degenerate 



stars with hydrogen from those with 
helium atmospheres has been obtained 
by Greenstein, largely with the mul- 
tichannel spectrophotometer. With in- 



HALE OBSERVATORIES 



285 



creased numbers of helium stars, it be- 
came clear that convection to various 
depths must be episodic for degenerate 
stars in the range of temperature 
25,000° to 12,000°K. No helium-rich 
white dwarf atmosphere has been 
found hotter than 25,000°, where 
helium lines have their maximum 
strength. (Only one star, probably near 
50,000°, has both ionized helium and 
hydrogen in its spectrum.) Among the 
cooler helium-atmosphere white 
dwarfs, two have been found in which 
the lines of ionized calcium appear. 
This extraordinary range of excitation 
is still compatible with a reduced 
abundance of calcium compared to the 
opacity-producing elements. Most 
cooler white dwarfs with metallic lines 
are extremely metal deficient. A work- 
ing hypothesis is that degenerate stars 
start very hot, with or without hydro- 
gen at the surface, but in any case, ac- 
crete interstellar matter of normal 
composition. In a relatively short time, 
the high gravity results in a very com- 
plete separation, with hydrogen at the 
top, helium below, and the metals still 
deeper. Near 25,000° the stability of 
this molecular weight gradient is over- 
turned by convection, and both helium 
and varying small amounts of metal di- 
lute the accreted hydrogen for a time. 
The helium atmosphere with metals 
visible may represent a later stage as 
the molecular weight gradient reestab- 
lishes itself when convection stops. It is 
not obvious that this happens only once 
in the life of a white dwarf, nor that it 
requires the star to evolve in the direc- 
tion mentioned. An alternate view is 
that convection may start at a lower 
temperature and because of the de- 
crease of opacity result in a higher- 
temperature object that gradually cools 
again. 

An important feature of the hydro- 
gen white dwarfs has been discovered 
in collaboration with Drs. Alec Bok- 
senberg, Robert Cars well, and Keith 
Shortridge of University College Lon- 
don. The Image Photon Counting Sys- 
tem was used in the coude spectro- 
graph of the Hale reflector to study a 



phenomenon discovered earlier by 
Greenstein in 40 Eridani B (Fig. 1) and 
Wolf 1346. Abnormally sharp cores 
exist at the center of the hydrogen lines 
Ha and H/3. The efficiency of the IPCS 
system permitted observation of ten 
hydrogen-atmosphere white dwarfs, 
with proof of the existence of sharp 
cores in Ha or H/3 for seven of them. 
Observations by Greenstein with the 
coude image-tube spectrograph at 27 A 
mm -1 had already revealed the exis- 
tence of sharp Ha cores in five other 
white dwarfs of type DA. These cores 
arise from the large optical depth at 
the center of Ha and the low boundary 
temperature of hydrogen atmospheres 
caused by Balmer and Lyman line 
blanketing. Greenstein and Deane 
Peterson of the State University of 
New York at Stony Brook predicted 
nonlocal thermodynamic-equilibrium 
profiles with cores essentially of ther- 
mal width (about 15 km s ', mean ve- 
locity). If the projected rotational veloc- 
ity of a white dwarf is larger than this 
thermal velocity (30 or 40 km s" 1 ) the 
sharp core would be destroyed. It is 
now clear that very slow rotation, cer- 
tainly less than 40 km s _I and probably 
for some stars less than 10 km s _1 , is 
the rule for hydrogen white dwarfs. 
The implication is that during the 
red-giant phase considerable transfer 
of angular momentum between the 
center and envelope of the star must 
occur. The mass loss of the red giant 
should be accompanied by selective loss 
of matter of greater than average an- 
gular momentum per gram. The brak- 
ing of the object, possibly by interac- 
tion of the stellar wind with the in- 
terstellar medium via magnetic fields, 
seems to be very successful. The core is 
finally exposed as a hot subdwarf and 
eventually a white dwarf of small spe- 
cific angular momentum. If the sun 
were to shrink homologously to a white 
dwarf, the rotational velocities would 
be over 500 km s _1 . Since in many 
white dwarfs the mass lost is approxi- 
mately three quarters of the star's ini- 
tial mass, this mechanism is quite effi- 
cient. 



286 



CARNEGIE INSTITUTION 




6480 6520 6560 6600 6640 A 



CO 



15000 



12000- 



9000 



g 6000- 



3000 




4280 4300 4320 4340 4360 4380 4400 A 



Fig. 1. The line profiles of hydrogen in the classical white dwarf 40 Eridani B. These were obtained 
with the Hale Telescope at Palomar, using the Image Photon Counting System developed by A. 
Boksenberg of University College London. The plots show the photon counts per picture element with 
this modern television system. The background slope is caused by the sensitivity changes within the 
entire system. Significant is the enormous width of the Hy line, caused by Stark broadening in the 
high-pressure atmosphere. The unusual feature of the Ha line, however, is an extremely sharp deep 
core, less than 5 picture elements wide in toto. This deep core is produced at the top of the atmosphere 
where the pressure is so low that the Stark broadening is minimal. Part of the apparent width is 
instrumental in origin, in spite of the original dispersion of 15 A mm -I . The deep core is produced by 
effects of nonlocal thermodynamic equilibrium. However, its sharpness proves that the white dwarf 
cannot be rotating rapidly, since rotation of as much as 30 km s - ' at the equator would blur the core 
out of existence. The estimated rotation is certainly less than 20 km s" 1 and perhaps less than 10 km 
s _1 . Thus the period of rotation of the white dwarf must be longer than 30 min and possibly longer 
than 1 hr. Were the sun to contract without loss of angular mometum to the size of a white dwarf, its 
equatorial velocity would be about 300 km s~'. 



HALE OBSERVATORIES 



287 



As a further result of the sharp line 
cores in the hydrogen-line white 
dwarfs, it has been possible to derive 
rather accurate radial velocities. The 
correlation between these and the 
prime-focus spectrograph results as 
measured by Virginia Trimble of the 
University of California at Irvine and 
Greenstein is fairly good, with 0.84 
correlation coefficient and only small 
systematic difference. For the stars 
with sharp cores observed either with 
the coude image tube or the IPCS, a 
mean K-term (i.e., a systematic expan- 
sion due to the gravitational redshift) 
of +44 km s _1 is derived. This is within 
the probable errors of the photographic 
value of Trimble and Greenstein, +51 
km s" 1 . This confirms a mean mass of 
about 0.6 to0.8 9TI o for the white dwarfs. 

Individual white dwarfs of special 
interest are being searched for by the 
multichannel spectrophotometric pro- 
gram. In studies made in collaboration 
with Boksenberg, the hot star Feige 7 
seems almost certain to be a magnetic 
white dwarf, showing, without a 
Zeeman analyzer, resolved triplet pat- 
terns of H/3 and Hy. The 12,000° 
helium-rich white dwarf Gill -54 was 
found by Greenstein and Boksenberg to 
have only the strong, relatively sharp 
lines of ionized calcium. Several sus- 
pected late-type red degenerates were 
also studied for very weak metallic fea- 
tures. The difficulty of separating such 
metal-poor, helium-rich, cool degener- 
ate stars from extremely high-velocity, 
metal-poor, hydrogen-rich G or K sub- 
dwarfs is extreme. Only parallax and 
proper motion will decide. 

Greenstein has published data on 
189 multichannel spectra observed for 
various types of degenerate stars, in 
the form of a number of monochromatic 
colors and magnitudes. Study of the 
correlation of colorimetric properties 
and luminosity for stars of known paral- 
lax indicates that the degenerate stars 
form a relatively narrow sequence with 
a spread of about ±0.7 magnitude. 
These color-luminosity curves are pre- 
sented in the Astronomical Journal 
(81, 323-338, 1976) and predict the ex- 



pected photometric parallax to be com- 
pared with trigonometric parallaxes. A 
considerable number of cool degenerate 
stars exist with relatively large pre- 
dicted parallax but have not yet been 
measured. The space motions are those 
of the old disk population, although 
some Population I and about 9% Popu- 
lation II degenerate stars exist. The 
number of cool (<7000°) degenerate 
stars has been doubled, but this is not 
enough for an elementary theory of 
cooling. The low number of red degen- 
erates probably arises from the rapid 
drop in specific heat of the core below 
effective temperature 6000°. 

There is evidence that Population II 
stars more than 1.5 mag subluminous 
are rare. When allowance is made for 
differential line blanketing, part of this 
discrepancy is eliminated. A large gap 
remains between the faintest red sub- 
luminous star and the faint red degen- 
erates. 

Studies of Proper-Motion Stars 

A systematic program to improve the 
relation between luminosity and color 
for proper- motion stars has been con- 
tinued by Greenstein with the mul- 
tichannel spectrophotometer. About 
100 stars of known parallax are in- 
cluded in this program, and another 
300 of large proper motion. Prelimi- 
nary indications are that a sublumin- 
ous sequence approximately 1 mag 
below the main sequence exists in al- 
most any color-luminosity plot of paral- 
lax stars. With a still insufficient 
number of parallaxes for late-type, 
very high-velocity stars, this difference 
seems to disappear at dM2 or dM3. The 
earlier high-velocity M stars are dis- 
tinguished by a relatively greater ratio 
of hydride to oxide bands. At lower 
temperatures, the hydride bands 
weaken, and differential line blanket- 
ing may disappear. Alternatively, 
there may be few or no very high- 
velocity stars of late type and low 
luminosity. To illuminate the latter 
problem, basic for the existence of a 
possible galactic halo of low-mass M 



288 



CARNEGIE INSTITUTION 



dwarfs, a systematic program has been 
undertaken. The color-luminosity 
curves permit calibration of stars of 
known multichannel color. Green- 
stein's program will measure all Low- 
ell stars with proper motion larger 
than 1" per year that are brighter than 
magnitude 16.4. 

Rotation of B -Type Stars 

Preston has used 20 A mm -1 spec- 
trograms obtained with the Mount 
Wilson 2.5-meter telescope to estimate 
projected rotational velocities for 180 
stars of types B1-B5. This study com- 
pletes his survey of the rotational 
properties of the bright northern 
B-type stars. No new chemically pecul- 
iar stars were detected. Higher- 
dispersion (9 or 4.5 A mm -1 ) spectro- 
grams were subsequently obtained for 
some 40 of these stars with u sin i < 40 
km s~~' to better define the v sin i dis- 
tribution near the origin and to lower 
the detection limits for chemical 
peculiarities. This effort led to the dis- 
covery of only one peculiar star, HR 
7467, which exhibits enhanced lines of 
P II and P III and no detectable rota- 
tional broadening (v sin i< 5 km s '). 
As in the case of 3 Centauri A and t 
Orionis B, positive wavelength shifts 
for the He I lines XX 4921, 5015, and 
6678 indicate a gross overabundance of 
3 He ( He/ 4 He ~ 2), but unlike 3 Cen A, 
HR 7467 does not exhibit lines of Ga II 
or Kr II that can be detected at 4 A mm 
dispersion. 

P -He Stars 

The companion of Rigel is a double- 
lined spectroscopic binary with a 
period of 10 days. Preston has found 
lines of P II and P III in the spectrum of 
the primary component and a +0.25- A 
shift of He I X 6678 that indicates a 
He/ 4 He ratio comparable to those 
found for 3 Cen A, i Ori B, and HR 
7467. The P III/P II and Si Ill/Si II line 
ratios for Rigel B (primary) indicate a 
spectral type considerably earlier than 
B9, as estimated by Sanford, and more 



nearly like that of 3 Cen A. Lines of Ga 
II and Kr II are not visible on 4 A mm -1 
spectrograms. Instead, enhanced lines 
of A II are present, including the lead- 
ing lines of RMT multiplets 1, 2, 6, 7, 
and 10. Thus, of the four P- H stars 
known at present, no two are spectro- 
scopically alike. Three, curiously, are 
members of visual binary systems in 
young stellar associations, while one, 
HR 7467, is an isolated field star with 
MK type (B5 II-III) and (3, c indices 
that indicate a late stage of main- 
sequence evolution. All four are slow 
rotators, but the low incidence of such 
stars among slow rotators in the field 
sample suggests that slow rotation is 
not a sufficient condition for the pro- 
duction of the peculiarities of these ob- 
jects. 



Braking of Magnetic Ap Stars 

Hartoog has completed a study of the 
rotation of Ap stars in clusters and as- 
sociations to look for magnetic braking. 
No evidence for magnetic braking was 
found. Hot Ap stars seem to rotate 
more rapidly than cool Ap stars at all 
ages, so the observed decrease in aver- 
age rotational velocity with decreasing 
temperature must not be an age effect. 
It would appear that most of the angu- 
lar momentum is lost very early, prob- 
ably before the star reaches the main 
sequence. 



He Stars 

Hartoog, using plates from the Hale 
Observatories' plate collection, has 
identified He in a Sculptoris. The 
wavelengths of XX 6678, 4387, and 
5015 all indicate about 50% He. 
Jugaku and Sargent (Publ. Astron. 
Soc. Pacific, 73, 249, 1961) describe 
this star as having underabundances of 
oxygen and helium and overabun- 
dances of strontium, chromium, and 
titanium. In this respect a Sculptoris is 
quite different from other He stars. 



HALE OBSERVATORIES 



289 



Circumstellar Envelopes 

The direct measurement of the angu- 
lar diameter of the circumstellar en- 
velope around a Orionis (M2 lb) by A. 
P. Bernat and D. L. Lambert (As- 
trophys. J. [Lett.], 201, L153, 1975) led 
Munch and S. Roesler and J. Trauger of 
the University of Wisconsin to attempt 
a similar measurement for the more 
luminous and distant star /x Cephei 
(M2 la). The observations were carried 
out at the coude focus of the 5-meter 
telescope with the 150-mm PEPSIOS 
spectrometer. Using both an annular 
diaphragm with a central occulting 
disk of 2.7" diameter and the com- 
plementary aperture with the same 
diameter, scans of the K I \ 7699 and 
D 2 lines were obtained over a range of 
75 km s _1 under excellent seeing condi- 
tions. By subtracting one scan from the 
other after normalizing them to the 
same continuum level, an excess of 
radiation was found in the scan corre- 
sponding to the annular diaphragm; 
this has been ascribed to emission of 
the circumstellar shell. The circumstel- 
lar D 2 -line emission has an apparent 
width at half maximum of 15 km s" 1 
and its peak intensity is 10% of the 
stellar continuum. Since /x Cep is at a 
distance of no less than 1 kpc, it follows 



that the radius of its circumstellar en- 
velope is of the order of 2000 A.U. 



Stellar Spectroscopy at 1 /a: R Aquarii 



In the course of reducing spectra 
taken in the 1-micron spectroscopy 
program, Zirin found that the variable 
star R Aqr, which had been identified 
as a source of radio emission by P. C. 
Gregory and E. R. Seaquist {Nature, 
247, 532, 1974), also displayed the Fe 
XIII coronal line at 10747 A. The co- 
ronal line was positively identified on 
two plates obtained in 1971 and 1972. 
A new plate obtained in 1976 did not 
show this line. On the basis of the co- 
ronal line identification, its intensity, 
and the absence of the companion 
10798 A line, it was possible to make a 
rough model of the coronal cloud that 
surrounds R Aqr. At a temperature of 
1,000,000°K, the product of the cloud 
radius and electron density is 2 x 10' 8 , 
and the thermal radio flux at 3.5 cm is 
1.44 x 10" 15 R cm mJy. Thus, if the 
radius of the corona is less than 4 x 
10' 5 cm, the observed flux of 8 mJy can 
be obtained with a temperature of 2 x 
10" K. 



STELLAR CHROMOSPHERES 






Absolute Luminosities 

Wilson has completed and published 
his work on the absolute magnitudes of 
over 700 late-type giants, subgiants, 
and supergiants from the widths of the 
chromospheric K-emission compo- 
nents. 

H and K Fluxes in Main-Sequence 
Stars 

The investigation of variations in 
flux of the chromospheric H and K 



emission in main-sequence stars has 
continued. Several objects have com- 
pleted about one cycle of variation, and 
several others appear close to comple- 
tion. These cyclic variations are pre- 
sumably the stellar analogs of the solar 
cycle. It is anticipated that this work 
will be published sometime in the com- 
ing calendar year. 



290 CARNEGIE INSTITUTION 

INFRARED AND MILLIMETER-BAND PHOTOMETRY 



Extragalactic Objects 

This year marked the conclusion of a 
long-term infrared study of several ex- 
tragalactic objects carried out by 
Neugebauer and Becklin to set limits 
on the source of infrared energy by 
studying the objects' time history. The 
1967 and 1976 light curves for 3C273 
and 3C345 are shown in Figs. 2 and 3. 
For 3C273 no significant variability 
greater than the uncertainties of about 
10% is seen at either 2.2 or 3.5 /jl. Since 
the infrared energy distribution of 
3C273 appears to peak at 3.5 /jl, it can 
be concluded that there is no evidence 
for infrared variability from this QSO. 
For 3C345, 2.2-yu. variations of a factor 
of 3 are shown over the years; the vari- 
ations appear to have the same phase 
and amplitude as visible variations. 
Changes by a factor of 2 within one 
month's time rule out a simple thermal 
source for the 2.2-^t radiation from 
3C345. 



2.5-/U spectra of several of the 2.2-^u 
point sources shown on the high- 
resolution maps of Becklin and 
Neugebauer (Astrophys. J. [Lett.], 200, 
L71, 1975) have been made. Three of 
the sources, including the brightest, 
show strong carbon-oxide absorption at 
2.3 /jl, indicating that they are probably 
individual late-type giant or super- 
giant stars at the galactic center. Two 
sources do not show CO absorption; one 
is associated with the brightest 10-yu 
emission, while the other is near the 
nonthermal radio source found by B. 
Balick and R. L. Brown (Astrophys. J., 
194, 265, 1974). 

Observations at 10 \x have been 
made by Willner. The major result is 
that the 10-yu, silicate feature is shown 
to be mostly interstellar. Also, the Ne 
II line at 12.8 /jl has been mapped at 5" 
resolution; the results indicate that the 
10-/X emission is not correlated closely 
with the ionized gas. 



Galactic Center 

During the past year much work has 
been done on the central 3-pc core of 
the center of the Galaxy by Becklin and 
Neugebauer. In particular, the 2.1 to 



Nova Cygni 1975 



During the past year extensive in- 
frared observations of Nova Cygni 



LJ 

o 



9.0 



CD 

< 



10.0 



3.5^ 
2.2^ 



t H 



i r 

3C273 



♦ t + 



f + + V ^ i m V ^ 



H MH Ht 



J L 



67 68 69 70 71 72 73 74 75 76 

TIME 



Fig. 2. Photometry of 3C 273 at 2.2 /x and 3.5 tt over the time period 1967-1976. 



HALE OBSERVATORIES 



291 



i r 

3C345 



visual 
2.2/x 



V 



UJ 
Q 

Z> 



< 



♦♦fc. 



* it 



W 



t 



\ 



I » 



»♦ 



67 



68 



70 



71 
TIME 



72 



73 



74 



75 



Fig. 3. Comparison of visual light and 2.2-/U. variability of 3C 345 over the time period 1967-1975. 



1975 have been made by Neugebauer, 
Becklin, and many members of the in- 
frared group. The photometric results 
are in basic agreement with those of J. 
S. Gallagher and E. P. Ney (As trophy s. 
J., 204, L35, 1976) but are much more 
extensive, particularly in the early and 
later phases of the nova. In addition, 
spectroscopy with 1% resolution was 
obtained in the 2.2-fx region. The ob- 
servations are easily interpreted in 
terms of a uniformly expanding shell of 
ionized gas. The gas in initially opti- 
cally thick at infrared wavelengths and 
shows a Rayleigh-Jeans spectrum and 
the Brackett y line at 2.16 fx in absorp- 
tion. After four days the gas becomes 
thin, the infrared spectrum becomes 
flat, and the By line goes into emission. 
The flux decreased as t~ 2 until —200 
days, indicating that the gas is in a 
shell rather than uniformly filling an 
expanding volume. This dependence is 
now becoming closer to a t~' A function, 
which indicates that the shell has be- 
come more diffuse. 



1-mm Photometry of Compact 
Extragalactic Objects 

This program, headed by M. Werner, 
has resulted in the detection of five 
sources: 3C84, 3C120, 3C273, 3C279, 
and BL Lacertae. The derived fluxes lie 
in the range 5-20 janskys. Addition- 
ally, 3or upper limits of <5 Jy have 
been set for a number of objects, includ- 
ing NGC 1068, NGC 4151, and M82. 
None of the sources detected show any 
significant 1-mm flux over what is ex- 
pected from an extrapolation of the 
microwave spectrum. The latter limits 
are, however, just at the point where 
they are becoming significant in dif- 
ferentiating thermal from nonthermal 
origins for the large amount of infrared 
radiation present in these objects. Ad- 
ditionally, 3C273 has been monitored 
on a bimonthly basis from December 
1973 to March 1976 and has shown no 
firm evidence for variation greater 
than —30% of the mean flux, which is 
-13 Jy. 



GLOBULAR CLUSTERS 



Dynamical Models 

Theoretical dynamical models of 
globular clusters incorporating veloc- 



ity anisotropy and rotation have been 
constructed by Gunn to aid in interpre- 
tation of his and Dr. R. F. Griffin's 
(Cambridge Observatories, England) 



292 



CARNEGIE INSTITUTION 



radial-velocity data. The data for M3 
will be analyzed first. The analysis is 
not quite complete as of this writing, 
but it is clear that models constructed 
from central- velocity dispersions alone 
are woefully inadequate to describe the 
true dynamical state of clusters. M3's 
light in its central regions, as 
elsewhere, is dominated by giants, and 
the radial-velocity data clearly indi- 
cate that there is a "cold" (low- velocity 
dispersion) population of giants in the 
nucleus. Conventional dynamical ar- 
guments for systems with relaxation 
times as short as those for globular 
clusters require that any such popula- 
tion have high mass, and it is possible 



that these stars arise as the result of 
the evolution of close binaries, al- 
though no radial-velocity radiation 
survives to this epoch. 

For M13, rotation is evident, and an 
exciting possibility exists of combining 
the radial-velocity data with the preci- 
sion proper-motion measurements of 
Dr. Kyle M. Cudworth at Yerkes Ob- 
servatory to find complete space mo- 
tions for individual stars. This will not 
only give an almost completely unam- 
biguous look at the dynamics but will 
also yield a distance for the cluster in- 
dependent of all astrophysical assump- 
tions (except perhaps for 1/r 2 gravity 
and the Boltzmann equation). 



INTERSTELLAR MATTER AND GASEOUS NEBULAE 



High-Latitude Reflection 

Nebulosities Illuminated by Stars 

of the Galactic Plane 

A new survey of the north and south 
galactic polar caps has been in progress 
for several years by Sandage, using the 
1.2-meter Palomar Schmidt with fine- 
grain IllaJ and 127-04 high- 
information-capacity plates. Its pur- 
pose is to find remote clusters of 
galaxies with which to extend the 
HubBle diagram in a program con- 
ducted jointly with Kristian and 
Westphal. 

During the five years of the survey, 
there have been found occasional 
high-latitude fields that show bright 
nebulous regions, often in connective 
patterns that are correlated over dis- 
tances of degrees. 

A particularly interesting region oc- 
curs near the galaxies M81 and M82 at 
/ = 142°, b = +41°, shown in the very 
high contrast print of Fig. 4. The fila- 
mentary nebula of highest surface 
brightness at a 5() = 9 h 48 m , S, = +71° 
10' is almost due north of M81 and 
M82, which are shown as very overex- 
posed images near the lower left border 
of the print. 

The nebulosities over the print form 
only part of a very extensive network of 



bright structures that extends over the 
adjacent Sky Survey field at +72°, 9 h 
4 m ; it can be seen on the original Sky 
Survey prints. 

The fact that regions are nearly 
equally visible on the red and the blue 
prints, and especially that they are 
equally bright on a 103aD emulsion 
behind a Wratten-16 filter (XX 5200- 
6000 A) where no emission lines occur, 
is strong evidence that the structures 
are reflection nebulae. The source of il- 
lumination at these high-galactic 
latitudes is of much interest. 

The observed surface brightness of 
the regions in Fig. 4 averages SB = 25 
mag/D" for the brightest area. Calcula- 
tions of the flux due to the light from 
the galactic plane show that the appar- 
ent magnitude of the plane at any 
height is -6.7 V magnitude. The sur- 
face brightness of dust that scatters 
this light depends on the optical thick- 
ness of the structure. But if the same 
dust that obscures the background 
galaxies by Am also scatters the light 
from the galactic plane, then the sur- 
face brightness of the nebulosities 
should be SB V ~ 24.2 - 2.5 log Am 
mag/D". 

From galaxy counts on and off the 
brightest nebulosity in Fig. 4, and from 



HALE OBSERVATORIES 



293 










i'ijfc 








r fi:f, 


"&*/.""'» ':■'.' 


«'*' t& 



Fig. 4. A high-contrast print from part of a Illa-J plate (+ Wratten 4 filter) of a high-latitude region 
(a 3 „ = 9 h 49 m , 8 5( | = +71°10', / = 142°, b = +42°) that contains varied reflection nebulae. Overexposed 
images of the galaxies M81 and M82 are in the lower third of the print near the left border. The 
surface brightness of the brightest reflection nebulae is SB, — 25 mag per square sec, which agrees 
with predicted values from a model in which the illumination of the high-latitude dust is provided by 
the galactic plane. The scale of the print is 2°45' in width and 3°32' in length. 



294 



CARNEGIE INSTITUTION 



the 21-cm column density of iV H , = 5 x 
10 20 atoms cm -2 in the same region, as 
measured by Heiles, it is estimated 
that Am = 0.3 mag for the dense 
nebulae shown. Hence, if the galactic 
plane is the source of illumination, the 
surface brightness is predicted to be ~ 
25.5 mag/D". This is in such good 
agreement with the observations as to 
suggest that the plane is in fact the il- 
luminator. 

Because the plane is always there, 
everywhere, the presence or absence of 
faint nebulosities at high latitudes that 
have SB less than or equal to —25 
mag/D" should act as an indicator of 
dust clouds with extinction contrasts of 
Am =s 0.3 mag. Hence the presence or 
absence of reflection nebulae of this 
brightness should aid in new studies of 
the extinction through the halo in the 
polar directions. 

The polar survey is not yet complete 
but is far enough along to note that al- 
though some fields do have many re- 
flection nebulosities at this level and 
fainter, many parts of the north polar 
cap are free of detectable nebulosities 
to SB, ~ 26 mag/D", which means that 
the extinction is not lumpy on a scale of 
~ 0.2 mag in these regions. But, 
equally interesting, the presence of any 
reflection nebulae at high latitude 
shows the presence of dust above the 
galactic plane with optical depths that 
can be as high as r = 0.921 Am ~ 0.3. 

Interstellar Matter 

The study of several cometary reflec- 
tion nebulae by Greenstein had shown 
peculiarities in the form of differences 
in the spectrum between the reflection 
nebula and the central object. In col- 
laboration with Dr. E. Ye. Khachikyan 
of the Byurakan Observatory, a study 
of Greenstein's spectra of R 
Monocerotis and NGC 2261 showed an 
outward acceleration, increasing nega- 
tive velocities outward from the central 
object. There is also increasing 
strength of hydrogen and Ca II. The 
nebula is in general somewhat bluer 
than the central object and does not 



show a steep upturn toward the in- 
frared at 1 ix. Greenstein and Oke have 
discovered anomalously high reflectiv- 
ity near A 6500 in the "Red Rectangle" 
centered on the star HD 44179. The 
dust grains in the Red Rectangle differ 
substantially in physical properties, 
therefore, from those in NGC 2261. HD 
44179 has an enormous Balmer jump, 
requiring a temperature near 8000°, 
and surface gravity log g ~ 0. No 
available model atmosphere fully ex- 
plains the spectrum of HD 44179, al- 
though a circumstellar envelope of hy- 
drogen can produce a large Balmer dis- 
continuity. 

Herbig-Haro Object in the Orion 
Nebula 

Photographs of the Orion Nebula in 
radiation of the [O I] A. 6300 line show a 
nonstellar double object, 13" in p. a. 
280° from the star U 1782 (P. P. 
Parenago, Trudy Sternberg Astron. 
Inst., 25, 1954), which does not appear 
on photographs in recombination lines. 
A series of spectra of this object at 29 A 
mm _1 dispersion was obtained by 
Munch with the f/1.5 camera of the 
5-meter telescope coude spectrograph 
and an image intensifier. Guiding on 
the object was possible through the use 
of a two-stage electrostatically inten- 
sified eyepiece, filtered to cut down [O 
III] and Ha light. The spectra obtained 
show the characteristics of classical 
Herbig-Haro (HH) objects; namely, 
great strength of [N I], [Fe II], [O I], 
and [S II] with respect to Ha. The most 
remarkable feature of the object, how- 
ever, is that its spectral lines are dis- 
tinctly double, with one component 
shifted by -250 km s _l with respect to 
the other, which is intrinsically 
brighter in [O I] and which has nearly 
the same radial velocity as the mean of 
the Orion Nebula. It is conjectured that 
the two kinematically separated com- 
ponents correspond to the two distinct 
nuclei seen in the [O I] direct imagery. 
The large shift of the weaker line spec- 
trum with respect to the nebula allows 
recognition of its [N II] and Ha lines 



HALE OBSERVATORIES 



295 



within the greatly overexposed lines of 
the nebula. The intensity ratio be- 
tween [O I] A 6300 and Ha in this 
high-volocity component is 1.5. The 
large relative shift between the two 
components of this HH object implies a 
time scale of only 50 years for its for- 
mation as a double. The violent nature 
of the phenomena involved in the evo- 
lution and formation of the HH objects 
is thus exhibited in a more striking 
fashion than heretofore. 

The H II Region W3 

Further observations of the region 
W3, a site of recent active star forma- 
tion, have been carried out by S. Will- 
ner, a Caltech graduate student who 
discovered the presence of an emission 
line of Ar III at 8.99 /x. He also meas- 
ured more accurately the Ne II and S 
IV lines found previously. On the basis 
of the 8 to 13-/JL spectrophotometry and 
broadband measurements at 5 and 20 
fx, the compact sources in NGC 7538 
and W3 can be divided into two classes. 
The first class, which includes NGC 
7538, IRS-1, and W3-IRS-3, consists of 
compact sources whose emission from 5 
to 20 fx can be described by a single 
dust temperature. The second class, 
which includes all the other sources in 
the two H II regions, consists of both 
compact and extended objects that ex- 
hibit a range of dust temperatures. The 
mass of the emitting dust can be de- 
rived for those objects with a single 
temperature; for the others, only a 
lower limit can be obtained. The mass 
of ionized gas can also be deduced for 
those objects that have been observed 
at radio wavelengths. NGC 7538-IRS-l 
has a gas-to-dust ratio of 75; many of 
the other objects have upper limits 
below 1000. Other workers have found 
that the dust inside H II regions is de- 
pleted; little evidence for depletion has 
been found here. 

Infrared Properties of OH Masers 

The 1.5-meter telescope at Mount 
Wilson, which has recently become op- 



erational as a good infrared telescope, 
was used by S. Beckwith, a graduate 
student at Caltech, in the continued 
program related to OH maser sources. 
The work was done in collaboration 
with N. Evans of Owens Valley Radio 
Observatory. Discovery of the infrared 
sources associated with OH 1821-12 
(E. G. Hardebeck, Astrophys. J., 172, 
583, 1972) and OH 21.4-0.7 (Evans e* 
at., in preparation), as well as meas- 
urements of ON-4 at 10 fj. and 20 /x, 
concluded the observations of 1612 
MHz, Type II OH maser emitters (W.J. 
Wilson, A. H. Barrett, and J. M. Mo- 
ran, Astrophys. J., 160, 545, 1971). The 
major results of the study are: 

1. Of the eight OH maser sources in- 
cluded in the study, every one has an 
associated infrared source. 

2. Of five sources followed for more 
than one year, four were found to vary 
both in the infrared (2-20 /x) and at 18 
cm. In all cases the infrared and radio 
fluxes varied in the same sense. 

3. One of the infrared sources, that 
associated with OH 30.2-0.3, is shown 
to have 2A-fx CO band, characteristic 
of stars with spectral type later than K. 

4. Based on the observed flux at 3.4 
[X and 12.5 \x, infrared pumping of the 
1612 MHz radio line is possible at 34 fx 
but not at 2.8 /x. 

Work continued on the properties of 
1665 MHz OH masers with the dis- 
covery of two more infrared sources as- 
sociated with the 1665 MHz masers OH 
35.6 and OH 40.6 (Evans etal., in prep- 
aration). The infrared sources are very 
weak and there appears to be no corre- 
lation between near-infrared flux and 
1665 MHz line strength in all of the 
sources studied. No property character- 
istic of infrared sources associated with 
these masers has been found. 

Dark Clouds 

Scanning of dark clouds has con- 
tinued by J. Elias, a Caltech graduate 
student. To date, three small cloud 
complexes have been scanned, those 
near IC 5146, NGC 7023, and Sharp- 
less 239. In addition, scanning of the 



296 



CARNEGIE INSTITUTION 



two closest large complexes, in Taurus 
and Ophiuchus, has continued. Scan- 
ning of these regions is from half to two 
thirds completed. 

The results to date demonstrate that 
the sources detected so far — about 10 :i 
in number — are for the most part 
background objects similar to those de- 
tected by earlier, lower-sensitivity sur- 
veys. Several interesting objects have 
been found. Some photometry and 
low-resolution spectrophotometry have 
been done on these, but the results are 
not yet fully analyzed, and definite 
conclusions cannot be drawn. 

Mapping of Thermal Emission from 
Molecular Clouds 

Maps with 1' resolution covering ~ 
5' x 5' have been made by Werner and 
his collaborators of the 1-mm emission 
from four sources, W3, Sgr B2, W49, 
and M42 (OMC-1). In each case, the 
distribution of 1-mm surface bright- 
ness shows a sharp peak superposed on 
an extended background having < 25% 
of the peak brightness. The peak 1-mm 
flux density into the 1' beam is 35 Jy 
for W3, 90 for Jy for W49, 215 Jy for 
OMC-1, and 310 Jy for Sgr B2. These 
fluxes are much too high to be due to 
free-free emission or unresolved 
molecular lines, and the 1-mm radia- 
tion is almost certainly due to thermal 
emission from dust. Thus it is notewor- 
thy that the peaks are seen at the posi- 
tions of compact H II regions and/or 
pointlike infrared sources, which are 
thought from other considerations to be 
the youngest and/or most dust- 
embedded objects in each of the regions 
mapped. 

The data have been analyzed in 
terms of a simple model of a central 
object heating a surrounding dust 
cloud in which the dust density de- 
creases radially outward as p(r) oc r~". 
The best fits to the data are found for 
values of n between 1.5 and 2. This 
type of density gradient arises very 
generally in a variety of gravitational 
collapse calculations, so the steep den- 
sity gradients inferred from the 1-mm 



data may have been produced by the 
collapse process that led to the forma- 
tion of the central luminosity source. 

NGC 7538 

The infrared group has continued its 
studies of the sites of recent star forma- 
tion. One area studied is NGC 7538 
(Sharpless 158), an object visible as a 
nebulosity on the Palomar Sky Survey. 
Aperture synthesis measurements at 
21 cm (F. P. Israel, H. J. Habing, and T. 
de Jong, Astron. Astrophys., 27, 143, 
1973) show a large (5' diameter) H II 
region associated with the nebulosity 
and a compact H II complex about 3' to 
the southeast. A 20-/x map of this com- 
pact region shows three infrared 
sources within a region about 30" 
square, which higher-resolution radio 
observations (A.H.M. Martin, Mon. No- 
tic. R. Astron. Soc, 163, 141, 1973) 
show to be young H II regions. The 
most compact of these sources is IRS-1, 
which spectroscopy has shown to have 
a deep silicate feature. 

During the past year, observations 
on NGC 7538, centered on the "in- 
frared cluster," have been made simul- 
taneously at 30, 50, and 100 jx with 1' 
resolution from the NASA C-141 Air- 
borne Telescope. An exciting aspect of 
these observations is the discovery of a 
new source not corresponding to any 
known radio or near-infrared feature, 
located about 1 arcminute to the south- 
east of the "infrared cluster," desig- 
nated here NGC 7538 (East). The far- 
infrared color of this source is similar 
to that of the "infrared cluster," but its 
luminosity is about 5 times smaller. 

Subsequently, a search at near in- 
frared wavelengths with the Mount 
Wilson 1.6-meter telescope has located 
this source, and broadband photometry 
shows its 1.6-13 (jl spectrum to be iden- 
tical in shape and intensity to that of 
IRS-1. However, no radio emission is 
seen from this region down to 10 mJy 
at centimeter wavelengths (C.G. 
Wynn-Williams, private communica- 
tion). Size measurements with the 
5-meter telescope show that NGC 7538 



HALE OBSERVATORIES 



297 



(East) and IRS-1 are both less than 1" 
in size at 10 /x. 

Recently a 1-mm map of this region 
was constructed by Werner from obser- 
vations made during twilight hours 
from the prime focus of the Hale 
5-meter telescope. At 1 mm the 
maximum emission comes from the po- 
sition of the "infrared cluster," but a 
significant extension is seen that in- 
cludes both NGC 7538 (East) and a re- 
gion about 1' to the south, which con- 



tains an OH maser designated OH(S). 
The region around OH(S) shows signif- 
icant 100-^t flux but is not seen at 30 or 
50 /x, implying that it is extremely cold. 
In summary, infrared observations of 
the region NGC 7538 between 
wavelengths of 1.6 fi and 1 mm have 
revealed a wealth of structures and 
varying conditions. Detailed analysis 
of these data should provide great in- 
sight into the evolution of young re- 
gions. 



PULSARS 



Search for the Hulse-Taylor 
Binary Pulsar 

Kristian, Clardy, and Westphal have 
given the first results of a search for 
the optical counterpart of the binary 
pulsar PSR 1913 + 16, discovered by R. 
A. Hulse and J. H. Taylor. The 59 msec 
period of this pulsar is the shortest 
known except for the Crab pulsar. The 
frequency shows cyclic variations with 
an 8-hour period, due to an orbital mo- 
tion about an undetected second body: 
The source is the radio analog of a 
single-line spectroscopic binary. It is 
the only pulsar known to occur in a bi- 
nary system and is of considerable 
interest. Among other things, it offers 
the possibility of the direct determina- 
tion of a pulsar mass and of a "labora- 
tory" for a number of new tests of 
gravitational theories. 

Early synchronous photoelectric 
measurements give a limit of V > 23 



mag for visible pulsed radiation. This 
limit scaled against the Crab pulsar is 
roughly the fourth power of the period 
ratio. A recent improvement of the 
radio position accuracy to 0.2" offers 
the possibility of (1) a search for fainter 
visible pulsations and (2) detection of 
nonpulsed visible light, including that 
of the companion, at the pulsar posi- 
tion. The Palomar Sky Survey plates 
are empty at the radio position, which 
sets limits of B > 21 mag and R > 20 
mag for any object, pulsating or not, 
associated with the pulsar. Rough es- 
timates of the distance and absorption 
give a limit ofM S3 for the absolute 
magnitude of the pulsar and/or the 
companion. Deeper data taken with the 
SIT area photometer show two faint 
red objects 3" to 4" from the pulsar posi- 
tion. Further observations of these 
stars and a deeper search for pulsed 
radiation are anticipated during the 
1976 observing season. 



X-RAY SOURCES 



Oke and Greenstein have made a 
study of the x-ray nova A0620-00, 
which appeared first as a strong x-ray 
source in August 1975. Observations 
between September 2, 1975, and March 
20, 1976, were made with the mul- 
tichannel spectrometer, the SIT digital 
spectrograph, or the Boksenberg cam- 
era in the coude spectrograph. Infrared 
observations at 1.65, 2.2, and 3.5 fx 



were also obtained by Neugebauer and 
Becklin. The flux below 600 A can be 
represented accurately by an energy 
distribution somewhere between that 
of a model atmosphere and a blackbody 
at 25,000°-30,000°K. This radiation 
appears to come from an accretion disk 
around the x-ray source. One possible 
source for the red and infrared radia- 
tion is a hot gas at 10"°K, also as- 



298 



CARNEGIE INSTITUTION 



sociated with the accretion disk; in this 
case, the density of the gas must be 
such that it becomes optically thick in 
the near infrared. A second possible 
source of the red and infrared radiation 
is a cool (4000°K) blackbody, which 
must be substantially larger than the 
binary system itself. The emission 
lines appear to come from the accretion 
disk. The derived reddening is consis- 
tent with E R _ V = 0.39 obtained from 
the Astronomical Netherlands Satel- 
lite UV observations. The distance is 
approximately 1200 pc. Using the ob- 
served data for September 2, 1975, it is 
found that the x-ray flux can be pro- 
duced by a flow of 1.2 x 10 18 gm s"' of 
matter into a neutron star. Using a 
simple accretion disk model, this same 
flow can generate the energy flux actu- 
ally observed from the disk. The model 
predicts too small ah energy release in 
the outer parts of the disk; this outer 
portion of the disk may be heated by 
the x rays themselves. The total mass 
in the disk is approximately 1.6 x 10 2! 
gm, which is much more than the mass 
of optically thin gas in the disk and is 
of the right order to explain the charac- 
teristic times associated with variation 
in the optical and x-ray flux. 

Oke has completed a study of the 
x-ray binary HZ Herculis (Her X-l), 



using spectrophotometric observations 
around the 1.7-day eclipse cycle. 
Analysis of the data at minimum light 
yields MIM Q =2.60, L/L Q = 80.7, and 
RIR Q = 4.2 for the large primary star, 
assuming the standard mass- 
luminosity law and conventional stel- 
lar evolution. Different but consistent 
results can be obtained, provided log g 
= 3.60. The hot x-ray heated cap of the 
primary star has an angle of 26° and an 
effective temperature of 25,000°K. 
Analysis of the energy distributions at 
phases other than minimum suggests 
an accretion disk with a diameter of 2.1 
x 10 11 cm; its radiation is consistent 
with half of the visual flux coming from 
a plasma at 10 ,;o K and N e = 10 14 cnr :; 
and the other half from an opaque body 
with a temperature of about 20,000°K. 
Another x-ray source, A0535 + 26, 
which has an x-ray period of 104 s but 
no detected optical period, is being 
studied by Wade and Oke. Both mul- 
tichannel observations and single- 
trailed SIT digital spectra were ob- 
tained around the 104 s period. The 
spectral energy distribution is nearly 
flat from 3300 to 10,000 A. Balmer 
emission lines are seen and occasion- 
ally A. 5876 of He I. No periodicity has 
been found so far either in the con- 
tinuum or in the emission lines. 



SUPERNOVAE 



Nine supernovae were discovered in 
the course of the Palomar supernova 
search during the report year, all with 
the 1.2-meter Schmidt telescope. Sar- 
gent decided to terminate this part of 
the search, which was mainly for 
statistical purposes, at the end of 1975. 
In all, 178 supernovae have been dis- 
covered with the 1.2-meter Schmidt 
since the systematic search was begun 
by Zwicky in 1959. The search on the 
46-cm Schmidt is to go on in an ex- 



panded form. The main aims will be to 
study the statistics of supernovae in Sc 
I galaxies and to maximize the chance 
of discovering a really bright super- 
nova of Type II. 

Kowal completed the work of setting 
up photoelectric UBV sequences in 20 
of the 1.2-meter supernova fields. Work 
is now proceeding on measuring light 
curves for several supernovae observed 
during the 17 years of the search. 



HALE OBSERVATORIES 



299 



THE GALAXY 



Metal Abundances in Globular 
Clusters 

Searle is making a comparative 
study of the chemical history of the 
globular cluster systems of the Galaxy, 
of M31, and of the Magellanic Clouds. 
In this connection, Zinn and Searle are 
mapping the spatial distribution of 
clusters of different metal abundances 
within the Galaxy. A new method of 
determining the metal abundances in 
individual red giants belonging to a 
cluster has been developed. It is based 
upon measurements of the integrated 
metal-line absorption from spec- 
trophotometric scans. This method is 



much more sensitive than any previ- 
ously used. The observations deter- 
mine an intrinsic (reddening-free) 
index that varies by a full 1.5 mag over 
the abundance range between M71 and 
M92. 

In this report year Searle and Zinn 
have calibrated the new system and 
have begun a program to determine the 
metal abundances of all globular clus- 
ters for which color-magnitude arrays 
are available. Particular emphasis has 
been placed on remote clusters and 
other stellar systems on the fringes of 
the Galaxy. So far in this program, 
abundances have been determined for 
200 stars belonging to 25 systems. 



GALAXIES 



New Redshifts for Bright Galaxies 

A long-range program to obtain red- 
shifts for all (—1200) galaxies in the 
Shapley-Ames catalogue (whose limit 
is m m = 13) in both hemispheres is 
nearing completion by Sandage. Dur- 
ing the report year, he prepared for 
publication new redshifts for 700 
galaxies from 1100 plates taken be- 
tween 1969 and 1975 with image-tube 
spectrograms on the Mount Stromlo 
1.9-meter ami the Hale 5-meter tele- 
scopes. 

All but about 50 Shapley-Ames 
galaxies now have measured redshifts, 
either from literature sources or from 
the new data; so the redshift program 
begun in 1935 by Humason and Mayall 
is essentially complete. 

Sandage and Dr. G. A. Tammann of 
the Astronomisches Institute der Uni- 
versitat Basel are preparing a redshift 
catalog for the complete Shapley-Ames 
list, where all available data will be 
combined. New two-dimensional mor- 
phological classifications of the same 
galaxies, using large-scale reflector 
plates from Mount Wilson, Palomar, 



and Las Campanas, have also been 
completed, and these, combined with 
the redshift data, are expected to pro- 
vide improved luminosity functions for 
the various Hubble-van den Bergh 
classes. 



Distances to Nearby Galaxies Derived 
from Brightest Stars 

Accurate photometric distances to 
nearby resolved galaxies just beyond 
the Local Group are still largely un- 
known. The brightest resolved stars 
provide the easiest fairly accurate 
method to find them, short of the very 
time-consuming search and photom- 
etry of Cepheid variables. 

Sandage has begun a long-range 
program to use brightest stars to find 
distances to many of these resolved ir- 
regular dwarf systems whose distance 
moduli are less than m — M ~ 28 {D < 
4 Mpc). 

The first phase of the program is a 
strengthening of the absolute mag- 
nitude calibration of the brightest 
stars. The color-magnitude diagram for 



300 



CARNEGIE INSTITUTION 



the Local Group irregular IC 1613 has 
been completed by Sandage and 
Katem. The galaxy is at the faint end 
of the calibration sample, with an inte- 
grated absolute magnitude of only M tt 
— -14.4. A special blink survey of 
5-meter plates for the reddest stars 
shows that the brightest M-type super- 
giant has M v = -8.0, M B = -6.0, and 
the brightest blue supergiant has M, = 
-7.4, M B = -7.6, in agreement with 
previous studies of this galaxy reported 
in Year Book 69 (p. 95). 

Other calibrating galaxies, such as 
M33 and the Large and Small Magel- 
lanic Clouds, are currently being 
searched for the brightest stars on 
plates taken with the Mount Wilson 
2.5-meter and the Las Campanas 1.0- 
meter reflectors. 

The strengthening of the present 
calibration is expected to take several 
years, but the main program for the 
unknown galaxies has also begun, with 
the first results on the Wolf- 
Lundmark-Mellotte (WLM) system 
completed by Sandage and Katem. The 
color-magnitude diagram from 5-m 
plates shows the brightest three blue 
stars to have (fi(3)> = 17.85, (V(3)) = 
18.00. The brightest red supergiants 
have (V) = 18.20. The previous calib- 
ration by Sandage and Tammann, done 
as a part of their steps toward the Hub- 
ble constant, gave, for luminosity class 
V galaxies, M fl(3) = -7.99 for the 
brightest blue supergiant, and M, (red) 
= -7.90, with concordant moduli of (m 
- M) AB = 25.95 and (m - M),, = 
26.10, for a mean of 26.02 (orD = 1.6 
Mpc). This puts the WLM system at the 
outer edge of the Local Group. 

Plates in good seeing were obtained 
with the 5-meter telescope for the addi- 
tional dwarf galaxies Pegasus A, Sex- 
tans A, Sextans B, Leo A, Leo B, GR 8, 
and Ho IX, which is the companion to 
M81. Photoelectric calibration of field 
stars to V = 17 was completed in each 
of these fields, using the Mount Wilson 
2.5- and 1.5-meter reflectors. The se- 
quences were extended to V = 22, 
using 5-m photographs that had sec- 



ondary images 5 mag fainter, made 
with the Racine wedge calibrator in the 
converging prime-focus beam. 

It is known that these very nearby 
dwarfs partake of the expansion (all 
have positive redshifts). The aim of the 
long-range program is to determine, 
from the distances, the deceleration ef- 
fects of the Local Group, and hence to 
find its mass from deviations of the 
dwarf velocities from the global Hubble 
rate. 

Supergiant cD Galaxies 

Gunn and Thuan are using a new di- 
gital spectrograph with a cooled inte- 
grating SIT-Vidicon camera at the 
Cassegrain focus of the 5-meter Hale 
telescope to obtain velocity dispersions 
and rotation curves of cD galaxies in 
clusters. A synthesis of the total light 
of these intriguing systems will also be 
performed. This dynamical informa- 
tion, along with the light distributions 
obtained from plates taken by Thuan 
with the Schmidt camera, should per- 
mit construction of dynamical models 
of the supergiant galaxies. 

Markarian Galaxies 

Becklin, Neugebauer, Oke, and 
Searle have obtained slit spectra in the 
visible and spectrophotometric and in- 
frared broadband observations of 18 
Markarian galaxies with emission 
lines. Eight of the program galaxies 
can be classified as Seyfert galaxies. In 
general, thermal, nonthermal, and 
stellar radiation components are pre- 
sent. Broadly speaking, one group of 
Seyfert galaxies is characterized both 
by the presence of a high-density re- 
gion of gas and by a continuum domi- 
nated by nonthermal radiation. The 
continua of the remaining program 
Seyferts that do not have a high- 
density region of gas are dominated by 
thermal radiation from dust and stellar 
continuum. The 10 galaxies that are 
not Seyfert galaxies are examples of 
extragalactic H II regions. There is 



HALE OBSERVATORIES 



301 



evidence for thermal emission at 10 fx 
from dust in four of these galaxies. 

In a continuing study, Sulentic has 
completed a morphological study of 
Markarian galaxies 1 through 604, 
using the Palomar Sky Survey. The re- 
sults have been correlated with radio- 
continuum measurements previously 
obtained with the National Radio As- 
tronomy Observatory 300-foot tele- 
scope. It is found that useful coarse 
structural information can be obtained 
for most of the bright (m =s 15.5) Mar- 
karian galaxies from the Sky Survey. 
The Markarian galaxies could be dif- 
ferentiated into spiral and irregular 
morphological types covering a wide 
range in absolute luminosity. 

Previously Sulentic had suggested 
the existence of unusually high radio 
luminosities and radio detection frac- 
tions for a sample of multiple galaxy 
systems. In pursuing this idea, photo- 
metric and spectroscopic data are being 
obtained for a large sample of these 
systems. The correlation of these data 
with radio emission and structural 
morphology is in progress. 

Seyfert Galaxies 

Dr. Gregory A. Shields of the Uni- 
versity of Texas at Austin and Oke 
have obtained and analyzed spectral- 
energy distributions for the two Zwicky 
compact galaxies ZW 2130+09 (II Zw 
136) and Zw 0051 + 12 (I Zw 1), as well 
as for 3C273. All of these have strong 
Fe II lines in emission. The permitted 
line intensities are consistent with a 
filamentary emission region having N ( 
~ 10 9 cm"', R ~ 1 pc, M ~ 10- 3TC , and 
v ~ 3000 km s" 1 . The observed H/3 in- 
tensities are smaller than expected on 
the basis of an extrapolation of the vi- 
sual continua to ionizing frequencies, 
and this suggests that the emission- 
line region does not completely sur- 
round the continuum source or that the 
continuum slope becomes steeper for A. 
< 3000 A. The Fe II line intensities can 
be explained by continuum fluores- 
cence if the observed line widths result 



from radial rather than circular mo- 
tion. The derived column density is 
~10 2i±1 cm - ' 2 , comparable with the 
value implied by the x-ray absorption 
in NGC 4151. The O I K 8446 emission 
may result from continuum fluores- 
cence; alternatively, it can be ex- 
plained by Lyman-/3 fluorescence if a 
portion of the broad-line region has 
t(H«) > 100 as a result of time- 
dependent ionization. 

Infrared Observations 

Persson, J. A. Frogel of the Cerro To- 
lolo Inter-American Observatory, and 
M. J. Aaronson of Harvard College Ob- 
servatory continued their studies of the 
stellar populations of elliptical 
galaxies by means of infrared photom- 
etry. The program of measuring the CO 
absorption at 2.2 /jl was largely com- 
pleted, and the results are being pre- 
pared for publication. 

New observational programs were 
initiated to investigate further the in- 
frared properties of galaxies with the 
1.5-meter and 2.5-meter telescopes at 
Mount Wilson, the 5-meter telescope, 
and the 1-meter telescope at Las Cam- 
panas. The first program consists of 
measuring the depth of the 2.0-fj. H 2 
band in spectra of galaxies and stars. 
The band-strength is quite sensitive to 
effective temperature in both giant and 
dwarf stars later than M2 and offers a 
means of studying the relative num- 
bers of very late-type stars in galaxies. 
A preliminary result of this work is 
that the H 2 band is strong in galaxies 
and, hence, that a substantial fraction 
of the galaxy light at 2 /x is contributed 
by M stars. The effect of this parameter 
upon stellar synthesis models is under 
study. 

A second program consists of deter- 
mining accurate V - [2.2 /jl] colors for 
galaxies. Some preliminary results 
were reported by Frogel et al. (As- 
trophys. J. [Lett.], 200, L123, 1975). 
The importance of these colors follows 
from an earlier finding that giant stars 
contribute much of the 2.2-^t light of 



302 



CARNEGIE INSTITUTION 



elliptical galaxies. The V - [2.2 fi] 
color should thus depend to some ex- 
tent upon the morphology of the com- 
posite giant branch through the effect 
of metal abundance on the luminosities 
and effective temperatures of the indi- 
vidual stars. A correlation between the 
line-blanketing-sensitive U—BorU — 
V colors and V - [2.2 /x] should there- 
fore exist. The data for three samples 
of galaxies, Virgo (30 galaxies), Coma 
(12 galaxies), and the field (40 
galaxies), show the effect in an unam- 
biguous way: A(£/ - V) is approxi- 
mately equal to A(V - 2.2 fx]. The V - 
[2.2 fi] color thus provides a means of 
studying metallicity, and perhaps also 
the relative distance moduli of clusters 
of galaxies. 

Nuclear Regions of M31 

Thuan and Oke have obtained spec- 
trophotometry measurements of the 
nuclear region of M31 under conditions 
of excellent seeing and with M31 near 
the zenith. A round 3"6-diameter aper- 
ture was used for the nucleus meas- 
urements, while an annulus with a 
diameter of 3" 8- 9" 9 was employed for 
the bulge observations. It is found that 
the nuclear region with r =s 1'.'8 is only 
slightly redder and shows marginally 
stronger CN blends that the bulge from 
r = 1'.'9 to 4"9. The data can be inter- 
preted in terms of an increase in 
heavy-element abundance toward the 
center of M31 by a factor substantially 
less than 2. 

Observational Search for the 
"Missing Mass" 

Thuan used the 1.2-meter telescope 
at Palomar to obtain plates of several 
edge-on galaxies. They will be used to 
test observationally the Ostriker- 
Peebles proposal that massive halos 
may exist around spiral galaxies and to 
study color-gradients in the halos of 
galaxies. Photoelectric data will also be 
obtained to supplement the photo- 
graphic material. 



Redshifts, Magnitudes, and Colors of a 
Sample of Galaxies 

Thuan, in collaboration with G. R. 
Knapp of the Owens Valley Observa- 
tory and J. Huchra, has started a pro- 
gram to obtain redshifts, UBVR 
photometry and morphological data for 
a complete sample of about 1100 
galaxies selected from the Zwicky 
catalogue of galaxies and groups of 
galaxies, such that m vg =£ 14 m , 6 11 5s 
40°, and 8 5= 0°. Since most of the 
galaxies (—90%) are spirals, the possi- 
bility exists of measuring their red- 
shifts via their H I radio emission. 
Measurements of a large number of 
redshifts are being made quite accu- 
rately (—10 km s _1 ) with the sensitive 
21-cm system on the 300-foot telescope 
at the National Radio Astronomy Ob- 
servatory. Redshifts of ellipticals, and 
the photometry, are being obtained 
with the Palomar 1.5-meter reflector 
and the Kitt Peak 91-cm reflector. 

The investigations that can be car- 
ried out with these data are many, and 
all have important implications for 
cosmology and for the theories of for- 
mation and evolution of galaxies: 

1. The study of systematic dif- 
ferences (if there are any) in properties, 
such as colors between cluster and 
noncluster galaxies. 

2. The color-magnitude relation 
(Sandage, Astrophys. J., 176, 21, 1972) 
for elliptical and SO galaxies may be 
improved. If the tightness of the corre- 
lation holds, it may prove to be a very 
powerful cosmological tool. 

3. The distributions of colors and ab- 
solute magnitudes of galaxies from a 
statistically complete sample could be 
compared with simple models of star 
formation in galaxies (Searle, Sargent, 
and Bagnuolo, Astrophys. J., 179, 427 ', 
1973). 

4. By mapping the local velocity 
field, we can apply a "cosmic" virial 
theorem to deduce a local mean density 
and hope to make headway toward the 
outstanding problem of an open or 
closed universe. 



HALE OBSERVATORIES 



303 



5. In addition to the radial-velocity 
information, the H I data will provide 
virial masses and total H I content. 
Such a large sample should provide an 
interesting statistical study of H I con- 
tent as a function of morphological 
type. 

Chemical History of Galaxies 

As part of an investigation into the 
comparative chemical histories of the 
Galaxy, of M31, and of the Magellanic 
Clouds, Searle has begun a program of 
classification of the integrated spectra 
of their globular clusters. In collabora- 
tion with A. Boksenberg of University 
College London, spectra were obtained 
of 12 globular clusters in M31. These 
spectra, obtained with the University 
College London image-photon- 
counting system, cover the range aa 
3500-5000 with a spectral resolution of 
5 A. Douglas Rabin, a Caltech 
graduate student, is collaborating with 
Searle and Boksenberg in the interpre- 
tation of these spectra. In a related in- 
vestigation, Searle has obtained low- 
resolution spectrophotometric scans of 
70 of the globular clusters in M31. So 
far, all of the observations obtained ap- 
pear to be consistent with the idea that 
the M31 globular clusters differ from 
one another in one parameter only — 
presumably the metal abundance. 

The situation is essentially different 
in the Magellanic Clouds. During the 
report year, Searle obtained five-color 
photometric observations of the inte- 
grated light of 30 Magellanic Cloud 
globular clusters at Las Campanas. 
These show that there are intrinsic dif- 
ferences between the globular clusters 
of M31 and of the Galaxy on the one 
hand and those of the Magellanic 
Clouds on the other. There are also in- 
trinsic differences between the clusters 
of the Small and Large Clouds. 

A possible explanation of the obser- 
vations is that in the Galaxy and in 
M31 metal enrichment occurred sud- 
denly, so that all globular clusters, of 
whatever composition, have closely 



similar ages; whereas, in the Magel- 
lanic Clouds enrichment was slower 
and the metal-poor globular clusters in 
the Clouds have a substantial range of 
age. 

Large Magellanic Cloud 

The importance of the study of stel- 
lar populations in the Magellanic 
Clouds, and in particular of the old 
stellar component of the LMC disk, has 
been previously emphasized. Knowl- 
edge of the population structure of the 
LMC would make that galaxy, our 
closest neighbor, an ideal testing 
ground for some aspects of the kind of 
synthetic models that must be used 
with the unresolved, distant galaxies. 

Hardy has studied a number of re- 
gions, 3.'4 in diameter each, free of 
supergiants, and smooth in the distri- 
bution of stars, in and near the bar of 
the LMC. Observations were made 
photographically in B and V and 
photoelectrically in the integrated 
light of the UBVR and DDO systems. 
His main conclusions are: 

The surface-brightness distribution 
shows the existence of an exponential 
disk of which the bar is a narrow 
axisymmetric perturbation. The blue- 
length scale for the exponential disk 
was found to be a H ~ l = 1.36 ± 0.42 kpc, 
while the central surface brightness 
was ^ B (0) = 21.93 ± 0.22 mag/D". 
There are no significant color gradients 
over the sampled baseline of 1.5 kpc, 
which roughly corresponds to one 
e-folding length over the surface 
brightness. But color asymmetries due 
to either population or reddening ef- 
fects were detected when the bar was 
taken as an axis of symmetry. 

Color-magnitude diagrams to M v = 
0.5 were constructed, and it was found 
that the observed giant branch resem- 
bles that of intermediate Population II 
clusters such as M4 or M75. The sur- 
face colors, which include new observa- 
tions in the R region obtained at Las 
Campanas, are compatible with this 
interpretation. There are indications of 



304 



CARNEGIE INSTITUTION 



the existence of a horizontal branch of 
Population II, but an important con- 
tribution from intermediate-age stars 
cannot be ruled out only on the basis of 
the colors and the color-magnitude 
diagrams. 

Luminosity functions were con- 
structed by Hardy and then normalized 
to the total, light contained in the area 
counted (count- brightness-ratios: 
CBR). A method was devised that in- 
volved comparison of the observed CBR 
in both V and B with those of known 
globular clusters of different metallic- 
ity to determine the fraction of the 
total light in Population II stars. It was 
found that if the giant branch is inter- 
preted as belonging to an M4-M75 
type of population, then Population II 
contributes 70% ± 10% of the total 
light in V and 80% ± 20% in B in the 
regions studied, thus leaving only a 
small fraction of the light to be ac- 
counted for by stars of the solar- 
neighborhood type that were too faint 
to be detected photographically. 

The alternative, that the LMC disk 
is a scaled version of the galactic disk, 
was discussed. It was shown that the 
observed CBR in B light did not fit the 
CBR derived from the van Rhijn func- 
tion for the solar neighborhood, and it 
was argued that this was not only an 
effect on the young stellar population 
but involved the old component as well. 

Alternatives for the formation and 
evolution of the LMC disk were consid- 
ered. Hardy concluded that the ob- 
served disk is likely to be as old as the 
galactic disk but, being more metal 
poor, resembles an enriched globular- 
cluster population rather than an 
M67-NGC 188 population, as in the 
galactic disk. 

New data obtained with the 4-meter 
telescope at Cerro Tololo Inter- 
American Observatory in January 
1976 are now being analyzed. 

Population Synthesis 

Turnrose has determined the stellar 
content of the nuclear regions of seven 
nearby Sc galaxies by means of popula- 



tion syntheses, utilizing narrow-band 
spectrophotometry covering the 
wavelength range XX3300- 10,400. A 
linear-programming fitting technique 
was used in conjunction with simple 
models of star formation to provide 
physically consistent population mod- 
els incorporating multiple generations 
of stars. Evidence is found for substan- 
tial intrinsic reddening in the nuclear 
regions studied. Upper-main-sequence 
stars are significant contributors to the 
light in most cases, and the lower main 
sequence contributes significantly in 
all cases. All available indicators are 
consistent with evolved- M-star domi- 
nance at the longest wavelengths and 
low photometric mass-to-light ratios 
(0.3 < MIL r < 10). Using absolute 
measurements of the emission-line 
spectra of the nuclear regions, it is 
found that the O-B stars, which arise 
naturally in the population models, are 
just sufficient to provide the observed 
nuclear ionization in all cases except 
that of NGC 5194, which may be colli- 
sionally ionized. The inferred stellar 
populations are largely consistent with 
simple evolutionary models of star 
formation, using a "local" initial mass 
function and a variety of time depend- 
ences ranging from exponential decays 
with time constants —3-6 billion years 
to constant rates. A possibly significant 
correlation between nuclear stellar 
content and overall dynamical prop- 
erties among four of the program 
galaxies is pointed out. 



Variable Stars in the Dwarf Spheroidal 
Galaxies 

A rare class of variable stars that 
seem to occur preferentially in the 
dwarf spheroidal galaxies has been the 
subject of two investigations by Zinn in 
the current report year. 

Together with Searle, he obtained 
spectrophotometric scans of five of 
these peculiar Cepheids in the Draco 
system. Comparing these with similar 
observations of RR Lyrae variables in 
this system, Zinn and Searle have 



HALE OBSERVATORIES 



305 



found that the masses of the anomalous 
Cepheids are from two to three times 
the masses of the RR Lyrae stars. This 
result, which was unexpected in an old 
stellar system whose evolved stars 
were believed to have the same mass, 
confirms the hypothesis put forward by 
J. Norris of Yale University and Mt. 
Stromlo-Siding Spring Observatory 
and Zinn on the basis of theoretical ar- 
guments. How the anomalous Cepheids 
acquired their large masses remains 
something of a puzzle, however. Either, 
as Norris and Zinn argued (see Year 
Book 74, p. 338), star formation oc- 
curred recently in Draco or, as 
suggested as an alternative by Zinn 
and Searle, the large masses were pro- 
duced by the fusion of the components 
of close binary systems. 

Until recently, the anomalous 
Cepheids had been found in only the 
dwarf spheroidal galaxies and the 
Small Magellanic Cloud. This past 
year Zinn and Conrad C. Dahn of the 
Naval Observatory showed that Vari- 
able 19 in the globular cluster NGC 
5466 obeys the same period-luminosity 
law and has the same mass as the un- 
usual Cepheids in the Draco system. 
The as yet unidentified characteristic 
of the dwarf spheroidal galaxies, the 
Small Cloud and NGC 5466 that dis- 
tinguishes them from other systems is 
clearly a key to understanding the ori- 
gin of these peculiar variables. 

Image Processing of Galaxy 
Photographs 

There has always been interest in 
extracting maximum information from 
astronomical photographs. In an ex- 
perimental project that utilizes the 
capabilities of modern digital com- 
puters, Jean Lorre of the Jet Propul- 
sion Laboratory's Image Processing 
Laboratory and Arp have collaborated 
in the analysis of a number of galaxy 
photographs. The photographs were 
made by Arp on fine-grain plates and 
under especially good observing condi- 
tions with the 4-meter reflector at 
Palomar. The plates have been scanned 



with a PDS microphotometer and pro- 
cessed by a variety of mathematical al- 
gorithms by Lorre at JPL. Techniques 
have been demonstrated that (1) en- 
hance the contrast and clarity of 
nebulous features, (2) detect previously 
invisible features, (3) moderately im- 
prove resolution (through partial see- 
ing compensation), (4) add information 
from different plates to improve 
signal-to-noise ratios, and (5) compare 
response at different wavelengths in 
order to portray color differences. 

In certain well-known and interest- 
ing groups of galaxies, new features 
were detected by these procedures. For 
example, in the region of NGC 7331 
and Stephan's Quintet, luminous 
nebulous filaments were discovered 
(see Astrophys. J., 210, No. 1, 1976, 
November 15). Additional detail is also 
revealed in some of the galaxies in 
Seyfert's Sextet. In one of the most 
striking results, a photograph of the 
famous jet in the spherical galaxy M87, 
taken with the 5-meter telescope on 
fine-grain emulsion, has been pro- 
cessed for maximum resolution. The 
deconvolution algorithm has reduced 
the smallest image sizes by about a fac- 
tor of 2 and compressed the dynamic 
range so that the jet is seen in this pic- 
ture as consisting of a very small well- 
spaced and well-aligned series of 
luminous points emerging from the nu- 
cleus of M87 (see Fig. 5a, b). 

Ejection from the Spiral Galaxy 
NGC 1097 

While cataloging peculiar southern 
galaxies in Edinburgh, using plates 
from the U.K. 1.2-meter Schmidt in 
Australia, Arp learned of the discovery 
by R. D. Wolstencroft and W. J. Zealey 
of luminous jets emerging from the 
bright southern spiral galaxy, NGC 
1097. A few months later, with the new 
4-meter telescope at Cerro Tololo in 
Chile, he obtained a series of deep 
plates on this object which reveal that 
it is one of the clearest examples of 
ejection from a galaxy that has so far 
been encountered. The 4-m plates show 



306 



CARNEGIE INSTITUTION 




Fig. 5a. A photograph of the jet in M87 with the 5-meter telescope on fine-grain Illa-J emulsion. 
This photograph has been subjected to an image deconvolution algorithm that improved the obtaina- 
ble resolution by about a factor of 2 and increased the dynamic range. 



that the three jets all point exactly 
back to the nucleus of the galaxy and 
suggest, therefore, that material has 
been ejected from the nucleus. 



Neighborhoods of Galaxies 

A two-year collaborative project was 
initiated by Arp with Francesco Ber 



HALE OBSERVATORIES 



307 




Fig. 5b. The resulting image-processed picture shows the well-known jet resolved into a series of 
evenly spaced, well-aligned points of light. 



tola and Cesare Barbieri of Padova Ob- 
servatory. Its purpose is to study the 
contents of fields in the vicinity of 
bright galaxies. Ninety-nine bright 
spiral galaxies between magnitudes 10 
and 12 have been selected, as well as 99 
control fields carefully chosen to be 



more than 6° distant from any bright 
galaxies. The region within 1° of each 
of the spirals has been carefully 
studied in order to catalog companion 
galaxies and disturbed or nonequilib- 
rium galaxies, including interacting 
double galaxies in which a quantita- 



308 



CARNEGIE INSTITUTION 



tive measure of relative separation 
may be recorded. Jack W. Sulentic, 
postdoctoral Hale Observatories Fel- 
low, has completed the initial survey. 
The same kind of census has been com- 
pleted for each of the associated control 
fields. 

In May 1976, Dr. Graziella de Tullio 
of the Padova Observatory visited 
Pasadena to work on this same prob- 
lem, comparing her independent 
cataloging of the fields with Sulentic's. 
Now a program is under way in both 
Pasadena and Padova to obtain photo- 
metric magnitudes, colors, spectro- 
scopic classifications, and redshifts of 
these objects with the Palomar 1.5- 
meter and the Asiago 2.3-meter reflec- 
tors. A secondary phase of this program 
is to determine whether there are any 
photometric or spectroscopic dif- 
ferences that distinguish companions 
of the bright spirals from the back- 
ground galaxies in the control fields. 

With A. G. Willis and H. de Ruiter of 
Leiden Observatory, Arp has com- 
pleted optical identifications of accu- 
rate Westerbork radio positions around 
a number of bright galaxies. Some of 
the more interesting identifications in- 
clude a new 16.5-17.0 mag object of the 
BL Lacertae type very near the large 
spiral galaxy NGC 6503, a quasar that 
brightened almost 4 magnitudes in less 
than one month, and a number of 
quasars with redshifts greater than 2, 
including one with z = 3.19 (see Year 
Book 74, p. 333). 

Sulentic and Arp have obtained U, B 
double-image plates of a number of 
their galaxy and control fields. These 
plates permit selection of ultraviolet 
excess stars that are quasar candidates 
and will allow an independent deter- 
mination on the value of radio-quiet 
quasar numbers in the general field. 



M87 and Its Globular Clusters 

Sargent collaborated with C. R. 
Lynds of Kitt Peak National Observa- 
tory, F. D. A. Hartwick of the Univer- 
sity of Victoria, and A. Boksenberg and 
R. F. Carswell of University College 
London in an attempt to measure the 
dispersion in velocity of the system of 
globular clusters around M87. The ob- 
servations were made with Boksen- 
berg's IPCS detector on Lynds' "Gold" 
spectrograph at the Cassegrain focus of 
the Kitt Peak 4-meter telescope. Be- 
cause of indifferent seeing conditons, it 
proved possible to obtain a good spec- 
trum of only one suspected cluster 
(IV-122 in an unpublished catalog by 
D. A. Hanes of David Dunlap Obser- 
vatory). This object, which has B = 
20.6 mag, proved to be a globular clus- 
ter (based on its redshift). Remarkably, 
IV-122 turns out to be more metal-rich 
than the galactic globular cluster M71, 
despite the fact that it is in the halo of 
M87, about 34 kpc from the center. 
This object was measured in a 3-hour 
exposure. It thus appears to be feasible 
to carry out the original project, given 
good seeing. A further attempt on the 
problem will be made on the Hale Tele- 
scope in 1977. 

In view of the poor seeing during the 
run at Kitt Peak, the above team de- 
voted a considerable effort to M87 it- 
self. Lengthy exposures were made to 
determine the radial dependence of the 
velocity dispersion of the stars in M87 
out to 150" from the center. Usable 
data were obtained despite the fact 
that at this radius M87 is about 1 mag 
fainter than the sky. They are now 
being analyzed. The ultimate aim of 
the observations of M87 and its clus- 
ters is to measure the total mass of the 
galaxy and its radial distribution. 



CLUSTERS OF GALAXIES 



Redshifts and Magnitudes 

Kristian, Sandage, and Westphal are 
continuing a program to extend the 
Hubble diagram to redshifts greater 



than 0.2. The main candidate lists in 
the first stage are (1) the fainter Abell 
clusters, (2) clusters found on a special 
high-latitude 1.2-meter deep survey 
with Illa-J and 127-04 plates, and (3) 



HALE OBSERVATORIES 



309 



faint clusters associated with 3C radio 
sources. Redshifts are being measured 
with the SIT-prism digital spectro- 
graph and BVR magnitudes with an 
S20 photomultiplier and the SIT area 
photometer. Redshifts have been 
measured for 80 clusters, including 60 
with z > 0.2, 15 with z > 0.3, and 3 
with^ > 0.5 (3C330 at^ = 0.53, 3C123 
at z = 0.63, and 3C343.1 at z = 0.75 
[preliminary!). With observations by 
Spinrad, using the image-dissector 
scanner at Lick Observatory, by Gunn 
and Oke, using the multichannel scan- 
ner, and those reported here, the 20- 
year-old "wall" atz = 0.2 has collapsed, 
and redshifts to 2Vz or 3 times larger 
distance are now routine. The principal 
limitation at present is not measuring 
the spectra but finding faint candidates 
and positioning them on the spectro- 
graph slit. It now appears likely that 
about 100 clusters in the important 
range z = 0.3-0.5 can be measured in 
the next few years. Larger redshifts re- 
quire rare nights of exceptionally good 
seeing and are likely to come more 
slowly. 

Analysis of some early results in the 
range z = 0.2 to 0.3 shows several fea- 
tures: (1) In the absence of evolu- 
tionary corrections, the Friedmann 
models with q u — 1 continue to fit the 
data reasonably well. (2) The cosmic 
dispersion of 0.25 mag in the absolute 
magnitude of the brightest member 
and cluster richness found earlier by 
Sandage and Hardy is confirmed but is 
remarkably small over a large range of 
richness. This implies that if the mag- 
nitude of the brightest member is gov- 
erned by a general luminosity function, 
then the slope of the function at the 
bright end must be very steep: d log 
f(M)ldM 5= 5. 

Preliminary analysis of 35 addi- 
tional clusters to z > 0.5 supports the 
earlier conclusions and begins to show 
the expected turnover to bluer B — V 
colors at large redshift, as the pass 
bands move into the ultraviolet regions 
of the galaxy rest spectrum. This color 
change is expected to be a useful dis- 
covery discriminant, and a program 



has been started to use it to find ex- 
tremely distant clusters. 

The Hubble Diagram 

The survey by Gunn and Oke with 
the 90-mm image tube and the 5-meter 
Hale Telescope now consists of 250-300 
plates. On these plates approximately 
60 faint clusters have been found. The 
south galactic hemisphere sample is 
essentially complete, but considerable 
work needs to be done on one or two of 
the spring fields, where progress has 
been slow due to bad weather. Since 
their first analysis of the Hubble dia- 
gram, an additional 38 galaxies in 
faint clusters have been observed, and 
redshifts and energy distributions now 
exist for most of these. A substantial 
fraction of these 38 objects have red- 
shifts of 0.40 or more. The SIT spec- 
trograph with on-line data handling 
and reduction system is now fully op- 
erational and is being used to obtain 
spectra of very faint galaxies; its ex- 
pected speed advantage over the mul- 
tichannel spectrometer is realized. 

In their first analysis of the Hubble 
diagram, Gunn and Oke found that a 
significant uncertainty was introduced 
because of poor statistics for the rela- 
tively close clusters. This has been rec- 
tified by work with Thuan on a com- 
plete sample of nearby Abell clusters 
(all with distance class «s 4, richness 
5=1, and |6 | > 30°, 102 in all). Photom- 
etry with an intermediate-band sys- 
tem, designed to be easily tied to the 
spectrophotometric system used for the 
distant objects with the 5-meter, has 
been carried out on the Palomar 1.5- 
meter telescope, and redshifts have 
been obtained, mostly with the image- 
tube spectrograph and more recently 
with the SIT digital spectrograph on 
the 5-meter telescope. This sample 
should provide very good statistics at 
the brighter end of the Hubble dia- 
gram. Since the sample is in some 
sense complete, it also allows more 
careful statistics to be done on sys- 
tematic effects with richness and mor- 
phological type than have been possi- 



310 



CARNEGIE INSTITUTION 



ble before. Some of this work is being 
underta