Origin and Distribution of the Elements

1133 Pages·1968·28.457 MB·English

by  L. H. Ahrens (Eds.)

ORIGIN AND DISTRIBUTION
OF THE ELEMENTS
Editor
L. H. AHRENS
University of Cape Town, South Africa
Co-editors
Section I Theories of origin
DR. A. G. W. CAMERON,
Institute of Space Studies,
New York, U.S.A.
Section II Solar, stellar and interstellar
abundances
PROFESSOR WM. P. BIDELMAN,
University of Michigan, U.S.A.
Section III Meteorites
DR. J. F. LOVERING,
Australian National University
Canberra, Australia
Section IV Planets, asteroids, comets, tektites
DR. AUDOUIN DOLLFUS,
Observatoire de Paris,
Paris, France
Section V Terrestrial abundances
PROFESSOR M. FORNASERI,
University of Rome,
Rome, Italy
Editorial Assistant
MME G. PROTAS,
C.R.P.G. Nancy-Vandoeuvre, France
tara
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Preface
THE suggestion to hold an international symposium on The Origin and Distri-
bution of the Elements originated at the London (July 1963) meeting of
IUPAC's Commission of Geochemistry. The Commission was dissolved in
1965, and the suggestion for the symposium was passed on to the embryonic
International Association of Geochemistry and Cosmochemistry (IAGC).
IAGC's interim Statutes Committee met at UNESCO Headquarters in Paris
in November 1965 and the idea of a symposium and the theme itself appealed
so strongly both to UNESCO and IUGS, that they decided to co-sponsor the
symposium; this was held at UNESCO headquarters from 8 to 11 May 1967.
Some 235 scientists attended and 90 papers were read.
The symposium was perhaps unique. We are only too aware these days that
science has become more and more specialized and this tendency will in-
evitably grow because of our inability to assimilate the outpouring of the
vast amounts of information in each of the numerous scientific disciplines
which now exist. At times, prospects for the future of science may appear to
some of us to be depressing—are we all to become narrowly informed ultra-
specialists and is science to become pigeon-holed and fragmented into a
myriad separate disciplines ? If this were entirely so, much of science would
become sterile and inert. Fortunately for us, tendencies counter to that of
specialization are also in operation: a continuous coalescence, as seemingly
rigid boundaries between different well-established disciplines break down—
biophysics, biochemistry, geophysics, geochemistry and cosmochemistry are
examples—and it is because of this counter tendency to extreme specializa-
tion, that IAGC's first symposium was so invigorating and refreshing.
Scientists representing an extremely wide range of interests met to discuss a
theme common to them all. Some who from a given country and, in one in-
stance, a single university, had not met each other before, did so for the first
time in Paris. In this respect, it is interesting to recall comments made to me
by Professor T. W. F. Barth, President of IUGS, at the end of the symposium;
he remarked: "Here at this symposium in Paris, a whole spectrum of sciences
has been united." The contents of the Proceedings volume of the Paris
Conference reflects both this statement and many of the thoughts expressed
above.
It is indeed true that IAGC has a uniting effect, and this is all to the good.
It is true also, however, that IAGC will help developments in very specific
fields. The establishment of a working group on the geochemistry of
O.D.E.—1*
X Preface
sediments is an example. In my opinion, much of the strength, vigour and
excitement of IAGC is due to these two characteristic features : its uniting
effect coupled with an ability to encourage and sponsor developments in
highly specialized fields.
Difficulties in editing have—not surprisingly—arisen. Authors are from a
wide range of disciplines and from many countries. Some, who are not
English-speaking, have endeavoured to present their contributions in English.
Parts of these articles may not be clear despite considerable editorial efforts
and the interested reader should consult the author for clarification, should
the need arise.
The reader will note that references and some other items have not been
presented in a uniform manner. Physicists and astronomers tend to use a
bibliographical style distinctly different from geologists and geochemists and
much time would have been spent in ensuring uniformity of presentation.
My thanks are due to the many who have facilitated the production of this
Proceedings volume of the Paris Symposium. Each of the five Section Chair-
men agreed to act as a co-editor and undertook the task of editing the papers
in his section. Much of the heavy burden of attending to editorial detail and
that of arranging and despatching manuscripts fell to Mme G. Protas,
Editorial Assistant; these duties were undertaken expeditiously and efficiently.
The ready cooperation of members of Pergamon Press is acknowledged.
Dept. of Geochemistry, L. H. AHRENS
University of Cape Town
Introductory Remarks
EARL INGERSON
University of Texas, Austin, Texas
As a brief introduction to the Proceedings of the Symposium on the Origin
and Distribution of the Elements, I should like to mention some types of
problems of abundance and distribution that were not emphasized in the
symposium, but which must be encouraged by the IAGC and pursued by
various types of organizations.
These are some of the problems of applied geochemistry, which some
"pure" geochemists consider beneath their dignity, but which are assuming
increasing practical importance as our known and readily accessible mineral
resources diminish dangerously and as experience and better determinative
techniques demonstrate essential relations between element distribution and
agronomy, nutrition, health and related subjects.
It is appropriate that this first symposium of the Association should deal
with elemental distribution. Until a scant generation ago, geochemistry and
abundance and distribution of the elements were essentially synonymous.
Detailed studies of trace elements and isotopes and the use of these data
with the methods of physical and organic chemistry to make important
interpretations in geology are recent innovations. Partly for this reason and
partly because they allow us to solve geologic problems that were insoluble
by older methods, they receive the lion's share of time on programs and in
geochemical publications.
Nevertheless, the study of distribution itself is by no means passé; witness
the 80-odd papers on distribution in this symposium and the wide interest the
subject engendered in groups of diverse backgrounds throughout the world.
I should like to suggest that there are two factors that will make studies of
abundance and distribution important in geochemistry and cosmochemistry
indefinitely in the future :
1. Continuing refinement of analytical techniques allows the detection and
determination of smaller and smaller amounts of elements, which will bring
to light new facts and make possible new interpretations in old fields and
the application of trace element geochemistry to new problems. The same ap-
plies to increasing accuracy in the determination of isotope ratios, especially
in heavier elements where no isotope fractionation has yet been detected.
2. Studies of distribution in new areas will continue to be exciting as long
xii Introductory Remarks
as new areas can be found and for many purposes major element distribu-
tion will be as interesting and important as minor and trace elements.
Refinement of Techniques
As examples of the first of these factors, let us choose two of the many
fields in applied geochemistry that could be mentioned: (1) Geochemical
prospecting and (2) the part of geochemistry that deals with the effects of
trace elements on plants and animals. In geochemical prospecting anomalies
in the parts per million range have been used for some decades in the search
for new ore bodies and extensions of known ones. The techniques are well
established, widely used and have scored many successes.
Spectrographic, colorometric and Chromatographie techniques can be used
to detect most metals in the ppm range and have been the work horses of
geochemical prospecting in the past. More sophisticated methods, however,
are opening new vistas in prospecting. For example, the mercury "sniffer" can
determine mercury in the parts per billion range.
In country rocks not affected by ore-forming solutions the mercury back-
ground is of the order of 10 ppb, which is beyond the range of ordinary
analytical methods. The mercury "sniffer" can follow it, however, and
anomalies in the hundreds of parts per billion range are significant, not as
indicators of deposits of mercury, but they can lead to anomalies of less
mobile elements such as Pb, Zn, Ag, Au, which, in turn, can point to work-
able concentrations of those elements.
Some anomalies can now be sought directly in the ppb range. For example,
gold and silver, determined by atomic absorption show significant variations
and anomalies in the tens of parts per billion range, which can serve as guides
to commercial deposits. More recently neutron activation has been applied
to problems of geochemical prospecting, not only in checking standards and
techniques in the laboratory, but also in analyzing samples for active projects,
both in the laboratory and with at least two different types of mobile units
for field use.
Mineral statistics show that in recent decades mineral consumption has
been doubling approximately every thirty years. Even at the present rate of
use known reserves of many critical materials will last only a few years to a
few tens of years. It is evident, therefore, that determinative innovations and
modified prospecting techniques must do more than just improve methods of
looking for the types of mineral deposits currently being worked. There are
at least two other important applications that must be made :
1. A much better geochemical census than is currently available is needed.
This might well take the form of isoclarke maps for important elements for
materials readily reached from the Earth's surface. For evaluation in terms of
production and needs such maps should be accompanied by statements of
Introductory Remarks
Xlll
clarkes of concentration necessary for each element to allow its commercial
production from each type of material concerned. In order to look toward
the future, attempts should be made to predict the clarkes necessary with
improved metallurgical techniques for specified times in the future. This last
would be difficult and uncertain for some elements, but might be based in
part on predicted needs, which could serve to stimulate research on extractive
techniques in desired directions.
2. Greater efforts must be made in the near future on methods of locating
deposits that are not readily reached, or at least detected, from the Earth's
surface. They can be screened from the surface by great thicknesses of barren
sedimentary or volcanic rocks, or by much thinner covers of recent materials
such as glacial drift, latérite, or basin fill in arid or semi-arid areas, where
there has not been time for geochemical anomalies to reach the surface, or
where conditions in the cover do not allow them to do so.
In any of these situations the most sensitive detection methods available
must be used and anomalies of mobile tracer elements looked for first in
order to hold the amount of drilling required to a minimum.
Geophysical techniques should also be used in conjunction with such a
search to define favorable and unfavorable areas, where this is possible, and
thus eliminate as much exploratory drilling as possible.
The importance attached to this search for deposits at depth by the Council
of IAGC is shown by the fact that the only symposium topic they selected for
the meeting in Prague in 1968 is Deep Geochemical Prospecting.
The importance of other aspects of applied geochemistry has been recog-
nized by the establishment of a Study Group on Applied Geochemistry.
Similarly, in the biogeochemistry of trace elements it has long been recog-
nized that in addition to the dozen or so major elements listed in textbooks as
"essential" for plant and animal growth there are many trace elements whose
presence in infinitesimal amounts appears to be just as "essential" for normal
healthy growth as are some of the more abundant elements.
For example, a considerable area in Australia that had conditions of soil,
moisture, etc., favorable for agriculture consistently failed to produce crops.
Research finally revealed that the soil was practically devoid of molybdenum
and addition of a few ppm of Mo to the soil solved the problem completely.
On the other hand too much of a trace element can be very harmful. It
has long been known that concentrations of selenium in excess of 100 ppm
will produce the crippling so-called "alkali" disease and eventually death.
Only very recently, however, has it been realized that minute traces of this
same element are necessary for healthy growth. Concentrations of less than
about 30 ppb appear to produce muscular dystrophy. Where chromium is
less than about 200 ppb a diabetes-type disease develops.
There are at least three areas where continuing work in this field is most
important:
XIV Introductory Remarks
(a) Additional work in the ppm range on trace elements that have not
been thoroughly studied in this connection.
(b) Extension into the ppb range, especially for elements where no threshold
values are apparent in the higher ranges.
(c) Determination of optimum concentrations of elements in any range.
This last item has been largely neglected. We tend to determine a minimum
requirement for "normal" growth and then see that this minimum is exceeded
without being sure whether the correction is inadequate or excessive. We have
just noted the minimum and maximum quantities for selenium, but does
anybody know the ideal amount? No.
Perhaps the best example of this principle is the substance most necessary
for plant growth—water. For a given soil and climate there is a rate of water
supply below which plants are stunted and may turn brown and die. Farmers
try to supply water in excess of this minimum so that their crops appear to
have normal growth. Carefully controlled experiments have shown, however,
that some crops produce at least five times the amount considered "normal"
in a given area if supplied with not just an "adequate" quantity of water but
the optimum amount at the optimum rate. Too much water, of course,
damages the plants and yields go down. The same principle undoubtedly
applies to trace elements.
Let us consider fluorine as an example. An endemic population that has
subsisted on a water supply containing less than about 0.3 ppm F not only
suffers from widespread dental caries, but is also much more susceptible to
the very serious diseases of osteoporosis and osteoarthritis than a comparable
population that has had a water supply with adequate fluoride. A smaller
proportion of the population also develops nervous disorders. What is
"adequate"? Authorities do not agree. Children brought up in an area
where water contains 5-10 ppm F may develop brown spots on their
teeth, but apparently show no other abnormalities. Concentrations
much higher than 10 ppm taken over a long period can produce crippling
fluorosis.
We should strive to find the optimum concentrations of trace elements in
soil and water for plant and animal life and then devise methods to approach
these best values as closely as possible. These are clearly problems of geo-
chemical distribution and the immediate approach, at least for man, should
probably be more detailed studies of trace element distributions in nature and
their effects on stable populations. Controlled experiments must also be
conducted, but for an evaluation of eventual effects on the human body
interpretation of controlled studies for a given element may require a genera-
tion or more. Nature has already run the experiments for us in areas of stable
population that have different abundances of trace elements.
Introductory Remarks
xv
New Areas
Never before has man been able to envisage the imminent possibility of
determining the composition of such exotic and fascinating material as may
become available in the near future. We can characterize these briefly as
samples from inner and outer space. Distribution of chemical elements in
these samples will be of far-reaching theoretical and practical importance.
It is unfortunate that the Mohole Project was stopped, even temporarily.
Samples through the oceanic crust and of the upper mantle could have
answered questions of long standing, and perhaps posed others just as baffling!
For example, analysis of these samples of lower crust and mantle should
reveal the cause of sharp changes in density at depth. Studies of element
distribution in the deep sediments, especially if there is a fairly complete and
continuous section could make possible the use of trace elements, such as
nickel and associated metals, in estimating age, rates of accumulation of
sediments, etc. In the parlance of American football, the U.S. appears to
have fumbled on the Mohole; perhaps our colleagues in the U.S.S.R., who
have had much experience in deep drilling, will pick up the ball and run
with it.
In the other direction, outer space, the possibilities are without limit.
Emphasis at the moment is on the nature of the surface of the moon. Whole
books, and countless articles have been written on the subject; increasingly
better pictures of the surface have become available through satellites and
soft landings, but we have little if any more specific information about the
chemical composition than before.
One's idea of the Moon's composition still depends largely on the hypo-
thesis of its origin that he accepts. If it were thrown out of the Pacific Basin
it would probably be largely basaltic; if it condensed from the same part of a
primordial dust cloud that gave rise to the Earth, it might be expected to have
an over-all composition similar to that of the Earth; if it accumulated in
another part of the solar system and was captured, such similarity is less
likely, etc.
Most recent studies of the lunar surface have been concerned with such
things as texture, topography and bearing-strength; that is, with physical
properties. Even the recent Surveyor III with its little sampling shovel made
no attempt to analyze the samples scratched up and No. IV was not equipped
to do more chemically. Certainly this physical approach is essential in plan-
ning manned landings on the Moon, but after the landings the distribution
of elements on and near the surface will assume increased, and perhaps
critical, importance.
Few are now naive enough to believe that early justification put forth for
the space program which suggested that there might be ore deposits on the
Moon that could supplement our dwindling terrestrial supplies. Nevertheless,
XVI Introductory Remarks
there is the possibility of material on the Moon that would be useful in
prolonged visits to its surface.
Foremost among these is water. It has been suggested that there is a perma-
frost zone near the Moon's surface where water from degassing of the interior
accumulated under such low temperatures that it would not sublime even
over geologic time.
If such a zone does exist there are problems of element distribution that must
be solved before it can be used, and not just problems of where the water is and
how to extract it. What other compounds are frozen in with the water? Will it
be fit for human consumption and/or hydroponic production of vegetables
simply on melting, or must it be purified ? Can it be purified readily enough and
in large enough quantities for a "lunar colony" ? Answers to these questions
depend largely on the abundances of other elements (compounds) in the ice.
Water could also be important, of course, if there is enough of it, to provide
hydrogen and oxygen as propellants for return trips to Earth, or for further
exploration of the solar system.
The composition of the lithic part of the crust is also important for practical
purposes as well as for theoretical considerations of the origin of the Moon
and the development of its surface features. Any usable material present on
the Moon's surface would represent a tremendous saving of energy, time and
money over having to transport it from the Earth.
If it became important to establish greenhouses on the Moon with lunar
"soil", elemental abundances and availability would be critical. Availability
could be attained by accelerated weathering at high temperatures in the
presence of water, but if critical trace elements were present in dangerously
large quantities it would pose problems of processing that would be extremely
serious, if not insuperable; if in inadequate quantities, they would have to be
transported from the Earth.
There will almost certainly not be encountered any usable concentrations
of raw materials for atomic energy on the Moon. There is a much better
chance for concentrations of other heavy metals. The grades probably will
be very much below those of terrestrial ores, but recovery from them could
still be much more economical than transporting metals from the Earth. Iron,
for example, might be recovered and fabricated by using solar energy as a
heat source and electrolytic hydrogen from lunar permafrost, or hydrous rocks
or minerals, as a reducing agent.
It is difficult to imagine that any considerable amount of structural steel
could be produced by such a process; the predominant material for con-
struction will undoubtedly be cast stone, basaltic or whatever other composi-
tion is readily available at the surface. The material can be melted in a solar
furnace and cast in such a way as to render it highly impervious to prevent
the escape of the artificial atmosphere. The process will depend on composi-
tion and melting characteristics of the rock(s).