Frank A. Podosek
Professor
Ph. D., University of California at Berkeley, 1969
Isotope Geochemistry
Professor Podosek and his colleagues use high-precision mass
spectrometry to study isotopic compositions and elemental abundances in
a variety of terrestrial, lunar, and meteoritic materials. His prior experimental
work has focused on noble gas mass spectrometry; more recently, he and
his colleagues have established a thermal ionization mass spectrometry
facility and associated clean laboratory for sample preparation. The principal
experimental technique is thermal ionization mass spectrometry, along
withthe associated clean-laboratory chemical processing; Podosek has also
previously employed the related techniques of noble gas mass spectrometry.
There are a few isotopes (e.g., 87Sr,
143Nd, 206Pb)
whose natural abundances change continuously because they are the daughters
of naturally-occurring long-lived radioactive isotopes (e.g.,
87Rb, 238U). and whose
natural isotopic abundances are measurably variable among different samples
due to favorable combinations of factors involving nuclear physics, geochemistry,
and instrumental technology. In a given sample, the relative abundances
of trace isotopes reflect the geochemical history of that sample, and
measurement of isotopic abundances thus allows constraints to be placed
on that history. Sometimes the constraint is on the nature of the reservoir
from which that sample was derived; sometimes it is a chronological constraint
on when the sample formed. The rate of change of these isotopes
is different in different samples because geochemical fractionations produce
samples with different parent- daughter elemental ratios. When there is
a favorable combination of geochemical factors, nuclear physics and instrumental
technology, these variations become measurable and thereby provide interesting
geochemical constraints. For some samples, the constraint is on the nature
of the reservoirs from which the samples were derived; in other cases
it is possible to determine the time at which the samples formed.
For example, Professor Podosek and his colleague Joyce Brannon
have used isotopic analyses of Sr and Pb to study the origin and evolution
of the hot brines that formed major and economically important base metal
sulfide ore deposits of the so-called Mississippi Valley Type. The initial
isotopic compositions of these elements in the ores and associated minerals
yield information on the origins of these fluids and on their fluid flow
pathways. In addition, some of the minerals have sufficiently high Rb/Sr
or U-Th/Pb ratios that it is possible to determine the time of their formation
and thereby their relation to major tectonic events.
In meteorites the roster of parent-daughter pairs which produce
interesting isotopic effects is greater than it is in terrestrial (or
lunar) rocks because many meteorites are very old and formed when some
short-lived radionuclides, extinct by the time of formation of any terrestrial
rock, were still extant (e.g., 129I, which
decays to 129Xe). Study of these systems provides
important information about the formation and earliest history of the
solar system. Moreover, some meteorites exhibit isotopic compositional
variations in elements (e.g., Cr, Ba) which are believed to be isotopically
uniform in the earth and for whose production there is no known mechanism
operating in the different isotopic compositions with which different
stars synthesize the elements, variations which were preserved throughout
all the processes involved in planetary body formation in the early solar
system. Study of these effects offers insights into conditions in the
formative processes of the solar system, as well as nucleosynthetic processes
in the stars which made most of the materials that constitute the terrestrial
planets.
Dr. Podosek is the executive editor of Geochimica
et Cosmochimica Acta, the journal of the Geochemical
Society.
Ozima M., Miura Y. N., and Podosek F. A. (2004) Orphan
radiogenic noble gases in lunar breccias: evidence for planet pollution
of the Sun? Icarus 170, 17-23.
Podosek F. A., C. A. Prombo, S. Amari, and R. S. Lewis
(2004) s-Process Sr isotopic compositions in presolar SiC from the Murchison
meteorite. The Astrophysical Journal 605, 960-965.
Kitts B. K., F. A. Podosek, R. H. Nichols Jr., J. C. Brannon,
J. Ramezani, R. L. Korotev, and B. L. Jolliff (2003) Isotopic composition
of surface-correlated chromium in Apollo 16 lunar soils. Geochimica
et Cosmochimica Acta 67, 4881-4893.
Podosek F. A. and Ozima M. (2000) The xenon age of the
Earth. In Origin of the Earth and Moon , R. Canup and K. Righter,
eds, p. 63-72, Univ. Arizona Press.
Podosek F. A., R. H. Nichols, J. C. Brannon, B. S. Meyer,
U. Ott, C. L. Jennings,
and N. Luo (2000) Potassium, stardust, and the last supernova. Acta
64, 2351-2362.
See also Department Publications
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314-935-7367 
fap@levee.wustl.edu
314-935-7361 
