Robert N. Clayton, Enrico Fermi Distinguished Service Professor Emeritus, Departments of Chemistry and the Geophysical Sciences, Enrico Fermi Institute, and the College

Abstracts

Oxygen Isotope Studies of Achondrites
Geochimica et Cosmochimica Acta 60, 1999-2017, 1996

Robert N. Clayton and Toshiko K. Mayeda

Abstract
Oxygen isotope abundances provide a powerful tool for recognizing genetic relationships among meteorites. Among the differentiated achondrites, three isotopic groups are recognized: (1) SNC (Mars), (2) Earth and Moon, (3) HED (howardites, eucrites, diogenites). The HED group also contains the mesosiderites, main-group pallasites, and silicates from IIIAB irons. The angrites may be marginally resolvable from the HED group. Within each of these groups, internal geologic processes give rise to isotopic variations along a slope-1/2 fractionation line, as is well known for terrestrial materials. Variations of from one planet to another are inherited from the inhomogeneities in the solar nebula, as illustrated by the isotopic compositions of chondrites and their constituents. Among the undifferentiated achondrites, five isotopic groups are found: (1) aubrites, (2) winonaites and IAB-IIICD irons, (3) brachinites, (4) acapulcoites and lodranites, and (5) ureilites. The isotopic compositions of aubrites coincide with the Earth and Moon, and also with the enstatite chondrites. These bodies apparently were derived from a common reservoir, the isotopic composition of which was established at the chondrule scale by nebular processes. Isotopic similarities between chondrites and achondrites are seen only for the following instances: (1) enstatite chondrites and aubrites, (2) H chondrites and IIE irons, and (3) L or LL chondrites and IVA irons. The isotopic data also support the following genetic associations: (1) winonaites and IAB-IIICD irons, (2) acapulcoites and lodranites, and (3) ureilites and dark inclusions of C3 chondrites. An attempt to reconcile the whole-planet isotopic compositions of Earth, Mars, and the eucrite parent body with mixing models of their chemical compositions failed. It is not possible to satisfy both the chemical and isotopic compositions of the terrestrial planets using known primitive Solar System components.

s-Process Zirconium in Presolar Silicon Carbide Grains
Science 277, 1281-1283, 1997

Gunther K. Nicolussi, Andrew M. Davis, Michael J. Pellin, Roy S. Lewis, Robert N. Clayton, and Sachiko Amari

Abstract
The isotopic composition of zirconium in silicon carbide grains from the Murchison meteorite was measured by resonant ionization mass spectrometry of laser-ablated neutral atoms. These grains are condensates from the atmospheres of red giant stars that existed before the formation of our sun and solar system, and they contain records of nucleosynthesis in these stars. The r-process-dominated isotope zirconium-96 was depleted by more than a factor of 2 compared with the s-process-dominated isotopes zirconium-90, zirconium-91, zirconium-92, and zirconium-94, in agreement with expectations for neutron capture nucleosynthesis in asymptotic giant branch stars.

Molybdenum Isotopic Composition of Individual Presolar Silicon Carbide Grains from the Murchison Meteorite
Geochimica et Cosmochimica Acta 62, 1093-1104, 1998

G. K. Nicolussi, M. J. Pellin, R. S. Lewis, A. M. Davis, S. Amari, and R. N. Clayton

Abstract
We report the isotopic composition of molybdenum in twenty-three presolar SiC grains from the Murchison meteorite which have been measured by resonant ionization mass spectrometry (RIMS). Relative to terrestrial abundance (and normalized to s-process-only ), the majority of the analyzed grains show strong depletions in the p-process isotopes and and r-process isotope . Sixteen of these grains have -values < -600% for these three isotopes. The observed isotopic patterns of Mo from mainstream SiC grains clearly reveal the signature of s-process nucleosynthesis. Three-isotope plots of all grain data ( vs. ) show strong linear correlations with characteristic slopes. This finding suggests mixing of solar-like material and pure s-process material in the parent stars. Comparison with evolutionary calculations of nucleosynthesis and mixing in red giants suggests that low-mass thermally-pulsed symptotic giant branch (TP-AGB) stars are the most likely site for the observed s-process nucleosynthesis.

Oxygen Isotope Studies of Carbonaceous Chondrites
Geochimica et Cosmochimica Acta 63, 2089-2104, 1999

Robert N. Clayton and Toshiko K. Mayeda

Abstract
The carbonaceous chondrites display the widest range of oxygen isotopic composition of any meteorite group, as a consequence of the interaction of primordial isotopic reservoirs in the solar nebula. These isotopic variations can be used to identify the reservoirs and to determine conditions and loci of their interactions. We present a comprehensive set of whole-rock analyses of CV, CO, CK, CM, CR, CH, and CI chondrites, as well as selected components of some of these meteorites. A simple model is developed which describes the isotopic behavior during parent-body aqueous alteration processes. The process of thermal dehydration also produces a recognizable effect in the oxygen isotopic composition.

Evaporation of Single Crystal Forsterite: Evaporation Kinetics, Magnesium Isotope Fractionation, and Implications of Mass-Dependent Isotopic Fractionation of a Diffusion-Controlled Reservoir
Geochimica et Cosmochimica Acta 63, 953-966, 1999

Jianhua Wang, Andrew M. Davis, Robert N. Clayton, and Akihiko Hashimoto

Abstract
Single crystals of forsterite were evaporated in a vacuum furnace at temperatures of 1500-1800°C to study evaporation kinetics, magnesium isotopic fractionation, and magnesium diffusion in forsterite. The evaporation of single crystal forsterite revealed that the evaporation process is kinetically hindered, in agreement with the results of Hashimoto (1990) on polycrystalline forsterite. The activation energy of forsterite evaporation obtained from this study is 628 kJ/mole. Forsterite can thus be much more refractory at low temperatures than expected from thermodynamic predictions.

The evaporation of solid forsterite supports a model of isotopic fractionation under diffusion-controlled conditions such that isotopic fractionation during the evaporation process is restricted to the vicinity of the evaporating surface. The measured solid-gas fractionation factor of is smaller than the theoretical prediction, suggesting more complicated gas speciation than a monatomic Mg gas. Diffusion coefficients of forsterite at high temperature (1500-1800°C) were obtained based on measurement of isotopic profiles in the evaporation residues. Mg diffusion in forsterite along its crystallographic a-axis has a very high activation energy (608 kJ/mole).