Nucleosynthetic osmium isotope anomalies in acid leachates of the Murchison meteorite

Abstract

We present osmium isotopic results obtained by sequential leaching of the Murchison meteorite, which reveal the existence of very large internal anomalies of nucleosynthetic origin (ε¹⁸⁴Os from - 108 to 460; ε¹⁸⁶Os from - 14.1 to 12.6; ε¹⁸⁸Os from - 2.6 to 1.6; ε¹⁹⁰Os from - 1.7 to 1.1). Despite these large variations, the isotopic composition of the total leachable osmium (weighted average of the leachates) is close to that of bulk chondrites. This is consistent with efficient large-scale mixing of Os isotopic anomalies in the protosolar nebula. The Os isotopic anomalies are correlated, and can be explained by the variable contributions of components derived from the s, r and p-processes of nucleosynthesis. Surprisingly, much of the s-process rich osmium is released by relatively mild leaching, suggesting the existence of an easily leachable s-process rich presolar phase, or alternatively, of a chemically resistant r-process rich phase. Taken together with previous evidence for a highly insoluble s-process rich carrier, such as SiC, these results argue for the presence of several presolar phases with anomalous nucleosynthetic compositions in the Murchison meteorite. The s-process composition of Os released by mild leaching diverges slightly from that released by aggressive digestion techniques, perhaps suggesting that the presolar phases attacked by these differing procedures condensed in different stellar environments. The correlation between ε¹⁹⁰Os and ε¹⁸⁸Os can be used to constrain the s-process ¹⁹⁰Os/¹⁸⁸Os ratio to be 1.275 ± 0.043. Such a ratio can be reproduced in a nuclear reaction network for a MACS value for ¹⁹⁰Os of ~ 200 ± 22 mbarn at 30 keV. More generally, these results can help refine predictions of the s-process in the Os mass region, which can be used in turn to constrain the amount of cosmoradiogenic ¹⁸⁷Os in the solar system and hence the age of the Galaxy. We also present evidence for extensive internal variation of ¹⁸⁴Os abundances in the Murchison meteorite. A steep anti-correlation is observed between ε¹⁸⁴Os and ε¹⁸⁸Os. Since ¹⁸⁴Os is formed uniquely by the p-process, this anti-correlation cannot be explained by variable addition or subtraction of s-process Os to average solar system material. Instead, this suggests that p-process rich presolar grains (e.g., supernova condensates) may be present in meteorites in sufficient quantities to influence the Os isotopic compositions of the leachates. Nevertheless, ¹⁸⁴Os is a low abundance isotope and we cannot exclude the possibility that the measured anomalies for this isotope reflect unappreciated analytical artifacts. © 2008 Elsevier B.V. All rights reserved.