Stable Isotope Variations in Extraterrestrial Materials

Abstract

The materials of the planets and the small bodies of the solar system contain a rich record of stable isotope variations in the light elements. As in terrestrial isotope geochemistry, this record reflects physical and chemical processes involving isotopic mixing among different reservoirs as well as fractionations arising in chemical reactions. The processes that influence the isotopic records of extraterrestrial materials range widely in environmental conditions from very high-energy events such as formation of refractory inclusions and chondrules by evaporation, condensation and melting in the solar nebula to lower temperature fluid-rock interactions in asteroids and planets. In addition, however, stable isotope cosmochemistry must consider issues that are beyond the scope of isotope geochemistry. For example, in the terrestrial sphere one may assume the existence of an isotopic reservoir that was originally homogenized during planet formation and the actual isotope compositions of the bulk earth do not need to be known in order to study the differences in stable isotope compositions that have been generated subsequently by various geochemical processes. This assumption of homogenization cannot be made for extraterrestrial samples, and in fact stable isotopes in meteorites preserve some of the most dramatic evidence for the incomplete nature of the mixing of distinct presolar materials during formation of the solar system. Such 'isotopic anomalies' are present in the isotopic distributions of H, C, N, and O on all spatial scales—from microscopic zoning in certain meteoritic minerals to the bulk compositions of asteroids and planets. Thus, some of the isotopic heterogeneities of 'primitive' solar system materials represent vestiges of primordial differences that could not be fully erased during the processing of presolar materials. In other cases, isotopic heterogeneities reflect the preservation of unique clues to processes occurring during formation of the solar system and planetary accretion, including early 'geologic activity' on planetesimals and even pre-biotic organic chemistry. Deciphering whether an isotopic signature is primordial or has been modified or even generated by solar system processes is often possible (e.g. by comparison to astronomical observations), but is sometimes fraught with ambiguity. The most extreme example of isotopic anomalies is provided by the laboratory analyses of individual preserved presolar dust grains extracted from primitive meteorites (Anders and Zinner 1993). These micron-size or smaller grains of SiC, graphite, and (less commonly analyzed) refractory oxides formed in the outflows of evolved stars and their isotopic compositions of C, N, O and other major and even trace elements quantitatively reflect the unique nucleosynthetic environment of that particular star, which may differ from average solar system compositions by one or more orders of magnitude. That such materials could have survived the journey from star, through the interstellar medium, the solar accretion disk and residence in the asteroidal parent bodies of meteorites, and finally to terrestrial laboratories is one of the most remarkable discoveries in all of twentieth century astronomy. The isotopic compositions of these particles have great importance for constraining models of stellar evolution and nuclear astrophysics, but because these materials are so rare and are distinctive by virtue of their lack of interaction with average solar materials, their study gives little indication of the nature of the most important processes that transformed primordial materials into the stuff that built the planets. Hence the subject of isotopic variations in presolar grains is beyond the scope of this chapter, and the reader is directed to Zinner (1998), Ott (1998), and Hoppe and Zinner (2000) for excellent reviews of this rapidly evolving subject.

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