Isotopic anomalies and presolar grains: Probes of nebular processes

The solar system formed by gravitational collapse of a molecular cloud core composed of gas and dust. Some of the dust survived solar system formation to be incorporated into meteorites. Known types of presolar dust include diamond, SiC, graphite, Al2O 3, SiN, hibonite, spinel, organic compounds, and silicates. The chemical properties and thermal resistance of these materials cover a large range. Thus, different degrees or styles of chemical and thermal processing in the solar nebula should produce different fractionations among the presolar materials. Compositional classes of chondrites originated through processing in the solar nebular. Volatility-based and metalsilicate fractionations are the best-known signatures of this processing. The assemblage of presolar grains in a chondrite class correlates with the degree of volatility-controlled fractionation reflected in its bulk composition. For example, CI chondrites and CM2 matrices have bulk compositions most like the solar photosphere. They also have the highest abundances of thermally labile but chemically resistant presolar graphite and the P-3 noble-gas component in diamonds, they have the highest SiC abundances and among the highest diamond abundances, and they have D-rich organics and presolar oxides. Thus, they contain the widest variety of presolar materials and the highest abundances of labile components in meteorites. In contrast, CV3 and CO3 chondrites show some of the largest refractory lithophile element enrichments. These meteorites have no detectable presolar graphite, very low P3 contents in diamonds, little Drich organics and SiC, but among the highest matrixnormalized diamond abundances. Thus, CV3s and CO3s contain among the most fractionated assemblages of presolar materials in chondrites. Other meteorites show similar correlations. These correlations have important implications. First, presolar grains experienced the same processes that produced the volatility-controlled chemical fractionations among chondrite classes. Second, most of the material that makes up chondrites did not evaporate and recondense in the solar system. Instead, chondrites formed primarily from thermally processed presolar dust, only some of which remains recognizable. Third, assumed relationships among chondrite classes should be re-evalutated.