Correlated Si isotope anomalies and large ^(13)C enrichments in a family of exotic SiC grains

Abstract

A suite of morphologically distinctive silicon carbide (SiC) grains from the Orgueil and Murchison carbonaceous chondrite meteorites contains Si and C of highly anomalous isotopic composition. All of the SiC grains in this suite are characterized by a distinctive platy morphology and roughly developed hexagonal crystal forms that allow them to be distinguished from other types of SiC found in the host meteorites. The δ^(29)Si and δ^(30)Si values of individual SiC crystals deviate from those of normal solar material by more than 100‰, while the δ^(13)C values range from 150 to 5200‰. Isotopically normal C and Si are not found in any of these SiC crystals. The SiC grains belonging to this morphological suite are isotopically distinct from fine-grained SiC aggregates and other morphological types of SiC in unequilibrated meteorites. The ^(29)Si/^(28)Si and ^(30)Si/^(28)Si ratios of these platy grains are well correlated and define a linear array that does not pass through the composition of normal, solar Si. This behavior contrasts sharply with the diverse and poorly correlated Si isotopic compositions shown by the total SiC population. We suggest that the distinctive morphological characteristics and comparatively simple Si isotope systematics identify the platy SiC crystals as a genetically related family, formed around a single, isotopically heterogeneous presolar star or an association of related stars. The enrichments in ^(13)C and the Si isotope systematics of the platy SiC are broadly consistent with theoretical models of nucleosynthesis in low-mass, carbon stars on the asymptotic giant branch. The Si isotope array most plausibly reflects mixing between ^(28)Si-rich material, inherited from a previous generation of stars, and material enriched in ^(29)Si and ^(30)Si, produced in intershell regions by neutron capture during He-burning. ^(13)C is also produced in intershell regions by proton reactions on ^(12)C seed nuclei and is carried withs-process nuclei to the stellar envelope by convection which penetrates down to the He shell. The absence of a correlation between the Si and C isotopic compositions of the SiC suggests either episodic condensation of SiC, extending over several thermal pulses, in the atmosphere of a single star, or derivation of the SiC from several stars characterized by different rates of ^(13)C production. In the multiple star scenario, the linear correlation of the ^(29)Si/^(28)Si and ^(30)Si/^(28)Si ratios among the platy SiC indicates that these stars evolved from a common Si seed composition under similar conditions of neutron-capture nucleosynthesis. The ^(29)Si/^(30)Si ratio of the SiC, inferred by us to be produced by neutron capture in the stellar interior, is distinct from values calculated from models of nucleosynthesis in AGB stars.

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