Silicon, carbon, and nitrogen isotopic studies of silicon carbide in carbonaceous and enstatite chondrites

Abstract

Both carbonaceous and enstatite chondrites contain complex populations of silicon carbide (SiC) grains. SiC in carbonaceous chondrites contains highly anomalous Si, C, and N. Individual SiC grains and aggregates of sub-micron SiC analysed in this study show variations in δ²⁹Si and δ³⁰Si exceeding 120 per mil, δ¹³C values ranging from -300 to +24,500, and δ¹⁵N values between -390 and -960. This range of isotopic compositions suggests that the total SiC population consists of families of grains formed in a variety of stellar sites (ZINNER et al., 1989; TANG et al., 1989). One such family, common to Orgueil and Murchison, and distinguished by a platy surface morphology, contains consistently anomalous Si defining a binary mixing array in the Si three-isotope diagram. On formation, these grains incorporated varying proportions of two exotic Si components, one enriched in ²⁸Si, the other in ²⁹Si and ³⁰Si relative to normal Si. Grains belonging to this family are consistently enriched in ¹³C and ¹⁴N relative to the solar isotopic compositions of these elements. The most plausible source for these grains is the circumstellar envelope of a low-mass red giant star on the Asymptotic Giant Branch (AGB)(GALLINO et al., 1990). In this case, the ²⁸Si-rich component represents the seed composition preserved in the stellar envelope, to which was added ²⁹Si and ³⁰Si produced by neutron capture during He-burning. Enrichments in ¹³C and ¹⁴N are attributable to proton capture following convective dredge-down of H into the top of the He layer. The poor correlation of C and N with Si isotopes in the platy SiC grains suggests condensation over a number of convective mixing episodes, which enriched the stellar envelope in ¹³C and ¹⁴N to differing degrees. Silicon carbide grains in Indarch differ in size and surface texture from those found in the carbonaceous chondrites. None of the Indarch grains analysed in this study contains isotopically anomalous Si or C, an observation difficult to reconcile with high concentration of the ²²Ne-rich component, Ne-E(H), found in Indarch acid residues (Huss, 1990). The grains analysed in this study either belong to a SiC family distinct from that presumed to host Ne-E(H), or formed in a region characterised by isotopically normal Si and C but anomalous Ne. In the former case, a solar-system origin for the grains analysed in this work cannot be ruled out.