ISOTOPIC, OPTICAL, AND TRACE-ELEMENT PROPERTIES OF LARGE SINGLE SIC GRAINS FROM THE MURCHISON METEORITE

Forty-one large SiC grains from the Murchison CM2 chondrite, ranging up to 15 x 26-mu-m, were analyzed by ion probe mass spectrometry for the isotopic compositions of C, N, Mg, and Si, and the concentrations of Al, Ti, V, Fe, Zr and Ba. Most grains were also examined by Raman spectroscopy. The majority have large isotopic anomalies, with C-13/C-12 and N-14/N-15 up to 30x and 9x solar, and Si-29,Si-30 enriched by up to 102 parts per thousand. Only two grains, characterized by extremely heavy carbon (delta-C-13 = 28,582 and 18,883 parts per thousand) give evidence for fossil Mg-26, with (Al-26/Al-27)0 ratios of 2.1 x 10(-3) and 3.9 x 10(-3). On the basis of C and Si isotopic composition, twenty-nine of the grains fall into three compact clusters, presumably from three discrete sources. Two of these clusters are anomalous and comprise only grains of cubic structure (according to their Raman spectra). The third, isotopically, normal cluster contains only anhedral, noncubic grains; and although contamination cannot be categorically excluded, an origin in a reducing environment in the early solar system is a viable possibility. The reality of these clusters is further supported by differences in morphology, size, N-content, and Al/N. This clustering of coarse-grained (> 6-mu-m) SiC stands in sharp contrast to the quasicontinuous distribution of finer grained SiC and suggests that the top approximately 0.1% of the mass distribution is a distinct population. A few conclusions can be reached about the astrophysical origin of the coarse-grained SiC. The C and N isotopic compositions of the anomalous grains are not very diagnostic, being consistent with H-burning in the CNO cycle. The very existence of SiC requires C-rich stars, of C/O > 1. The Si-isotopic compositions qualitatively show the signature of neutron capture in He-burning shells of highly evolved stars, narrowing the choice to asymptotic giant branch (AGB) or Wolf-Rayet stars. AGB stars are the more likely candidates, as only they can (during their final, planetary nebula phase) provide high mass loss rates and hence the high gas densities required for growth of large SiC grains.