CARBON, NITROGEN, MAGNESIUM, SILICON, AND TITANIUM ISOTOPIC COMPOSITIONS OF SINGLE INTERSTELLAR SILICON-CARBIDE GRAINS FROM THE MURCHISON CARBONACEOUS CHONDRITE

Seven hundred and twenty SiC grains from the Murchison CM2 chondrite, ranging in size from 1 to 10 mum were analyzed by ion microprobe mass spectrometry for their C-isotopic comPositions. Subsets of the grains were also analyzed for N (450 grains), Si (183 grains), Mg (179 grains ), and Ti (28 grains) isotopes. These results are compared with previo us measurements on 41 larger SiC grains (up to 15 x 26 mum) from a dif ferent sample of Murchison analyzed by Virag et al. (1992) and Ireland , Zinner, & Amari (1991a). All grains of the present study are isotopi cally anomalous with C-12/C-13 ratios ranging from 0.022 to 28.4 x sol ar, N-14/N-15 ratios from 0.046 to 30 x solar, Si-29/Si-28 from 0.54 t o 1.20 x solar, Si-30/Si-28 from 0.42 to 1.14 x solar, Ti-49/Ti-48 fro m 0.96 to 1.95 x solar, and Ti-50/Ti-48 from 0.94 to 1.39 x solar. Man y grains have large Mg-26 excesses from the decay of Al-26 with inferr ed Al-26/Al-27 ratios ranging up to 0.61, or 12,200 x the ratio of 5 x 10(-5) inferred for the early solar system. Several groups can be dis tinguished among the SiC grains. Most of the grains have C-13 and N-14 excesses, and their Si isotopic compositions' (mostly excesses in Si- 29 and Si-30) plot close to a slope 1.34 line on a deltaSi-29/Si-28 Ve rSUS deltaSi-30/Si-28 three-isotope plot. Grains with small C-12/C-13 ratios (< 10) tend to have smaller or no N-14 excesses and high Al-26/ Al-27 ratios (up to 0.01). Grains with C-12/C-13 > 150 fall into two g roups: grains X have N-15 excesses and high Si-29 and Si-30 deficits a nd the highest (0.1 to 0.6) Al-26/Al-27 ratios; grains Y have N-14 exc esses and plot on a slope 0.35 line on a Si three-isotope plot. In add ition, large SiC grains of the Virag et al. (1992) study fall into thr ee-distinct clusters according to their C-, Si-, and Ti-isotopic compo sitions. The isotopic diversity of the grains and the clustering of th eir isotopic compositions imply distinct and multiple stellar sources. The C- and N-isotopic compositions of most grains are consistent with H-burning in the CNO cycle. These and s-process Kr, Xe, Ba, and Nd su ggest asymptotic giant branch (AGB) or Wolf-Rayet stars as likely sour ces for the grains, but existing models of nucleosynthesis in these st ellar sites fail to account in detail for all the observed isotopic co mpositions. Special problems are posed by grains with C-12/C-13 < 10 a nd almost normal and heavy N-isotopic compositions. Also the Si- and T i-isotopic compositions, with excesses in Si-29 and Si-30 relative to Si-28 and excesses in all Ti isotopes relative to Ti-48, do not precis ely conform with the compositions predicted for slow neutron capture. Additional theoretical efforts are needed to achieve an understanding of the isotopic composition of the SiC grains and their stellar source s.