FINGERPRINTS OF CARBON, NITROGEN, AND SILICON ISOTOPES IN SMALL INTERSTELLAR SIC GRAINS FROM THE MURCHISON METEORITE

We report ion microprobe determinations of the carbon, nitrogen, and s ilicon isotopic compositions of small SiC grains (0.2-2.4 mum) from th e Murchison CM2 chondrite. Analyses were made on samples containing va riable numbers of grains (of sizes 0.2-1 mum) and on fourteen individu al grains (1.3-2.4 mum). In some cases the multiple-grain sample compo sitions were probably dominated by only one or two grains. Total range s observed are: 2.9 < C-12/C-13 < 106, 92 < N-14/N-15 < 2700, -152 < d eltaS-29 < 141 parts per thousand, and -216 < deltaSi-30 < 104 parts p er thousand. Only a few grains show values near the range limits. Both the total ranges of carbon and nitrogen isotopic compositions, and ev en the narrower ranges typical for the majority of the grains (C-12/C- 13 is similar to 40-100, N-14/N-15 is similar to 400-2000), are simila r to those observed for larger (>2 mum) SiC grains. Two rare component s appear to be present in the smaller-size fraction (0.2-1 mum), one c haracterized by C-12/C-13 is similar to 12-16 and the other (not direc tly observed) by very heavy nitrogen (N-14/N-15 < 100). The carbon and nitrogen isotopic compositions qualitatively may reflect hydrostatic H-burning via the CNO cycle and He-burning in red giants (most grains) , as well as explosive H-burning in novae. The silicon isotopic compos itions of most grains qualitatively show what is the signature of He-b urning (enrichment in Si-29 and Si-30). The silicon isotopic compositi on of one grain, however, suggests a different process.