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 silicon isotopic compositions of small SiC grains (0.2-2.4 mum) from the Murchison CM2 chondrite. Analyses were made on samples containing variable numbers of grains (of sizes 0.2-1 mum) and on fourteen individual grains (1.3-2.4 mum). In some cases the multiple-grain sample compositions were probably dominated by only one or two grains. Total ranges observed are: 2.9 < C-12/C-13 < 106, 92 < N-14/N-15 < 2700, -152 < deltaS-29 < 141 parts per thousand, and -216 < deltaSi-30 < 104 parts per thousand. Only a few grains show values near the range limits. Both the total ranges of carbon and nitrogen isotopic compositions, and even 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 similar to those observed for larger (>2 mum) SiC grains. Two rare components appear to be present in the smaller-size fraction (0.2-1 mum), one characterized by C-12/C-13 is similar to 12-16 and the other (not directly 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 compositions of most grains qualitatively show what is the signature of He-burning (enrichment in Si-29 and Si-30). The silicon isotopic composition of one grain, however, suggests a different process.