SMALL SIC GRAINS AND A NITRIDE GRAIN OF CIRCUMSTELLAR ORIGIN FROM THEMURCHISON METEORITE - IMPLICATIONS FOR STELLAR EVOLUTION AND NUCLEOSYNTHESIS

We report the results of SIMS isotopic analyses of carbon, nitrogen, o xygen, and silicon made on 849 small (approximate to 1 mu m) individua l silicon carbide grains from the Murchison meteorite. The isotopic co mpositions of the major elements carbon and silicon of most grains (ma instream) are similar to those observed in larger grain studies sugges ting an AGE star origin of these grains. In contrast, the trace elemen t nitrogen shows a clear dependency on grain size. N-14/N-15 ratios in crease with decreasing grain size, suggesting different stellar source s for grains of different size. Typically observed N-14/N-15 ratios in the small grains of this study are approximate to 2700, clearly large r than the values expected from model calculations of AGE stars. In ad dition to the three dredge-up episodes characteristic for the evolutio n of AGE stars, extra-mixing of CNO-processed matter in low-mass AGB s tars appears to be a promising possibility in order to explain the hig h N-14/N-15 ratios of the small circumstellar SiC grains. A small frac tion of grains shows a silicon isotopic signature not observed in larg er circumstellar SiC grains from Murchison. Their stellar origin is st ill uncertain. The minor type A, B, Y, and X grains were found to be p resent at a level of a percent, which is similar to their abundance in the larger-grain SiC separates from Murchison. Oxygen isotopic compos itions are normal within the experimental uncertainties of several 10% , indicating that oxygen of stellar origin is rare or even absent in t he SiC grains. We conclude that most of the oxygen is a contaminant wh ich was introduced into the SiC grains after their formation, e.g., du ring sample processing in the laboratoy. We identified a nitride grain , most likely Si,N,with little carbon, with highly anomalous isotopic compositions (C-12/C-13 = 157+/-33, N-14/N-15 = 18+/-1, delta(29)Si = -43+/-56 parts per thousand, delta(30)Si = -271+/-50 parts per thousan d). The isotopic patterns of carbon, nitrogen, and silicon resemble th ose of the rare SiC X grains suggesting that these two rare constituen ts of circumstellar matter formed in the same type of stellar source, namely, Type II supernovae.