INTERSTELLAR GRAINS IN METEORITES .2. SIC AND ITS NOBLE-GASES

We have analyzed He, Ne, Ar, Kr, and Xe in fourteen size fractions of interstellar SiC, isolated from the Murchison C2 chondrite. All are mixtures of a highly anomalous component bearing the isotopic signature of the astrophysical s-process and a more normal component, generally solar-like but with anomalies of up to 30% in the heavy isotopes. As these two components strikingly resemble predictions for the He-burning shells and envelopes of red giant carbon stars, it appears that the SiC grains are pristine circumstellar condensates from such stars. A number of elemental and isotopic ratios (such as Kr-80/Kr-82 and Kr-86/Kr-82) vary with grain size, suggesting that the SiC comes from carbon stars representing a range of masses, metallicities, temperatures, and neutron densities. The Ne-21-content of the SiC suggests a presolar cosmic-ray irradiation of up to 130 Ma, representing the interval between formation of the grains in a circumstellar shell and arrival in the solar system 4.6 Ga ago. Actually there is evidence that most of the Ne-21 (and Ne-22) is in less-than-or-equal-to 10% of the grains, suggesting that much of the SiC was degassed during or shortly before formation of the solar system. Thus the true cosmic-ray ages may be 7 to 18X longer. Apparently the gas-rich SiC grains predate the solar system by rat least 130 Ma and possibly up to 2000 Ma.