INTERSTELLAR EXPOSURE AGES OF LARGE PRESOLAR SiC GRAINS FROM THE MURCHISON METEORITE

We report the Li isotopic compositions of nine large, greater than 5 mu m, presolar SiC grains from the Murchison (CM) meteorite. Most of the SiC grains analyzed are isotopically similar in C and Si and morphologically distinct from other presolar SiC grains. Their Li-7/Li-6 ratios range from similar to 9.3 to the solar value (similar to 12). The Li-6 enrichments observed in the grains can only arise from galactic cosmic ray spallation off C atoms during travel through the interstellar medium (ISM) and before incorporation into the meteorite parent body. Using appropriate production rates, we calculate recoil-loss-corrected individual exposure ages of eight grains with Li-6 excesses that range from 40 Myr to about 1 Gyr. The long exposure ages (> 500 Myr) are consistent with calculations of grain survival lifetimes in the ISM, while the shorter ages (< 100 Myr) of two grains are more consistent with previous cosmogenic noble gas studies. Although the sample size is only eight grains, there appears to be little evidence for clustering of exposure ages and no obvious correlation between exposure age and grain size.