Interstellar exposure ages of large presolar sic grains from the murchison meteorite

We report the Li isotopic compositions of nine large, greater than 5 m, presolar SiC grains from the Murchison(CM) meteorite. Most of the SiC grains analyzed are isotopically similar in C and Si and morphologicallydistinct from other presolar SiC grains. Their 7Li/6Li ratios range from ̃9.3 to the solar value (̃12). The6Li enrichments observed in the grains can only arise from galactic cosmic ray spallation off C atoms during travelthrough the interstellar medium (ISM) and before incorporation into the meteorite parent body. Using appropriateproduction rates, we calculate recoil-loss-corrected individual exposure ages of eight grains with 6Li excessesthat range from 40 Myr to about 1 Gyr. The long exposure ages (>500 Myr) are consistent with calculationsof grain survival lifetimes in the ISM, while the shorter ages (<100 Myr) of two grains are more consistentwith previous cosmogenic noble gas studies. Although the sample size is only eight grains, there appears to belittle evidence for clustering of exposure ages and no obvious correlation between exposure age and grain size.