Ubiquitous interstellar diamond and SiC in primitive chondrites: abundances reflect metamorphism

Abstract

THE nature and distribution of interstellar grains in meteorites reflect both the material inherited from the Sun's parent molecular cloud and processes that operated in the early Solar System. The discovery of interstellar diamond and silicon carbide (SiC) in carbonaceous chondrites^1,2 demonstrated that presolar grains were incorporated into at least some meteorites. Here I show that interstellar diamond and SiC were incorporated into all chondrite groups. Abundances rapidly go to zero with increasing metamor-phic grade (with diamond typically surviving better than SiC), suggesting that metamorphic destruction is responsible for the apparent absence of these grains in most chondrites. In unmetamor-phosed chondrites, abundances normalized to matrix content are similar for different classes (500–1,000 p.p.m. for diamond; 10–18 p.p.m. for SiC). Diamond samples from chondrites of different classes have remarkably similar noble-gas contents and isotopic compositions, although constituent diamonds may have come from many sources. SiC seems to be more diverse, in part because grains are large enough to measure individually, but average characteristics seem to be similar from meteorite to meteorite. These observations suggest that various classes of chondritic meteorites sampled the same Solar-System-wide reservoir of interstellar grains.