Stardust silicates from primitive meteorites

Primitive chondritic meteorites contain material (presolar grains(1)), at the level of a few parts per million, that predates the formation of our Solar System. Astronomical observations(2) and the chemical composition of the Sun(3) both suggest that silicates must have been the dominant solids in the protoplanetary disk from which the planets of the Solar System formed, but no presolar silicates have been identified in chondrites(4-6). Here we report the in situ discovery of presolar silicate grains 0.1 - 1 mum in size in the matrices of two primitive carbonaceous chondrites. These grains are highly enriched in O-17 (delta(17)O(SMOW) > 100 - 400%), but have solar silicon isotopic compositions within analytical uncertainties, suggesting an origin in an oxygen-rich red giant or an asymptotic giant branch star(7,8). The estimated abundance of these presolar silicates ( 3 - 30 parts per million) is higher than reported for other types of presolar grains in meteorites(1), consistent with their ubiquity in the early Solar System, but is about two orders of magnitude lower than their abundance in anhydrous interplanetary dust particles(9). This result is best explained by the destruction of silicates during high-temperature processing in the solar nebula.