Presolar silicate grains: Abundances, isotopic and elemental compositions, and the effects of secondary processing

Close to a thousand presolar silicate grains have been identified since their initial discovery in interplanetary dust particles (IDPs) just over ten years ago. Studies have shown that silicates are the most abundant type of presolar grain other than nanodiamonds, with abundances of similar to 200 ppm in the most primitive meteorites and upwards of similar to 400 ppm in anhydrous IDPs. The oxygen isotopic compositions of presolar silicates are similar to those of presolar oxides, with the majority of the grains originating in low-mass red giant or asymptotic giant branch stars of close-to-solar metallicity. The vast majority of the grains are ferromagnesian silicates with high Fe concentrations. This, together with TEM studies indicating that many presolar silicates have amorphous structures with heterogeneous and non-stoichiometric compositions, suggests that conditions in the stellar environments in which these grains formed were variable and rapidly changing, with grain condensation under non-equilibrium kinetic conditions. Presolar silicates also reflect secondary processes taking place in the solar nebula and the parent bodies of the meteorites in which they are found. Abundance variations within individual meteorites provide constraints on secondary processes, and both thermal metamorphism and aqueous alteration result in changes to the elemental compositions of the grains. Studies of presolar silicates complement those of other presolar grain types, providing additional constraints on stellar environments and nucleosynthetic processes.