An investigation into the origin of Fe-rich presolar silicates in ACFER 094

Presolar silicate and oxide grains from primitive meteorites are recognized as "stardust" on the basis of their extremely anomalous O isotopic compositions. We report data on 48 O-anomalous grains that were identified in grain size separates of the ungrouped carbonaceous chondrite Acfer 094. A majority of these grains exhibit high ¹⁷O/¹⁶O isotopic ratios along with solar to sub-solar ¹⁸O/¹⁶O ratios and may have originated in low-mass stars with close-to-solar metallicity. Four silicate grains that contain ¹⁸O enrichments were also measured for their Si isotopes. A comparison of their O and Si isotopic compositions with model predictions indicates that these ¹⁸O-rich grains may have formed in supernova ejecta. Four of the O-anomalous grains are oxides while the remaining 44 are silicates, based on elemental compositions determined by Auger spectroscopy. The presolar oxides include a TiO₂ grain and a grain with spinel stoichiometry. The silicate grains largely exhibit ferromagnesian compositions, although a few grains also contain small amounts of Ca and/or Al. Stoichiometric silicates were further classified as either olivine-like or pyroxene-like, and in this study pyroxene-like grains are more abundant than olivine-like ones. The majority of silicates contain more Fe than Mg, including a few grains with Fe-rich end-member compositions. Spectroscopic observations indicate the presence of Mg-rich silicates in the atmospheres of stars and the interstellar medium. Mg-rich minerals such as forsterite and enstatite form by equilibrium condensation in stellar environments. However, non-equilibrium condensation can result in higher Fe contents and the occurrence of such processes in the outflows of stars may account for the Fe-rich grains. Alternatively, secondary processes may play a role in producing the Fe enrichments observed in the presolar silicate grains identified in the matrix of Acfer 094.