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Abstract:
We report high-resolution (<100 nm) Mg and Si isotope data of 12 presolar silicate grains (230–440 nm) from red giant and/or asymptotic giant branch stars that were previously identified based on their anomalous O-isotopic compositions (11 Group 1 grains and one Group 2 grain) in five primitive meteorites. The data were acquired by NanoSIMS ion imaging with the new Hyperion ion source that permits Mg and Si isotope measurements of presolar silicates with higher precision than was possible before. For a subset of five Group 1 ("category A") grains, 25Mg/24Mg and 29Si/28Si ratios correlate with the inferred initial 18O/16O ratios of their parent stars, a measure of stellar metallicity. The Mg and Si isotope data of category A grains show positive correlations in the δ 25Mg–δ 26Mg, δ 29Si–δ 30Si, and δ 25Mg–δ 29Si spaces. The correlations between O-, Mg, and Si-isotopic compositions are best explained by Galactic chemical evolution (GCE), with only minor imprints of nucleosynthetic and mixing processes in the grains' parent stars. Six Group 1 silicate ("category B") grains have close-to-normal Mg and Si isotopic compositions, possibly the result of isotope exchange in interstellar space or the meteorite parent bodies. For Si in category A grains, we find, with ~2σ significance, a slightly shallower slope in the δ 29Si–δ 30Si space for the GCE than inferred from presolar SiC mainstream grains. The 2σ upper limit on the slope for the linear trend in the δ 25Mg–δ 26Mg space of category A grains is slightly lower than the slope-1 predicted by GCE models around solar metallicity.
