Isotope Systematics of Presolar Silicate Grains: New Insights from Magnesium and Silicon

We report on Mg and Si isotope data of 86 presolar silicate grains identified through NanoSIMS oxygen ion imaging in thin sections of carbonaceous and ordinary chondrites. The O, Mg, and Si isotope data of 106 presolar silicates (including grains studied previously by our group) suggest division of O isotope Group 1 grains into four subpopulations: (i) "normal," (ii) ²⁵Mg-rich, (iii) ²⁶Mg-rich, and (iv) ²⁵Mg-poor. Normal Group 1 grains (∼60% of Group 1 grains) formed in the winds of low-mass asymptotic giant branch (AGB) stars, with Mg and Si defining linear arrays with slopes of ∼0.9 and 1.37, respectively, in three-isotope representations, most likely representing Galactic chemical evolution (GCE). The ²⁵Mg-rich grains (∼25%) show enrichments in ²⁵Mg of up to a factor 2.4 relative to solar composition and most likely formed in supernova (SN) ejecta or the winds of intermediate-mass AGB stars. The ²⁶Mg-rich and ²⁵Mg-poor Group 1 grains lie below the Mg GCE line and their isotopic compositions favor origins from supergiants or SNe. The O isotope Group 2 grains show a wide range of Mg-isotopic compositions, similar to Group 1 grains, with likely origins from massive AGB stars, super-AGB stars, supergiants, and SNe. The Mg- and Si-isotopic compositions of Group 4 grains are compatible with previously proposed SN origins. Our results suggest that >30% of presolar silicates formed in the winds of supergiants and in SN ejecta, and that low-mass AGB stars appear to have contributed only some 50% to presolar silicates, less than previously thought.