Microstructural and Chemical Investigations of Presolar Silicates from Diverse Stellar Environments

Abstract

We report the structural and chemical investigation of nine presolar silicate grains from the CH3/CB(b)3 chondrite Isheyevo and CR2 chondrite Northwest Africa (NWA) 801. Five of these grains belong to group 1, likely condensed in low- to intermediate-mass asymptotic giant branch (AGB) stars, super-AGB stars, or core-collapse supernovae, while the remaining four grains belong to group 4 and have a supernova origin. The advanced transmission electron microscopy and associated electron spectroscopy analyses show a diverse range of chemical and structural compositions for presolar silicates. Two GEMS (glass with embedded metal and sulfide)-like silicates, each from different groups, condensed under nonequilibrium conditions in stellar outflows. Two nonstoichiometric silicates from group 1 have dissimilar formation and alteration histories. An amorphous silicate from group 1 with olivine-like [(Mg,Fe)(2)SiO4] composition likely formed as a crystalline olivine that subsequently amorphized in the interstellar medium. An oldhamite (CaS) grain within a stoichiometric enstatite (MgSiO3) from group 1 probably formed by heterogeneous condensation in circumstellar outflows. Of the two crystalline grains from group 4, one is an antigorite [(Mg,Fe)(3)Si2O5(OH)(4)], while the other is a nontronite [Na,Fe-2(Si,Al)(4)O-10(OH)(2).nH(2)O], both formed as a crystalline forsterite and later altered to have hydrated silicate composition. A group-4 silicate has a chemical composition similar to a low Ca-pyroxene [(Ca,Mg)(Si,Al)(2)O-6]. Our data imply that presolar grains from different groups can have a similar range of grain-formation conditions. ; We thank Vikram Goyal at Physical Research Laboratory (PRL), India for his assistance on the NanoSIMS; Daniel Wielandt and Lalit Shukla for initial NanoSIMS measurements at PRL; Takeshi Kasama and Berit Wenzell at DTU-CEN, Copenhagen, Keulen Nynke and Alaei Mojagan at GEUS, Copenhagen, Deepak Panda at PRL, Yoshiyuki Iizuka at Academia Sinica, Taipei, and Zina Fihl at SNM, Copenhagen for help with the SEM. We also thank Larry Nittler at the Carnegie Institution for Science for help on the L'image software and interpretation of some of the grain data. Funding for this project was provided by the Carlsberg Foundation (CF18-1105), the Danish National Research Foundation (DNRF97), and the European Research Council (ERC Advanced grant Agreement, #833275-DEEPTIME) grants to M.B. The authors acknowledge the use of instrumentation provided by the National Facility ELECMI ICTS ("Division de Microscopia Electronica," Universidad de Cadiz, DME-UCA). L.L. acknowledges funding from the Andalusian regional government (FEDER-UCA-18-106613), the European Union's Horizon 2020 research and innovation program (grant agreement 823717-ESTEEM3), and the Spanish Ministerio de Economia y Competitividad (PID2019-107578GA-I00).