Coordinated chemical and microstructural analyses of presolar silicate grains from AGB/RGB stars and supernovae in the CO3.0 chondrite Dominion Range 08006

Abstract

We report the structural and chemical analyses of six presolar silicate grains identified in situ in the CO3.0 carbonaceous chondrite Dominion Range (DOM) 08006. Two of the grains have O-isotopic compositions consistent with origins in the circumstellar envelopes of low-mass (<2M☉) asymptotic giant branch (AGB)/red giant branch (RGB) stars, although without Mg-isotopic data, origins in supernovae (SNe) cannot be ruled out. The other four grains have O-isotopic compositions consistent with origins in the ejecta of type-II SNe. Transmission electron microscopy analyses reveal that all grains are crystalline (single crystal or polycrystalline) and have varied compositions. The analyzed AGB/RGB grains include an Fe-rich crystalline olivine with an Fe-sulfide inclusion and a chemically zoned olivine grain that also contains an Fe-oxide rim. The grains derived from SNe include two polycrystalline assemblages with structures that overlap with both olivine and pyroxene, an assemblage composed of both a single crystal of forsterite and polycrystalline forsterite, and an orthopyroxene grain with an embedded Fe-sulfide crystal. The thermodynamic origins of both AGB/RGB and SN grains are also diverse. The structure and compositions of two grains are consistent with equilibrium thermodynamic predictions of condensation, whereas four are not, suggesting formation through nonequilibrium or multistep processes. Our observations of presolar silicate grains suggest that the circumstellar envelopes of AGB/RGB stars and the ejecta of SNe can produce grains with comparable structures and compositions.