An in situ investigation on the origins and processing of circumstellar oxide and silicate grains in carbonaceous chondrites

We report on the isotopic, chemical, and structural properties of four O-rich presolar grains identified in situ in the Adelaide ungrouped C2, LaPaZ Icefield (LAP) 031117 CO3.0, and Dominion Range (DOM) 08006 CO3.0 chondrites. All four grains have oxygen-isotopic compositions consistent with origins in the circumstellar envelopes (CSE) of low-mass O-rich stars evolved along the red-giant and asymptotic-giant branch (RGB, AGB, respectively) of stellar evolution. Transmission electron microscope (TEM) analyses, enabled by focused-ion-beam scanning electron microscope extraction, show that the grain from Adelaide is a single-crystal Mg-Al spinel, and comparison with equilibrium thermodynamic predictions constrains its condensation to 1500 K assuming a total pressure <= 10(-3) atm in its host CSE. In comparison, TEM analysis of two grains identified in the LAP 031117 chondrite exhibits different microstructures. Grain LAP-81 is composed of olivine containing a Ca-rich and a Ca-poor domain, both of which show distinct orientations, suggesting changing thermodynamic conditions in the host CSE that cannot be precisely constrained. LAP-104 contains a polycrystalline assemblage of ferromagnesian silicates similar to previous reports of nanocrystalline presolar Fe-rich silicates that formed under nonequilibrium conditions. Lastly, TEM shows that the grain extracted from DOM 08006 is a polycrystalline assemblage of Cr-bearing spinel. The grains occur in different orientations, likely reflecting mechanical assembly in their host CSE. The O-isotopic and Cr-rich compositions appear to point toward nonequilibrium condensation. The spinel is surrounded by an isotopically solar pyroxene lacking long-range atomic order and could have served as a nucleation site for its condensation in the interstellar medium or the inner solar protoplanetary disk.