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An in situ investigation on the origins and processing of circumstellar oxide and silicate grains in carbonaceous chondrites
Author(s) -
Zega Thomas J.,
Haenecour Pierre,
Floss Christine
Publication year - 2020
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/maps.13418
Subject(s) - chondrite , olivine , spinel , crystallite , pyroxene , interplanetary dust cloud , geology , silicate , mineralogy , materials science , meteorite , astrobiology , chemistry , solar system , physics , paleontology , metallurgy , organic chemistry
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 CO 3.0, and Dominion Range ( DOM ) 08006 CO 3.0 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.