Characterization of the ungrouped pallasite Choteau

Abstract

Choteau is classified as an ungrouped pallasite due to its dissimilarity to other pallasite groups and its oxygen isotopic similarity to the acapulcoites/lodranites. This study uses oxygen isotope analyses, mineral chemistry, and trace element analyses to determine Choteaus petrogenesis and if it is related to any ungrouped pallasites or the acapulcoites/lodranites. We found that Choteau contains plagioclase, being only the second plagioclase-bearing pallasite. This suggests that Choteau formed at the core-mantle boundary of a parent body with a large core-to-mantle ratio, which would facilitate pressures low enough for plagioclase to remain stable in the mantle. When compared with other pallasites, Choteau is unique in mineral composition and oxygen isotope ratios, therefore representing its own group of pallasites. Choteau shows similarities to the acapulcoites/lodranites in terms of oxygen isotope ratios, mineral chemistry, and trace element abundances, but not in texture. Choteau could represent a pallasite from the acapulcoite/lodranite parent body.--Abstract.