Geochemistry and genesis of the angrites

Abstract

The angrites Angra dos Reis, LEW86010, and LEW87051 are petrologically and compositionally similar achondrites. All angrites have high FeO MnO ratios of 80–94 and very low CI normalized Na/Sm ratios of 0.001–0.003. High abundances of oxidized Fe and low abundances of moderately volatile Na most likely resulted from parent body processes, such as magmatic outgassing, rather than nebular processes. All angrites have fractionated Ca/Al ratios, with Angra dos Reis exhibiting the most extreme ratio (3.1 × CI). This Ca/Al fractionation may be due to small amounts of residual spinel in the source region after removal of the angrite melts or to crystallization of spinel from precursor melts. For LEW86010 and LEW87051, the fractionated Ca/Al ratios are not caused by the presence of significant cumulus material. For Angra dos Reis, cumulus material may be the cause of the high Ca/Al ratio. Refractory element (Ca, Eu, Al, Sc, La, Ce, Sm, Tb, Yb, Lu, Ti, Hf, Th, and Ta) abundances of LEW86010 and LEW87051 show similar patterns, while Angra dos Reis has both greater enrichments in these elements and more fractionated patterns. Compositional and petrologic constraints indicate that LEW86010 and LEW87051 are related via olivine control. The refractory element abundances and mg# of LEW86010 can be approximated by removal of olivine from LEW87051, suggesting that LEW86010 may be a residual melt from a LEW87051-like precursor. Alternatively, LEW87051 may have formed via olivine accumulation from a LEW86010-like precursor. The origin of the unusual refractory element pattern in Angra dos Reis remains obscure. The differences between the LEW86010-LEW87051 duo and Angra dos Reis suggest that either the angrite parent body was heterogeneous or that Angra dos Reis was formed on a separate parent body. Based on FeO MnO ratios and normative mineralogies, the angrite parent body(ies) may be similar in bulk composition to one of the carbonaceous chondrite groups, particularly CI-CM-CO.