Olivinites in the angrite D'Orbigny: Vestiges of pristine reducing conditions during angrite formation

Abstract

Olivinites, together with olivine megacrysts, are the most magnesian phases found in angrites. Their chemical composition (mg# 90) is out of equilibrium with the groundmass and far away from that of possible precipitates from angrite parent melts. Therefore olivinites, as well as olivine megacrysts, were considered as xenoliths and xenocrysts. We report here a detailed study of five olivinites from the angrite D'Orbigny. Our results indicate that D'Orbigny experienced metasomatic alteration processes, which led to enrichments in FeO and MnO (relative to the original composition), changing the initial Mg-rich composition of the olivines to the one seen now. As this process took place in equilibrium with a chondritic reservoir (e.g., Fe/Mn ratios spreading around primitive values), the primitive (Mg-rich) olivine chemical composition was changed towards a more fayalitic one while preserving a chondritic signature. This chondritic signature was preserved in the Fe/Mn ratio of the olivinites, olivine megacrysts, augite grains in olivinites and groundmass olivine of D'Orbigny. Therefore the fayalite content of about 35 mol.% that characterizes the groundmass olivine of this rock – as well as other angrites-does not correspond to its original composition but may be the result of a late metasomatic process that affected these rocks. If so, olivinites and Mg-rich olivines might not be compositionally exotic phases but are an early constituent phase that retained the pristine more reducing conditions that have been preserved in some angrites, where they form either a small part of the rock (e.g., Asuka 881371 and D'Orbigny) or the majority of it (NWA 8535).