A transmission electron microscopy study of the olivine coronas in the vaca

Introduction: Mesosiderites are breccias usually composed of approximately equal amounts of Fe-Ni metal and silicates. The latter contains mineral and lithic clasts in a fine-grained matrix [e.g., 1-3]. Based on silicate textures, mesosiderites are subdivided into four subtypes (1 to 4), which are considered to reflect an increasing degree in metamorphism [4]. Coronas in olivine, a mineral always present in scares amounts (~2 vol%), are considered the result of thermal metamorphism formed by the reaction of olivine with a mesosiderite-like matrix assemblage [4-7|. Coronas consist mainly of orthopyroxene with abundant chromite and merrillite and minor plagioclase, clinopyroxene and ilmenite [5]. Although the coronas are texturally similar, they can be divided in different mineralogical distinct zones with variable development even within the same mesosiderite. Here, a transmission electron microscopy (TEM) coupled with energy dispersive X-ray (EDX) analysis was performed to shed light on the main process under which the Vaca Muerta coronas were formed.