Olivine in Diogenites: Evidence for Multiple Diogenitic Lithologies on Vesta

Introduction: Traditionally, the diogenite meteorites have been viewed as orthopyroxenite cumulates with small amounts of olivine [1]. Several olivine-rich, harzburgite diogenites have recently been recognized and may provide insight into the occurrence of olivine on Vesta and in the HED meteorite suite [2,3]. In both the harzburgite and orthopyroxenite diogenites, olivine has been interpreted as a primary phase [1]. This study examines twelve diogenites with a range in olivine abundance from 1-18% by volume. Within the samples selected are the harzburgites, orthopyroxenites with olivine, and some orthopyroxenites with no olivine. We conclude that the occurrence of olivine in or-thopyroxenite diogenites is not primary, but instead is due to the brecciation and mixing of harzburgite. Results and Discussion: Two distinct phases of pyroxene, Mg and Fe-rich, were detected in the orthopyroxenites with oli-vine. In these samples, olivine occurs in non-brecciated contact and in close proximity to Mg-rich pyroxene. Fe/Mg distribution coefficients indicate that the Mg-rich pyroxene, not the Fe-rich, is in equilibrium with the olivine. Textural relationships and chemical evidence strongly suggest that Mg-rich pyroxene and olivine were derived from a distinct lithology, separate from the Fe-rich, olivine-free, orthopyroxenite fragments. We believe that the olivine and Mg-rich pyroxene parent lithology is represented by the two harzburgite samples selected for this study. These two samples have: entirely Mg-rich pyroxene, abundant olivine, non-brecciated textures between phases, and equilibrated oli-vine/pyroxene assemblages. The Fe-rich pyroxene and olivine-free fragments are interpreted as coming from typical, olivine-free, diogenite materials. Fractional crystallization is the likely process separating the harzburgite and orthopyroxenite lithologies on Vesta. While partial melting restites have been proposed as the origin of the harz-burgite diogenites [3], we believe that the presence of plagioclase and the range in Mg# among major phases necessitates a frac-tionated origin. Furthermore, the aforementioned Mg# variations , and ranges in REE abundances between samples [4], are likely the result of fractionation from different diogenitic plutons. While batch injection into a single magma chamber may produce compositional layering, other diogenite REE patterns strongly support multiple parent plutons as well [5]. Conclusions: Olivine occurs as a primary phase only in the harzburgite diogenites. In orthopyroxenite diogenites, minor oli-vine has been introduced by brecciation and mixing of harzbur-gite lithologies. The process separating these two diogenite lithologies on Vesta is likely fractional crystallization within different plutons.