Petrology and Geochemistry of Olivine-Bearing Diogenites and a Group of Paired Howardites

Asteroid 4 Vesta, the largest differentiated body in the asteroid belt, is a protoplanet, much like those that accreted to form the Earth. Understanding the geology of Vesta furthers understanding of early differentiation processes that occurred on Earth and helps define igneous processes occurring on other differentiated bodies in the early solar system. Howardite, eucrite and diogenite (HED) meteorites, which are thought to have originated from Vesta, can be analyzed to better understand the geology of that asteroid. Here my colleagues and I investigate the petrology and geochemistry of two groups of HEDs. This work is timely, in that the insights gained from these studies can be used to interpret data from the Dawn spacecraft, which has just been placed into orbit around Vesta. In the first four parts of this dissertation we investigate the origin of olivine in diogenites, which are ultramafic cumulates from Vesta. We discover that the majority of these samples are dimict (two-component) breccias, composed of harzburgitic and orthopyroxenitic lithologies. This is contrary to the traditional belief that all diogenites are orthopyroxenites with small amounts of cumulus olivine. Using bulk and in situ trace element chemistries, along with mineral major/minor element compositions, we demonstrate that these two lithologies were likely related through fractional crystallization. We also examine an anomalous achondritic dunite, and use geochemistry and petrology to demonstrate that it is the first recognized dunite belonging to the HED group. This sample likely also fractionated from a melt prior to the fractionation of harzburgitic and orthopyroxenitic diogenites. In the final part of this dissertation, we investigate compositional and textural heterogeneity in a large group of paired howardites, Vestan regolith breccias composed of diogenite and eucrite. We find significant compositional and textural variation within the group, and a preferential distribution of eucritic material in the finer grain sizes. This suggests an immature regolith, and has implications for interpretation of spectral data to be collected by the Dawn orbiter at Vesta.