Studies of Regolithic Vestan Samples: Brecciated Eucrites and Howardites

Brecciated eucrites and howardites represent samples of the regolith of asteroid 4 Vesta. As such, they are a valuable source of data for understanding the products of surface alteration. Two different processes are investigated here: impact mixing of comminuted rocks to produce regolith samples, and formation of glasses in the regolith.

Chapter 1 describes four newly discovered eucrite breccias: three presumably paired meteorites, all named NWA 6105, and NWA 6106. For each meteorite, major- and minor-element compositions of minerals were determined using the electron microprobe. Pyroxene Fe-Mn co-variations and bulk-rock oxygen isotope compositions confirm their classification as eucrites. Variations in mineral compositions and textures are attributed to differences in clast types present (i.e., basaltic or cumulate eucrite). The pyroxene compositions support the hypothesis that samples NWA 6105,1; 6105,2; and 6105,3 are paired polymict eucritic breccias, whereas sample NWA 6106 is a monomict basaltic eucritic breccia. Two-pyroxene geothermometry yields temperatures too low for igneous crystallization. The variation in temperatures among samples suggests that metamorphism occurred prior to brecciation.

Chapter 2 is an investigation of glasses in eight howardites, with the aim of distinguishing their origins as impact melt or pyroclastic. Although theoretical calculations predict that pyroclastic eruptions could have taken place on Vesta, the occurrence of pyroclastic glasses in HED meteorites has never been documented. This study involved petrographic examination of textures, electron microprobe analysis of major and minor elements, and LA-ICP-MS analysis for selected trace elements. Previously documented textural and compositional differences between lunar impact melt and pyroclastic glasses partly guided this study. This work yielded no positive identification of pyroclastic glasses. The most likely explanations are that pyroclastic glasses never formed, either because Vesta contains insufficient volatiles to have powered explosive eruptions, or because eruptive conditions produced optically dense fire-fountains which produced deposits that accumulated in lava ponds. The impact-melt glasses were grouped (K-rich, low-alkali, and Ca-rich) based on compositions. The K-rich group is postulated to result from impacts into previously unsampled, feldspar-rich lithologies, while the low-alkali and Ca-rich glasses are the result of impacts onto known HED lithologies though the latter formed from a preferential melting of plagioclase.