Abstract:
Ureilites are ultramafic igneous meteorites derived from planetesimals, and form the second largest group of achondrite meteorites. Ureilites represent asteroidal “mantle” rocks, although their mode of formation is poorly understood. Previous work on ureilites has been limited to whole rock chemical analyses and basic petrographic studies. This thesis presents a detailed chemical and isotopic study of 12 Antarctic ureilites. LA-ICP-MS analyses of olivine and pyroxene grains in ureilites reveal ranges in the concentrations of incompatible elements, which are most plausibly explained by differing degrees of silicate melt removal from the ureilite parent body of ca. 20%–30%. Ureilites also contain relatively high abundances of highly siderophile elements (HSE) and Fe metal. Analyses of platinum (Pt) stable isotope ratios in ureilites reveal a variety of compositions. Some ureilites still retain chondritic signatures and appear to be relatively undifferentiated, whereas others have heavy Pt stable isotope signatures indicative of varying degrees of differentiation and core formation. Ureilites have undoubtedly experienced smelting, and a number of previous studies have suggested that ureilites formed as a result of global, anatectic smelting, whereby the precursory material underwent both partial reduction and partial melting simultaneously. If global anatectic smelting had occurred ureilites would display a correlation between lithophile and siderophile elements in their olivine and pyroxene grains. This trend is not observable within ureilites. The HSE abundances of ureilites also do not appear to be consistent with the global smelting model. This study proposes that ureilites underwent smelting and partial silicate melt removal, where siderophile and lithophile elements were lost to the core and a basaltic melt phase, respectively. HSE were then injected back into the ureilite precursor material through the addition of foreign metallic material, or through the redistribution of metallic melts from the outer regions of the ureilite parent body.