Ross, Aidan Jennifer; (2024) Mineralogy and Geochemistry of Ureilite Meteorites and their Record of Early Solar System Planetary and Asteroidal Processes. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The aim of this study was to obtain information about the Ureilite Parent Body (UPB) concerning early solar system processes of accretion, differentiation and shock disruption. It provides detailed petrography and geochemistry of six ureilite samples from the Almahata Sitta fall, and two brecciated ureilite finds. Scanning electron microscopy (SEM) and computed tomography (CT) were used for petrographic examination of the samples, focusing on silicate minerals and metals/silicides. Electron probe microanalysis (EPMA) was complemented by laser ablation inductively coupled mass spectrometry (LA-ICP-MS). Mineral structures were examined by electron backscatter diffraction (EBSD) mapping. The Almahata Sitta meteorite was recovered quickly after asteroid 2008 TC3 was detected and tracked as it broke up over Sudan. They provide a unique opportunity to study unweathered ureilites. The multi-disciplinary approach of this dissertation places Almahata Sitta in the context of other ureilites. Several samples had not been studied in detail and are classified fully for the first time. Five are unbrecciated; one is dimict. Most show features that are well known in other ureilites but others contain new features that address silicate melt infiltration. Metals were analysed in several additional unbrecciated ureilites, providing an internally consistent dataset. Grain boundary metals and inclusions were the primary focus. EBSD mapping shows deformation within metal grains, even when shock is not indicated in the co-existing silicates. EBSD mapping shows recrystallisation and quenching of metals, indicating that metallic liquids were still present when the UPB was catastrophically disrupted. Siderophile analyses of metals provide evidence for core formation. Brecciated polymict ureilites contain Fe-silicides and Si-bearing metals which closely resemble metals in unbrecciated ureilites in their geochemistry and therefore were derived from metals in the UPB. This implies that silicides formed within the regolith of the present-day source of ureilites by space weathering, rather than originating from the UPB core.

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