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Abstract:

Impact cratering is increasingly recognized as a ubiquitous geologic process occurring from microscopic to planetary scales. In the latter case, the massive amount of energy released upon impact can have important consequences, including moon formation and mass extinctions, among other things. The 1.85 Ga Sudbury impact structure (Ontario, Canada) is one of the oldest, largest, and best exposed terrestrial impact structures, thus providing a unique window into the processes and products associated with massive impact events. Despite its recognized importance and long history of academic study, two fundamental aspects of the cratering process remain poorly constrained for the Sudbury event: the nature and fate of target and projectile materials. The lack of consensus on these matters prohibits the development of an accurate model for the event and limits our insight into solar system evolution. In this thesis, the depths of excavation and melting, and the nature of the bolide for the Sudbury impact are evaluated using new mineralogical, chemical and isotopic data. The samples used to address these questions are primarily from the Sudbury crater-fill (Onaping Formation) which is known to contain target rock lithic fragments, impact melt, and suspected meteoritic material. New zircon U-Pb geochronology and Pb isotope data challenge previous arguments in favour of a shallow depth of melting. The combined new data and a reevaluation of existing data indicate that a deeper (> 15 km) excavation and melting scenario is more likely. The zircon data also reveal a significant 2.6 Ga lithology not previously recognized amongst known or suspected target rocks. Whole-rock platinum group and trace element data expose three problems in attempting to determine projectile elemental ratios in the Sudbury impactites: variable mafic rock incorporation, alteration, and impact melt heterogeneity. Each of these problems are e↵ectively mitigated by geochemical and spatial filtering of the dataset. The projectile elemental ratios of a robust data subset match a non-carbonaceous chondrite. However, the distribution of platinum group elements and impact modelling seem to be consistent with a cometary bolide, suggesting that the refractory component of some comets may be similar to non-carbonaceous chondrites. The studies reported herein help to constrain aspects of one of the most important terrestrial impact craters.