Title: USING GEOCHRONOLOGY AND GEOCHEMISTRY OF KIMBERLITE AND CARBONATE-RICH OLIVINE LAMPROITE XENOLITHS TO ASSESS THE EVOLUTION OF THE SUB-CONTINENTAL LITHOSPHERIC MANTLE OF THE KAAPVAAL CRATON, SOUTHERN AFRICA

Program: Geosciences PhD

Committee Chair: Mark Schmitz

Committee: Mark Schmitz, Linda Reynard, CJ Northrup, Darin Schwartz

Abstract: The striking bimodal nature of Earth’s lithosphere — which can be subdivided into ancient, thick continents and young, tectonically recycled ocean basins — is a fundamental observation that makes our planet unique. This work seeks to understand how the ancient, stable continental regions of lithosphere were formed and the processes which have modified them since. The Kaapvaal craton of southern Africa is an ideal natural laboratory for this work, as its rich kimberlite- and lamproite-borne xenolith record readily samples both ancient and recent mantle processes. The first chapter of my dissertation focuses on mantle zircon as records of temporal and geochemical linkages between alkali magmatism and phlogopite-dominated, mica-amphibole-rutile-ilmenite-diopside (MARID) metasomatism. I suggest carbonate-rich olivine lamproites (CROLs) are derived from primary MARID metasomes, which may become introduced as early as the Mesoproterozoic during oceanic plate subduction beneath the craton. Additionally, pulses of kimberlitic melt infiltration prior to kimberlite eruption modify the compositions of MARID assemblages. My second chapter, while still focused on zircon isotope and trace element systematics, expands this metasomatic history into the Archean through the xenolithic eclogite record. Here, I document a ~3 Ga eclogitization event during assembly of the Kaapvaal craton, pulses of thermally and chemically activated metasomatism, and a pervasive carbonatitic overprint coeval with CROL magmatism, all within a uniquely metasomatized eclogite from the Roberts Victor kimberlite. The metasomatic history of the craton archived in this sample, as well as its periods of quiescence, are mirrored the diamond inclusion record. Lastly, in my third chapter I reassess the U-Pb isotope systematics of a perovskite standard from the Ice River Complex, Canada using high-precision U-Pb zircon geochronology and a novel single-grain separation protocol to constrain the initial Pb composition. This work is valuable to the scientific community due to the paucity of phases amenable to geochronology in ultramafic systems, which has encouraged the widespread use of U-Pb perovskite geochronology for dating kimberlites, and a pressing need for matrix-matched standards for in situ approaches.