HIGH-PRECISION U-PB ZIRCON GEOCHRONOLOGY AND TRACE ELEMENT GEOCHEMISTRY OF MANTLE-DERIVED XENOLITHS FROM THE KAAPVAAL CRATON

Kimberlite magmas and the mantle xenoliths they entrain contain geochemical insights into the relationship between tectonic processes, mantle petrology, and subcontinental lithospheric mantle (SCLM) evolution. There is a diverse suite of xenoliths entrained by kimberlite magmas, however this work will focus on a highly metasomatized subset of these from the Kaapvaal Craton mantle: mica-amphibole-rutile-ilmenite-diopside (MARID) xenoliths. MARID xenoliths, characterized by abundant phlogopite and the presence of k-richterite, are a direct product of intense mantle metasomatism and thought to be genetically linked to Group II kimberlite magmatism based on trace element and major element composition. Zircon is stable under high-grade metamorphic conditions and has a high closure temperature (>1000 deg C), making it one of the only accessory minerals that can document mantle events such as metasomatism. Despite the significance of high-temperature chronology to the goal of constraining the timing of major geochemical events in the mantle, there remain few high-precision zircon ages from MARID xenoliths which can be used to elucidate the timing of mantle metasomatism. In order to achieve this goal, we will combine high-precision U-Pb zircon geochronology with rare earth element and trace element data from the modal phases of MARID xenoliths from the Kimberley suite of kimberlites to address the hypothesis that these zircon record pulses of metasomatism in the SCLM under the Kaapvaal Craton of southern Africa. Cathodoluminescence images were used to qualitatively observe chemical zonation and micro-textural features, confirming these zircon are internally homogenous. Preliminary laser-ablation inductively-coupled plasma mass spectrometry (ICPMS) dates are Cretaceous and spatially resolved dates agree with qualitative observations of homogeneity. High-precision chemical abrasion isotope dilution-thermal ionization mass spectrometry (CA-ID TIMS) will be used to interrogate the zircon at a high temporal resolution. The petrographic and geochemical evidence will be combined with the geochronological data in order to determine which geologic events and processes produced the variance in the U-Pb systematics of the zircon.