Thallium isotopic composition of phlogopite in kimberlite-hosted MARID and PIC mantle xenoliths

Abstract

MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) and PIC (Phlogopite-Ilmenite-Clinopyroxene) rocks are rare mantle xenoliths entrained by kimberlites. Their high phlogopite modes (15 to ∼100 vol.%) and consequent enrichments in alkali metals and H2O suggest a metasomatic origin. Phlogopite also has high concentrations (>0.2 µg/g) of thallium (Tl) relative to mantle abundances (<3 ng/g). Thallium isotope ratios have proven useful in tracing the input of Tl-rich materials, such as pelagic sediments and altered oceanic crust, to mantle sources because of their distinct isotopic compositions compared to the peridotitic mantle. This study presents the first Tl isotopic compositions of well-characterised phlogopite separates from MARID and PIC samples to further our understanding of their genesis. The PIC rocks in this study were previously interpreted as the products of kimberlite melt metasomatism, whereas the radiogenic and stable N-O isotope systematics of MARID rocks suggest a parental metasomatic agent containing a recycled component. The ε205Tl values of phlogopite in both PIC (–2.7 ± 0.8; 2 s.d., n = 4) and MARID samples (–2.5 ± 1.3; 2 s.d., n = 21) overlap with the estimated mantle composition (–2.0 ± 1.0). PIC phlogopite Tl contents (∼0.4 µg/g) are suggestive of equilibrium with kimberlite melts (0.1–0.6 µg/g Tl), based on partitioning experiments in other silica-undersaturated melts. Kimberlite Tl-ε205Tl systematics suggest their genesis does not require a recycled contribution: however, high temperature-altered oceanic crust cannot be ruled out as a component of the Kimberley kimberlites’ source. Mantle-like ε205Tl values in MARID samples also seem to contradict previous suggestions of a recycled contribution towards their genesis. Recycled components with isotopic compositions close to mantle values (e.g., high temperature-altered oceanic crust) are still permitted. Moreover, mass balance mixing models indicate that incorporation into the primitive mantle of 1–30% of a low temperature-altered oceanic crust + continental crust recycled component or 1–50% of continental crust alone could be accommodated by the Tl–ε205Tl systematics of the MARID parental melt. This scenario is consistent with experimental evidence and existing isotopic data. One PIC phlogopite separate has an extremely light Tl isotopic composition of –9.9, interpreted to result from kinetic isotopic fractionation. Overall, phlogopite is the main host mineral for Tl in metasomatised mantle and shows a very restricted range in Tl isotopic composition, which overlaps with estimates of the mantle composition. These results strongly suggest that negligible high temperature equilibrium Tl isotopic fractionation occurs during metasomatism and reinforces previous estimates of the mantle’s Tl isotopic composition.