Multistage Mantle Metasomatism during the Generation of Kimberlite Melts: Evidence from Mantle Xenoliths and Megacrysts of the Grib Kimberlite, Arkhangelsk, Russia

Major and trace element compositions of garnet, clinopyroxene, orthopyroxene, phlogopite, and ilmenite from garnet peridotite, ilmenite-bearing peridotite (dunites), and clinopyroxene–phlogopite xenoliths, as well as megacrysts of these minerals from the Grib kimberlite, Arkhangelsk diamond province, Russia, have been analyzed. These data are used to propose a model for mantle metasomatism of lithospheric mantle by kimberlite melts, including their generation and evolution, geochemical enrichment of depleted lithosphere mantle, and formation of megacrystic assemblage. The lithospheric mantle beneath the Arkhangelsk diamond province, from its base (depth ~180–210 km) to a depth of ~100–120 km (corresponding to a pressure of 3.5 GPa) experienced extensive metasomatism along the main kimberlite melt channel. Petrography of the peridotite xenoliths indicates a progressive refertilization of depleted harzburgite into garnet lherzolite, phlogopite–garnet wehrlite, and clinopyroxene–phlogopite rocks. Metasomatic refertilization occurred shortly before the capture of these xenoliths by the kimberlite melt. The model melt compositions calculated from garnet–clinopyroxene equilibria in different types of xenoliths and megacrysts show that alkaline–carbonate–ultramafic kimberlite melt acted as a metasomatic agent in the sheared peridotite at the base of the lithospheric mantle. High-Ti garnet and high-Cr clinopyroxene megacrysts in the middle part of the lithospheric mantle, as well as the main volume of garnet lherzolite xenoliths were formed in geochemical equilibrium with the kimberlite melts, which demonstrate an increase of silicate components and fractionation of Fe–Ti phases. The modification could be related to the interaction of ascending carbonate-rich protokimberlite melts with surrounding lithospheric mantle. The similarities in the compositions of garnet, clinopyroxene, phlogopite, and ilmenite megacrysts with minerals of peridotite xenoliths in the Grib kimberlite suggest that these megacrysts are disintegrated fragments of coarsest grained metasomatized garnet lherzolite, ilmenite-bearing peridotite, and clinopyroxene-phlogopite mantle rocks or formed under the same conditions as xenoliths or directly crystallized from metasomatic melts.