New aspects of kimberlite-related metasomatism

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Petrological problems surrounding kimberlites are on the forefront of mantle petrology. We do not yet understand the genesis of kimberlites, as well as their melt compositions in the mantle and in the shallow crust before their emplacement. I will explore the role of metasomatism in 1) pre-conditioning the mantle to the birth of kimberlite melts, and in 2) controlling the mineralogy and textures of kimberlites in the crust.******Kimberlite melts are not equilibrated with the ambient mantle, and the enigmatic missing link between kimberlites and the mantle is believed to have formed through mantle metasomatism. We will study megacrysts and fenitized olivinites in contact with carbonatites of alkaline-ultramafic massifs as proxies for the geologic relationships between the metasomatized mantle and kimberlite melts, unavailable for the direct observation at depths >200 km. Independent constraints on metasomatic agents will be obtained from fluids trapped as diamond inclusions in fibrous and octahedrally-grown, gem-quality diamonds and deduced from geochemistry of metasomatic mantle minerals.******The second line of research deals with metasomatic reactions between xenoliths of silicic crustal rocks and host kimberlites. Crustal contamination in kimberlites can amount to more than half of the kimberlite volume, significantly impacting the diamond grade and the economics of the mine. Crustal contamination leads to melt hybridization and metasomatic replacement of silicic xenoliths, with mini-skarn reaction zones around the xenoliths. I will investigate the metasomatic transfer of elements in this process and subsequent kimberlite recrystallization and texture development. ******The proposed research will be based on detailed petrographic, geochemical and mineralogical studies as well as geochemical modelling and petrological experiments. The project is innovative in many ways. It is based on previously uncharacterized occurrences of megacrysts and diamonds, novel trap sites for mantle fluids in diamond fractures, novel massif analogues of kimberlite parent rocks and novel ideas about the genesis of pyroclastic kimberlites. ******The proposed research will be significant for the fundamental petrology and practical geology. The study of mantle metasomatism will contribute to solving the outstanding problem of the origin of carbonate-rich primary melts in the mantle, their composition, and the composition of their metasomatized mantle source. The practical significance of the proposal relates to the quantification of the diamond ore dilution and recognition of hybrid kimberlite minerals impacting the ore extraction. If we find the causal link between the kimberlite textures, the ore dilution and the country rock architecture, we will be able to predict many major characteristics of diamond deposits simply on the basis of crustal geology.*****