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While the role of carbonatite magmas and other CO2‐rich melts is crucial to the Earth’s global carbon cycle, their genesis remains highly controversial. Oldoinyo Lengai (OL, Tanzania, East African Rift) is the only active volcano erupting carbonatites and grants a theoretical access to the understanding of all of the magmatic steps for their genesis from the nature of the mantle source and related melting processes, through protracted differentiation, to the eruption of carbonatite lavas at surface. Several studies reveal that the OL carbonatites have their source located in a sub-continental lithospheric mantle domain influenced by asthenospheric metasomatic agents. In this area, mantle metasomatism appears in the form of ubiquitous metasomatic veins and pervasive hydrous phases. Such ‘fertile’ (low melting temperature) metasomatic lithologies are widespread in the lithospheric mantle beneath the OL magmatic province, which would imply an unavoidable influence on C‐rich magma production. By analyzing 110 mantle xenoliths from the OL magmatic province, our study aims to bring new constraints on the metasomatic stages that shaped the lithospheric mantle source of carbonatites and associated alkaline magmas. Most of the xenoliths host metasomatic phases (phlogopite, amphibole and Cr-diopside), and some contain cumulative phlogopite-bearing pyroxenite veins formed from the metasomatic agent. Thanks to a multi-mineral approach integrating chemical data of all the mineral phases present in those mantle xenoliths, we established a geochemical model where all xenoliths were refertilized by a single global metasomatic episode. The metasomatic agent is characterized as a silica under-saturated, Ti, Fe, Ca, OH, CO2-rich and Cr, Na-poor silicated melt, without any HFSE negative anomaly. The chemical covariations between the various phases of the peridotites made it possible to track the progressive effect of the metasomatic agent through the percolated mantle from a highly metasomatized mantle with metasomatic veins to an unmetasomatized protolith identified as refractory mantle. While the interaction between the metasomatic agent and the surrounding mantle progressively decreases, geochemical overprint changes (e.g., increase in the HFSE negative anomaly) along with the modal composition that evolves from phlogopite-bearing wehrlites (orthopyroxene and spinel consumed, and Cr-diopside and phlogopite crystallization (+)), to phlogopite-bearing lherzolites (+ Cr-diopside + phlogopite), to phlogopite-bearing harzburgites (+ phlogopite). The end-member of this metasomatic series appears in the form of rare earth elements (REE)-rich cryptic metasomatism in harzburgites. The unmetasomatized refractory mantle could be highlighted by one harzburgite that is devoid of Ti, Fe or REE enrichment. Hence, our multi-mineral approach makes it possible to track the evolution of metasomatism through the entire series of samples, even with the disappearance of clinopyroxene. This model simplifies the previous ones from the literature which hitherto proposed several distinct metasomatic episodes in the Tanzanian mantle. More generally our results highlight the added value of following a multi-mineral geochemical approach when studying mantle metasomatism, and question the widespread use of La/Yb versus Ti/Eu systematics in identifying carbonatite metasomatism. |
