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Manteau source et genèse des magmas parents des carbonatites : les principales ressources en terres rares sur Terre

Abstract : Rare Earth Elements (REE) are used in numerous industrialized products. The main sources of REE are carbonatite lavas and intrusions. Carbonatites are rare, carbonate-rich lavas, whose origin remains underconstrained: either directly derived from the melting of a C-rich mantle, or formed by protracted differentiation and immiscibility from silicate magmas. This project is focused on the petro-geochemical study of mantle xenoliths gathered around the Oldoinyo Lengai (OL; Tanzania), the only active volcano emitting carbonatites on Earth. Thanks to a multi-mineral approach integrating chemical data of all the mineral phases present in mantle xenoliths, this study shows that the Tanzanian lithospheric mantle is heterogeneous, with a complex metasomatic history. In addition, some xenoliths contain carbonates similar to those previously observed in the lithospheric mantle from the area. The isotopic compositions of carbonates from this study and of two samples from the literature reanalyzed herein display δ18O and δ13C isotopic compositions that support the pedogenic formation of these carbonates. This refutes an overall carbon enrichment, in the form of mantle carbonates, of the lithospheric mantle under the only active volcano emitting carbonatites. These results support the formation of Tanzanian carbonatites by extreme differentiation from alkaline magmas produced by the partial melting of a relatively C-poor mantle. In addition, alkaline silicate magmas (melilitite, Mg-nephelinite) are considered to be the potential parental magmas of carbonatites currently produced by the OL. Previous studies on these magmas have shown that they can potentially be generated by the partial melting of pyroxenite metasomatic lithologies carrying volatile-rich phases (CO2, H2O). Hence, the melting of a natural metasomatic vein from our sample set forms the most volatile-rich silicate alkaline magmas experimentally produced from a natural protolith. The melting of metasomatic phlogopite-rich pyroxenites veins produces magmas which compositions strongly suggest their involvement in the genesis of the primitive magmas of the area. Thus, these results suggest that the partial melting of the fertile veins of a metasomatized mantle participates in the formation of the parental magmas of carbonatites.
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Submitted on : Wednesday, April 6, 2022 - 5:06:55 PM
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Valentin Casola. Manteau source et genèse des magmas parents des carbonatites : les principales ressources en terres rares sur Terre. Sciences de la Terre. Université de Lorraine, 2021. Français. ⟨NNT : 2021LORR0198⟩. ⟨tel-03633075⟩

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