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Apports de la géochimie élémentaire et isotopique pour la compréhension des processus de serpentinisation : cas de la dorsale sud-ouest indienne

Abstract : Serpentinization, a reaction of hydration and oxidation of peridotites, occurs at slow to ultra-slow spreading ridges and is responsible of numerous chemical exchanges between the mantle, the oceanic crust and the ocean. In order to better understand these geochemical cycles, this thesis studies serpentinites from the smooth seafloor (SMS) of the ultraslow south west Indian ridge, an amagmatic end-member of the ridges system. We established profiles of serpentinization, showing an increase of the serpentinization degree with distance from the ridge. Serpentinization is accompanied by a modification of the peridotites bulk rock compositions in Si, Fe, Mg and Na without systematic evolution with the ridge distance, and by a progressive enrichment in light trace elements (LA, Pr), mobile elements (Li,B) and metals (As, Sb, Cd). The 56Fe composition of the SMS serpentinites does not show any evolution with the serpentinization degree, the partial melting rate, nor the refertilization by basaltic melts, indicating that these serpentinites can be used as analogs of the in situ oceanic mantle. Carbon isotopes compositions (13C) are similar to previous studies on serpentinites and do not show any evolution with the serpentinization degree. Few samples showing evidences of post-serpentinization alteration display however high organic carbon contents, probably coming from a stronger production of H2 through Fe-oxidation. This alteration is also responsible for supplementary enrichments in La, Pr, As, Sb and Cd.
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Lucile Dessimoulie. Apports de la géochimie élémentaire et isotopique pour la compréhension des processus de serpentinisation : cas de la dorsale sud-ouest indienne. Sciences de la Terre. Université de Lyon, 2019. Français. ⟨NNT : 2019LYSES013⟩. ⟨tel-02467017⟩

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