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Migration des liquides de fusion et formation des dunites dans le manteau de l’ophiolite d’Oman

Abstract : Mid-ocean ridges are the formation sites of ocean lithosphere covering two-thirds of the Earth's surface. Yet, melt transport to the ridge remains poorly understood. Ophiolites, segments of oceanic lithosphere thrust onto the continents, are ideal sites for studying mantle processes beneath ridges. They contain dunitic bodies, formed by interaction of the host peridotite with basaltic liquid causing pyroxene dissolution coupled with olivine precipitation. Using the example of the southeastern Oman ophiolite, this thesis explored the relationship between dunite distribution and melt transport to the ridge. The combination of structural observations with geochemical data on dunites and their host harzburgites allowed identification of three dunite types: (1) dunites of the Moho Transition Zone (MTZ), located immediately below the crust, (2) dunite veins and bands, scattered throughout the main mantle section, and (3) abundant dunitic bands in the banded unit at the ophiolite base. Our geochemical work focused on osmium (Os) isotopes, which are particularly suited for tracing melt flow through mantle peridotites. We found that harzburgites have fairly homogeneous compositions, similar to abyssal peridotite values, throughout the whole mantle section. In contrast, dunite compositions vary systematically with structural context. Basal dunites have highly residual character (e.g. high Mg#), and Os isotopic compositions equivalent to those of their host harzburgites, implying formation from these harzburgites by interaction with only small melt fractions. Dunites of the main mantle section have more radiogenic Os compositions, differing from those of adjacent harzburgites, suggesting more extensive melt interaction. MTZ dunites have interacted with large melt quantities, leading to low Os concentrations coupled with very high 187Os/188Os ratios, exceeding those of MORB. Modeling melt percolation through a dunite channel demonstrated that this transport mode cannot deliver melts with Os compositions equivalent to those of MTZ dunites and the overlying lower crust. This thesis works conclude that melt migration, from the mantle source to the Moho, occurred without equilibration with dunite or harzburgite, and was therefore rapid, or at least chemically isolated.
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Submitted on : Thursday, September 9, 2021 - 10:49:03 AM
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Delphine Klaessens. Migration des liquides de fusion et formation des dunites dans le manteau de l’ophiolite d’Oman. Sciences de la Terre. Université de Lorraine, 2021. Français. ⟨NNT : 2021LORR0062⟩. ⟨tel-03338943⟩



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