Résumé
Mantle peridotites represent at least 20% of (ultra-)slow spread oceanic lithosphere. They are serpentinized down to several km below seafloor, and the numerous ultramafic-hosted hydrothermal systems emphasize the importance of serpentinization as a mean of chemical exchange between the mantle and the ocean. Characterizing the chemical modifications of peridotites during oceanic serpentinization and associated reactions is thus determinant to estimate the chemical fluxes near the ridges axis and the resulting input budget in subduction zones. However, these chemical exchanges are still poorly constrained. They depend on the initial chemical budget of peridotites (melting), on fluid sources (seawater or hydrothermal fluids), on sinks of elements (mineralogy), and on the operating mass transfer mechanisms, all expected to vary from one site to another. In order to better constrain mechanisms, scale and timing of mass transfers during serpentinization of oceanic peridotites, we carried out a combined (micro)-structural, mineralogical and geochemical study of variably refertilized and serpentinized peridotites drilled at the MARK area (ODP Site 920, 23 degrees N Mid-Atlantic Ridge). The petrostructural study indicates that serpentinization is accompanied by adundant veining of different generations, characteristic of different mechanisms of deformation and transfer during the progressive tectonic exhumation of peridotites. Two main serpentinization stages are distinguished and rough constraints on the depth at which they occurred are provided by regional seismic velocity data. A diffusion-dominated stage occurs below approximately 2 km in depth. It is followed by an advection-dominated stage at shallower levels. This model is completed by a geochemical investigation of serpentinites using bulk and punctual analyses of traces elements by LA-HR-ICPMS, and iron redox state by XANES at iron K-edge. Results indicate that most samples preserve the bulk peridotite primary trace element signature except for highly mobile elements such as U (up to 10 X PM), B (4-45 ppm), As and Sb (up to 10 ppm). In situ analyses of serpentine indicate that those enrichments are strongly heterogeneous at the thin section scale. They are correlated to an increase in Fe3+/FeTot which follows non-linearly the local degree of serpentinization and tracks the local increase in water/rock ratio. Extensive chemical exchanges also occur for the supposedly less-mobile elements (e.g. HREE) at the thin section scale and, at the borehole scale for completely serpentinized samples: trace elements are progressively redistributed and homogenized between more and less enriched zones (e.g. Px/Ol, refertilized/refractory). These observations underline the pervasive and heterogeneous nature of fluid circulation occurring at the mm scale within the forming mesh texture and within late veins at the meter scale. These results are compared to ongoing studies on the Rainbow Massif that hosts an active hydrothermal field in order to characterize the effect of late and extensive hydrothermal fluid circulations on the chemical signature of oceanic serpentinites.