Caractérisation Géochimique des Péridotites de l'ophiolite d'Oman : processus magmatiques aux limites lithosphère/asthenosphère

The 93 samples of peridotite selected for this study in 3 massifs of the Oman ophiolite are representative of an area about 5000km2 of a fast spreading ridge. Relative to the segmentation, the sampling includes both the lithosphere of a new segment (0 to 4km below the moho), its limits (shear zones), and the young lithosphere (1 to 2 Ma) in which this new segment opens (between 0 to 12km below the moho).<br />* The totality of the mantle explored is composed of refractory peridotites, comparable to those exposed in other ophiolites and to the abyssal peridotites. Nevertheless, peridotites are enriched in very incompatible elements, involving a reactive percolation by fluids and a re-enrichment in clinopyroxene, as recorded by the occurrence of clinopyroxene-rich-harzburgites.<br />* The new segment has additional chemical characteristics, like high Cr number spinel (Cr#>50), which appears not to be assigned to dunites or to a particular geodynamic environment, and different pyroxene compositions, which involve higher temperatures of closure in relation with a process of magmatic percolation.<br />* The transition zone at the axis of the new segment is marked by the<br />importance of liquid/rock reactions which concern all rock types : harzburgites, dunites and impregnated dunites.<br />* The limits of the new segment (shear zones) and the base of the ophiolite are characterised by porphyroclastic BT textures (<1000°C) and by enrichment in clinopyroxene. These clinopyroxenes are interpreted as product of fractional crystallisation of magmas associated with a percolating front at limit between convective mantle/conductive mantle.<br />* All rocks show a contamination by seawater, traced by high radiogenic 87Sr and 207Pb isotopic ratios, acquired mostly before serpentinization process (T>500°C). <br /><br />This study points out the importance of ridge segmentation in the control of magmatic and hydrothermal processes at a fast spreading ridge.<br /><br />Key words : oceanic peridotites, asthenosphere/lithosphere limit, magmatic and hydrothermal processes, Oman ophiolite, petrology, geochemistry, Sr and Pb isotopes.