Oman diopsidites: a new lithology diagnostic of very high temperature hydrothermal circulation in mantle peridotite below oceanic spreading centres

Mafic–ultramafic dykes scattered in the mantle section of ophiolites are generally crystallisation products from common silicate melts. In the frame of a global survey of these melt migration relics in the Oman mantle harzburgites, we discovered a peculiar lithology made essentially of pure diopside (0.95 0.95) or traces of andradite. When not deformed, the diopsides are automorphic and their texture points to metamorphic growth in a matrix composed of antigorite and/or carbonate. Diopsidites have textures reminiscent to that of skarns developing in contact metamorphic halos or that of rodingite frequently present in the serpentine bodies of the ophiolitic crust. They frequently appear as dykes (former cracks), with a few mm to few tens of cm wide transitional zones, which contain high amounts of hydrous minerals, between the diopsidite facies and its host rock. The diopsidites are not randomly distributed in the Oman ophiolite, being more abundant near former asthenospheric diapirs emplaced at shallow depth in the lithosphere. We interpret the diopsidites as the footprint of very high temperature circulation of seawater and carbonated fluids (> 800 °C), which may have leached plagioclase rich lithologies before penetrating the mantle (as shown by a well developed positive Eu anomaly). Our data confirm the prediction of McCollom and Shock [T.M. McCollom, E.L. Shock, Fluid-rock interactions in the lower oceanic crust: Thermodynamic models of hydrothermal alteration, J. Geophys. Res. 103 (B1) (1998) 547–575.] who proposed that common anhydrous minerals like pyroxene, plagioclase and olivine may crystallise from high temperature fluids intermediate between silicate melts and supercritical water. This confirms that there is no clear-cut thermal and chemical boundary between the fields of magmatic and hydrothermal crystallisations.