Résumé
Outcrops of deeply-derived ultramafic rocks and gabbros are widespread along slow spreading ridges but the rheology and dynamics of the exhumation faults and of their uplifted footwalls are still poorly known. Previous studies of samples collected within meters of exposed exhumation fault surfaces in the Atlantic have shown that a gabbroic component was added to the primarily ultramafic material in the fault zone, allowing for the growth of abundant amphibole, chlorite and talc. The nature of this component (altered magmatic intrusions or metasomatic hydrothermal fluids) could not, however, be ascertained in the pervasively sheared fault material. In this abstract we report on a set of 474 samples collected at the Mid-Atlantic Ridge (MAR) during the Serpentine cruise (2007; RV Pourquoi Pas? PI Y. Fouquet) next to the ultramafic-hosted Ashadze (13 degrees N) and Logatchev (14 degrees 45'N) vent fields. Most of these 474 samples are weakly to moderately deformed and are interpreted as representing the upper few hundred meters below their respective exhumation fault zone, rather than the fault zone itself. The large number of samples, and their overall moderate degree of deformation gives us a chance to propose a semi-statistical study of plastic, brittle-plastic and brittle deformation in ultramafic rocks next to a MAR exhumation system, in relation with the magmatic and hydrothermal history. Our primary finding is that significant brittle-plastic deformation systematically involves amphibole+ or -chlorite+ or -talc-bearing ultramafic lithologies. Serpentine is commonly present in these deformed assemblages, but we did not find serpentine-only shear zones. Amphibole (in successive generations ranging from hornblende to tremolite) and chlorite occur in veins, many of which also contain zircon and some relict plagioclase, indicating a magmatic origin. Relicts of primary peridotite minerals in the most amphibole-rich samples indicate that magmatic injection followed on an episode of heterogeneously distributed brittle-plastic then brittle deformation of the unserpentinized ultramafics. We thus infer that melts injected near the brittle-plastic transition in fresh peridotite, were subsequently altered into amphibole-bearing assemblages which helped accommodate exhumation-related deformation further up in the axial lithosphere. We propose that this mechanism weakens the rheology of exhumation faults and their footwall prior to and during serpentinization, and could operate at most exhumation sites along the MAR. We further infer that exhumation systems at ridges that do not have a sufficient melt supply would lack this strain softening mechanism. Deformation experiments conducted at T 500-700 degrees C and P 100-300 MPa show that the coefficients of friction of tremolite are similar to those of serpentine in the same range of temperature and pressure.