Hydrous fluids down to the semi-brittle root zone of detachment faults in nearly amagmatic ultra-slow spreading ridges

At the Eastern part of the Southwest Indian Ridge (SWIR), plate divergence is accommodated by large offset normal faults, also called detachment faults, that exhume mantle-derived rocks on the seafloor. A third of the ultramafic samples dredged on- and off-axis in this nearly amagmatic ridge setting present amphibole-bearing secondary mineralogical assemblages indicative of hydration, and for the most part predating the growth of serpentine minerals. The deepest evidence of hydration is the occurrence of small amounts of syn-kinematic amphibole in microshear zones with strongly reduced grain size, which record deformation at high stress and high temperatures (>800 °C) at the root zone of the detachment. The composition of these amphiboles is consistent with a hydrothermal origin, suggesting that seawater derived fluids percolated down to the root of detachment faults, at the Brittle-Ductile Transition (BDT). We propose that the constant exhumation of new mantle material to the seafloor, and the limited lifetime of each detachment (1–3 Myrs) prevent a more pervasive deep hydration of mid-ocean ridge detachment root regions, as proposed at transform fault plate boundaries.