On the smooth seafloor of exhumed mantle-rocks of the slow-spreading Southwest Indian Ridge, the secondary oxide and carbonate mineralizations record specific chemical signatures of the alteration of serpentinites by low-temperature hydrothermal seawater-dominated fluids. The uncommon characteristics of Fe-oxides forming nodule cortex or thick encrustments are their high Si, low Mn contents, trace elements (e.g. Cr) and REE behavior. Oxides composition evolves through time. Associated to oxides, carbonates (aragonite, calcite, dolomite) characterize the matrix of young indurated serpentinite breccias found at ridge axis. Such mineralogical and geochemical sequences trace, as proxies, the ongoing mantle-exhumation and serpentinization process linked to the detachment faults, and the late alteration reactions that impact on crust–ocean transfers and oceanic budget.