Fracture-mediated deep seawater flow and mantle hydration on oceanic transform faults

•Seawater percolates and interacts with mantle to >875°C on transform faults.•Mantle fracturing up to >875°C leads to deep fluid percolation in these faults.•Fluid-peridotite interaction leads to continuous hydrous phase crystallization.•Mantle serpentinization extends down to 11–13 km and hydration to >20–25 km. Fluid-rock interaction on oceanic transform faults (OTFs) is important for both the deformation behavior of the lithosphere and volatile cycling in the Earth. Rocks deformed and exhumed at OTFs preserve information about the depth extent of fluid percolation and the nature of fluid-rock interactions within these fault zones. In this study, we focus on five dredges from the Shaka and Prince Edward OTFs on the ultraslow spreading Southwest Indian Ridge that recovered significant volumes of deformed mantle rocks. Samples are predominantly mylonites that have been deformed to high strains in the fault zone, but also contain several generations of fractures. Based on the mineral assemblages in fractures and shear bands combined with thermobarometry analysis, we identified three distinct temperature ranges of fluid-mantle interactions associated with deformation. At low temperature (LT), this leads to crystallization of serpentine (± talc ± amphibole ± chlorite) at