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

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 <500–550 °C. At medium temperature (MT), chlorite and amphibole crystallized at ∼500–750 °C. At high temperature (HT), amphibole (± second generation peridotitic minerals) crystallized. The composition of minerals in HT fractures and shear bands indicates that fracturing and fluid flow occur up to temperatures of at least 850–875 °C. Combining these results with modeled geotherms for both faults suggests that seawater percolation extended to depths of 20–25 km and that serpentinization extended to ∼11–13 km. The evolution of fault zone structure induced by deep fluid-rock interaction and progressive formation of LT, MT and HT mylonites on OTFs results in weakening and strain localization within the oceanic lithosphere, and suggests that the global transform system may represent a large reservoir of volatiles in the Earth's lithosphere.