Reaction Mechanism and Water/Rock Ratios Involved in Epidosite Alteration of the Oceanic Crust

Epidosites are a prominent type of subseafloor hydrothermal alteration basalts in ophiolites and greenstone belts, showing an end-member mineral assemblage epidote + quartz titanite Fe-oxide. known to form within crustal-scale upflow zones their fluids have been proposed as deep equivalents black-smoker seafloor vent fluids. Proposals the mass fluid per rock (W/R ratio) needed epidosites contradictory, varying from 20 (Sr isotopes) > 1,000 (Mg mobility). To test these proposals we conducted petrographic, geochemical reactive-transport numerical simulation study chemical reaction that generates km3-size epidosite lavas sheeted dike complex Samail ophiolite, Oman. At 250–400°C modeled epidosite-forming has near-neutral pH (? 5.2), high fO2, low sulfur very Fe (10?6 mol/kg) contents. These features argue against genetic link with Chemical buffering by enriches precursor spilites Ca depletes them Na Mg. Completion spilite-to-epidosite requires enormous W/R ratios 700–?40,000, depending on initial Mg content temperature. Collectively, variably altered rocks record flow ?1015 kg through each km3 spilite rock. This imposed Sr-isotope signature inherited previous rock-buffered evolution oceanic crust, thereby explaining apparently contradictory based Sr isotopes.