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

Epidosites are a prominent type of subseafloor hydrothermal alteration of basalts in ophiolites and greenstone belts, showing an end‐member mineral assemblage of epidote + quartz + titanite + Fe‐oxide. Epidosites are known to form within crustal‐scale upflow zones and their fluids have been proposed as deep equivalents of black‐smoker seafloor vent fluids. Proposals of the mass of fluid per mass of rock (W/R ratio) needed to form epidosites are contradictory, varying from 20 (Sr isotopes) to > 1,000 (Mg mobility). To test these proposals we have conducted a petrographic, geochemical and reactive‐transport numerical simulation study of the chemical reaction that generates km3‐size epidosite zones within the lavas and sheeted dike complex of the Samail ophiolite, Oman. At 250–400°C the modeled epidosite‐forming fluid has pH ∼ 5.2, fO2 ≥ 10−20, low sulfur and extremely low Fe (10−6 mol/kg) contents. These features argue against a genetic link with black‐smoker fluids. Chemical buffering by the epidosite fluid enriches the precursor spilites in Ca and depletes them in Na and Mg. Completion of the spilite‐to‐epidosite reaction requires enormous W/R ratios of 700–∼40,000, depending on initial Mg content and temperature. Collectively, the variably altered rocks in the Samail epidosite zones record flow of ∼1015 kg of fluid through each km3 of precursor spilite rock. This fluid imposed on the epidosite an Sr‐isotope signature inherited from the previous rock‐buffered chemical evolution of the fluid through the oceanic crust, thereby explaining the apparently contradictory low W/R ratios based on Sr isotopes. Key Points Epidosite alteration strips Na + Mg from precursor spilite rocks, adds Ca and minor Al + Fe, and conserves Si and Ti Alteration to end‐member epidosite entails water/rock ratios of 700–44,000 kg/kg Epidosite fluid chemistry is incompatible with black‐smoker type vent fluids and cannot form seafloor sulfide deposits