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

Weber, Samuel; Diamond, Larryn W.; Alt-Epping, Peter; Brett‐Adams, Alannah C. (2021). Reaction Mechanism and Water/Rock Ratios Involved in Epidosite Alteration of the Oceanic Crust. Journal of Geophysical Research: Solid Earth, 126(6) American Geophysical Union 10.1029/2020JB021540

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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 near-neutral pH (∼ 5.2), high fO2, low sulfur and very 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.

Item Type:

Journal Article (Original Article)


08 Faculty of Science > Institute of Geological Sciences

UniBE Contributor:

Weber, Samuel (A), Diamond, Larryn William, Alt-Epping, Peter, Brett, Alannah Charlotte


500 Science > 550 Earth sciences & geology




American Geophysical Union




Larryn William Diamond

Date Deposited:

10 Mar 2022 10:53

Last Modified:

29 Mar 2023 23:38

Publisher DOI:





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