Redox and pH constraints in the subseafloor root zone of the TAG hydrothermal system, 26° N Mid-Atlantic Ridge

Experiments were conducted at 375-400 °C, 500 bars to assess the role of redox and pH on Fe, Cu and H₂S concentrations at temperatures, pressures and total dissolved chloride concentrations applicable to the Trans-Atlantic Geo-Traverse (TAG) hydrothermal system at 26° N, Mid-Atlantic Ridge. The magnetite-bornite-anhydrite mineral assemblage imposed redox constraints, while pH was buffered with a 0.65 molal Na-K-1Cl-bearing aqueous fluid coexisting with K-feldspar, muscovite and quartz. Results indicate a strong inverse correlation between dissolved Cu and H₂S_(aq), whereas dissolved Fe is more sensitive to pH. Magnetite-bornite-anhydrite-fluid equilibria is characterized by unusually low mFe/mCu (molar) ratio and low H_2(aq)-H₂S_(aq) activities. To form this mineral assemblage during subseafloor hydrothermal alteration, however, requires elevated fluid/rock mass ratios, such that the ferrous iron and sulfur in the rock become sufficiently depleted to render pyrite and chalcopyrite unstable. The relatively high Cu concentrations, the low H₂S_(aq) and mFe/mCu ratios observed at TAG vent fluids are consistent with constraints imposed by magnetite-bornite-anhydrite-fluid equilibria at pH of approximately 4.9. Available chemical data confirm that the TAG hydrothermal system has experienced elevated fluid/rock mass ratios in keeping with the extent of mass transfer needed to generate oxidizing, low H₂S_(aq) conditions.