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–Cl-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 H2(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.