Phase equilibria constraints on the chemistry of hot spring fluids at mid-ocean ridges

Recent advances in experimental and theoretical geochemistry have made it possible to assess both homogeneous and heterogeneous equilibria involving a wide range of aqueous species at temperatures and pressures appropriate to model hydrothermal alteration processes at mid-ocean ridges. We have combined selected aspects of the chemistry of hot spring fluids with constraints imposed by a geologically reasonable assemblage of minerals in the system Na₂O-K₂O-CaO-MgO-FeO-Fe₂OrAl₂O₃-SiO₂-H₂O-HCl-H₂S to assess the effect of temperature on the composition of the aqueous phase and the activities of mineral components in plagioclase and epidote solid solutions. Assuming f{hook}O_2(g) and f{hook}S_2(g) controlled by pyrite-pyrrhotite-magnetite equilibria, a constant dissolved Ca concentration, and a dissolved Cl concentration equivalent to that of seawater, increasing temperature from 250 to 400°C at 500 bars results in systematic changes in the composition of mineral phases, which in turn constrain pH and the distribution of aqueous species. The model predicts that dissolved concentrations of Fe, SiO₂, K, H₂S, and H₂ increase, while Na and pH_(25°c) decrease with increasing temperature. pH_(in-situ) decreases slightly with increasing temperature, and has a value of 5.16 at 400°C. Dissolved Mg concentrations do not exceed 1 mmolal at any temperature investigated. Allowing for differences in pressure and total dissolved Cl, the predicted effect of temperature on fluid chemistry is in good agreement with results from basalt alteration and plagioclase + epidote + quartz recrystallization experiments, and in some cases, with results from hot spring fluids at mid-ocean ridges. Some vent fluids, in particular NGS (EPR, 21°N) and vent-4 (EPR, 11°N), however, reveal pH and/or dissolved Fe, H₂S, H₂, and SiO₂ concentrations, which are difficult to reconcile with measured temperatures in comparison with results of temperature dependent mineral solubility calculations, and suggest that these fluids have lost heat by conduction on ascent to the seafloor. That many hot spring vent fluids are characterized by variable degrees of conductive heat loss renders measured temperatures unreliable as indicators of the maximum temperature of subseafloor hydrothermal alteration processes. The implications of this are significant for hot spring fluids which reveal large Cl variations relative to seawater, since likely mechanisms to account for such variability typically require temperatures in excess of those inferred for subseafloor reaction zones by simply correcting measured temperatures for the effects of adiabatic cooling.