Homogeneous equilibria in hydrothermal fluids at near critical conditions.

Abstract

The activities of aqueous entities in fluids from active hydrothermal systems can provide detailed fluid-mineral equilibria information. Numerical integration of the equations describing homogeneous equilibria as a function of temperature and pressure is the best way to determine ion activities at elevated pressure-temperature conditions. Activities of most cations decrease with increasing temperature, but the activity of the hydrogen ion may either increase or decrease. The dominant control over changes in ion activity ratios as temperature and pressure increase is the evolution of H⁺. Changes in the activity of the hydrogen ion depend on equilibrium constants for reactions that produce or consume H⁺, the measured pH, and the concentration of elements such as chlorine and sulfur in the fluid. pH and ion to hydrogen activity ratios calculated at elevated temperatures display a wide range of behavior; they may be sensitive to the initial pH, insensitive to the initial pH, or a unique function of the initial pH. When pH is sensitive to the initial pH, uncertainties in the fluids composition are magnified at high temperature and pressure and quantitative information regarding fluid-mineral equilibria cannot be retrieved from the fluid. Although changes in activity ratios for a given fluid may be insensitive to the initial pH, fluid composition must be precisely determined for fluids where activity ratios are independent of temperature and pressure to precisely determine the temperature at which a given assemblage equilibrates. Activity ratios in acid sulfate-chloride solutions are a strong function of temperature and pressure and are sensitive to the initial fluid composition. Therefore, quantitative information cannot be obtained from these fluids. Activity ratios computed for mid-ocean ridge hydrothermal vent fluids and alkali chloride solutions are not sensitive to the initial pH, and can provide quantitative information regarding subsurface fluid-mineral equilibria. Analysis of mid-ocean ridge fluids in the H₂SO₄-H₂S-FeO-H₂O-HCl system suggests these fluids are only slightly undersaturated with anhydrite at vent conditions. Heating these fluids will move them into the magnetite stability field, while cooling moves them into the pyrite and pyrrhotite stability fields.