H2O activity in concentrated NaCl solutions at high pressures and temperatures measured by the brucite-periclase equilibrium

H2O activities in concentrated NaCl solutions were measured in the ranges 600 degrees-900 degrees C and 2-15 kbar and at NaCl concentrations up to halite saturation by depression of the brucite (Mg(OH)(2)) - periclase (MgO) dehydration equilibrium. Experiments were made in internally heated Ar pressure apparatus at 2 and 4.2 kbar and in 1.91-cm-diameter piston-cylinder apparatus with NaCl pressure medium at 4.2, 7, 10 and 15 kbar. Fluid compositions in equilibrium with brucite and periclase were reversed to closures of less than 2 mol% by measuring weight changes after drying of punctured Pt capsules. Brucite-periclase equilibrium in the binary system was redetermined using coarsely crystalline synthetic brucite and periclase to inhibit back-reaction in quenching. These data lead to a linear expression for the standard Gibbs free energy of the brucite dehydration reaction in the experimental temperature range: Delta G degrees (+/-120J)=73418-134.95T(K). Using this function as a baseline, the experimental dehydration points in the system MgO-H2O-NaCl lead to a simple systematic relationship of high-temperature H2O activity in NaCl solution. At low pressure and low fluid densities near 2 kbar the H2O activity is closely approximated by its mole fraction. At pressures of 10 kbar and greater, with fluid densities approaching those of condensed H2O, the H2O activity becomes nearly equal to the square of its mole fraction. Isobaric halite saturation points terminating the univariant brucite-periclase curves were determined at each experimental pressure. The five temperature-composition points in the system NaCl-H2O are in close agreement with the halite saturation curves (liquidus curves) given by existing data from differential thermal analysis to 6 kbar. Solubility of MgO in the vapor phase near halite saturation is much less than one mole percent and could not have influenced our determinations. Activity concentration relations in the experimental P-T range may be retrieved for the binary system H2O-NaCl from our brucite-periclase data and from halite liquidus data with minor extrapolation. At two kbar, solutions closely approach an ideal gas mixture, whereas at 10 kbar and above the solutions closely approximate an ideal fused salt mixture, where the activities of H2O and NaCl correspond to an ideal activity formulation. This profound pressure-induced change of state may be characterized by the activity (a) - concentration (X) expression: a(H2O)=X(H2O)/(1+alpha X(NaCl)), and a(NaCl)=(1+alpha)(1+alpha)[X(NaCl)/(1+alpha X(NaCl))]((1+alpha)). The parameter alpha is determined by regression of the brucite-periclase H2O activity data: alpha=exp[A-B/rho(H2O)]-CP/T, where A=4.226, B=2.9605, C=164.984, and P is in kbar, Tis in Kelvins, and rho(H2O) is the density of H2O at given P and T in g/cm(3). These formulas reproduce both the H2O activity data and the NaCl activity data with a standard deviation of +/-0.010. The thermodynamic behavior of concentrated NaCl solutions at high temperature and pressure is thus much simpler than portrayed by extended Debye-Huckel theory. The low H2O activity at high pressures in concentrated supercritical NaCl solutions (or hydrosaline melts) indicates that such solutions should be feasible as chemically active fluids capable of coexisting with solid rocks and silicate liquids (and a CO2-rich vapor) in many processes of deep crustal and upper mantle metamorphism and metasomatism.