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 ran ges 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)) - pe riclase (MgO) dehydration equilibrium. Experiments were made in intern ally heated Ar pressure apparatus at 2 and 4.2 kbar and in 1.91-cm-dia meter piston-cylinder apparatus with NaCl pressure medium at 4.2, 7, 1 0 and 15 kbar. Fluid compositions in equilibrium with brucite and peri clase were reversed to closures of less than 2 mol% by measuring weigh t changes after drying of punctured Pt capsules. Brucite-periclase equ ilibrium in the binary system was redetermined using coarsely crystall ine synthetic brucite and periclase to inhibit back-reaction in quench ing. These data lead to a linear expression for the standard Gibbs fre e energy of the brucite dehydration reaction in the experimental tempe rature range: Delta G degrees (+/-120J)=73418-134.95T(K). Using this f unction as a baseline, the experimental dehydration points in the syst em MgO-H2O-NaCl lead to a simple systematic relationship of high-tempe rature H2O activity in NaCl solution. At low pressure and low fluid de nsities near 2 kbar the H2O activity is closely approximated by its mo le fraction. At pressures of 10 kbar and greater, with fluid densities approaching those of condensed H2O, the H2O activity becomes nearly e qual to the square of its mole fraction. Isobaric halite saturation po ints terminating the univariant brucite-periclase curves were determin ed at each experimental pressure. The five temperature-composition poi nts in the system NaCl-H2O are in close agreement with the halite satu ration curves (liquidus curves) given by existing data from differenti al thermal analysis to 6 kbar. Solubility of MgO in the vapor phase ne ar halite saturation is much less than one mole percent and could not have influenced our determinations. Activity concentration relations i n the experimental P-T range may be retrieved for the binary system H2 O-NaCl from our brucite-periclase data and from halite liquidus data w ith minor extrapolation. At two kbar, solutions closely approach an id eal gas mixture, whereas at 10 kbar and above the solutions closely ap proximate an ideal fused salt mixture, where the activities of H2O and NaCl correspond to an ideal activity formulation. This profound press ure-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 pa rameter 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 dens ity 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 devia tion of +/-0.010. The thermodynamic behavior of concentrated NaCl solu tions at high temperature and pressure is thus much simpler than portr ayed by extended Debye-Huckel theory. The low H2O activity at high pre ssures in concentrated supercritical NaCl solutions (or hydrosaline me lts) indicates that such solutions should be feasible as chemically ac tive fluids capable of coexisting with solid rocks and silicate liquid s (and a CO2-rich vapor) in many processes of deep crustal and upper m antle metamorphism and metasomatism.