An experimental determination of the thermodynamic properties of H 2O-CO 2-NaCl fluids at high pressures and temperatures
Understanding the role of fluids in geological processes requires a knowledge of the mixing properties of common geological fluids. A technique for experimentally determining activity coefficients for H 2O in fluid mixtures at pressures and temperatures accessible to the internally heated pressure vessel (10 kbar, 1100°C) is presented which uses a new fH2O buffer. The fH2O buffer involves the simultaneous control of fO2 (with Ni/NiO) and fH2 (with a hydrogen membrane), thereby buffering fH2O (or equivalently, aH2O through the reaction H 2 + 1/2O 2 = H 2O . After quenching the run, the sample is analyzed to determine the XH2O and, thus, the activity-composition relation for H 2O ( aH2O- XH2O). Reversals are obtained by simultaneously running capsules loaded with low and high XH2O, thereby allowing the equilibrium XH2O to be approached from both directions. Several experimental problems encountered during this study and their implications for the precise control of fH2 in hydrothermal experiments are discussed. Data were collected primarily in the H 2O-CO 2 system at 2 kbars, 550-850°C, and at 4 kbars, 650°C. At those conditions, the H 2O-CO 2 system generally shows small positive deviations from ideality. These results are in good agreement with the predictions of the MRK equation of state of HOLLWAY (1977) at low T ( T = 550° C) and are in good agreement with the predictions of the MRK equation of state of KERRICK And JACOBS (1981) at higher temperatures. At two conditions (2 kbar, 750°C; 4 kbar, 650°C), negative deviations from ideality are indicated at high XH2O. Some data were also collected in the H 2O-NaCl and H 2O-CO 2-NaCl systems. In the former, H 2O shows small positive deviations from ideality at 2 kbars, 850°C, which increase with decreasing XH2O. In the H 2O-CO 2-NaCl system, isofugacity lines for H 2O were measured. In the two-phase region, these lines represent tie lines and, thus can be used to constrain the phase relations in the system.