An experimental study of phase equilibria in the system H2O-CO2-NaCl at 800 degrees C and 9 kbar

Phase equilibria in the ternary system H2O-CO2-NaCl were studied at 800 deg rees C and 9 kbar in internally heated gas pressure vessels using a modifie d synthetic fluid inclusion technique. The low rate of quartz overgrowth al ong the 'b' and 'a' axes of quartz crystals was used to avoid fluid inclusi on formation during heating, prior to attainment of equilibrium run conditi ons. The density of CO2 in the synthetic fluid inclusions was calibrated us ing inclusions in the binary H2O-CO2 system synthesised by the same method and measured on the same heating-freezing stage. In the two-phase field, tw o types of fluid inclusions with different densities of CO2 were observed. Using mass balance calculations, these inclusions are used to constrain the miscibility gap and the orientation of two-phase tie-lines in the H2O-CO2- NaCl system at 800 degrees C and 9 kbar. The equation of state of Duan et a l. (1995) approximately describes the P-T section of the ternary system up to about 40 wt% of NaCl. At higher NaCl concentrations the measured solubil ity of CO2 in the brine is much smaller than predicted by the EOS. A "salti ng out" effect must be added to the equation of state to include coulomb in teraction in the model of Anderko and Pitzer (1993) and Pitzer and Jiang (1 996). The new experimental data together with published data up to 5 kbar ( Shmulovich et al. 1995) encompass practically all subsolidus crustal P-T co nditions. A feature of the new experimental results is the large compositio nal range in the H2O-CO2-NaCl system occupied by the stability fields of ha lite + CO2-rich fluid +/- H2O-NaCl brine. The prediction of halite stabilit y in equilibrium with CO2-rich fluid in deep-crustal rocks is supported by recent petrological and fluid inclusion studies of granulites.