Experimental Determination of Phase Equilibria in the System H{sub 2}O-CO{sub 2}-NaCl at 0.5 Kb from 500 to 800C

An understanding of activity-composition (a/X) relations and phase equilibria for halite-bearing, mixed-species supercritical fluids is critically important to many geological and industrial applications. The authors have performed experiments on the phase equilibria of H{sub 2}O-CO{sub 2}-NaCl fluids from 500 C to 800 C at 500 bars, conditions of significant importance in studies of magma-hydrothermal systems, geothermal reservoirs and some ore deposits, to obtain highly accurate and precise data for this ternary system. These experiments are conducted using a double capsule technique. An excess of NaCl is placed in an inner Pt capsule, which is crimped shut and placed in an outer capsule containing H{sub 2}O and CO{sub 2}. During the experiment NaCl dissolves out of the inner capsule, and is deposited in the outer capsule during the quench. After the experiment the capsule is opened, and the amount of NaCl remaining in the inner capsule determined by dissolution. The difference between the initial and final amounts of NaCl in the inner capsule yields the solubility of NaCl at the P-T conditions of the experiment. At 500 C data from these experiments suggest that the vapor comer of the three-phase field lies near X(H{sub 2}O) = 0.760, X(NaCl) = 0.065, which is a significantly more water-rich composition than suggested by previous models. As expected, increasing temperature increases the solubility of NaCl in the NaCl-vapor field. For example, at intermediate H{sub 2}O/CO{sub 2} ratios the vapor field extends from approximately near X(H{sub 2}O) = 0.66, X(NaCl) = 0.06 at 500 C to near X(H{sub 2}O) = 0.65, X(NaCl) = 0.08 at 600 C