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. We have performed experiments on the phase equilibria of H₂O-CO₂-NaCl fluids from 500°C to 700°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 H₂O and CO₂. During the experiment NaCl dissolves out of the inner capsule, and is deposited in the outer capsule during quenching. 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. Test experiments reproduced the known values on the H₂O-NaCl join. At 500°C data from these experiments suggest that the vapor corner of the three-phase field lies near X(H₂O)=0.740, X(NaCl)=0.06, and at 600°C this corner is near X(H₂O)=0.770, X(NaCl)=0.143.

Research sponsored by the National Science Foundation, and by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy under contract DE-AC05-00OR22725, Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC .