Synthetic fluid inclusions XXI. Partitioning of Na and K between liquid and vapor in the H2O-NaCl-KCl system at 600-800 °C and 500-1000 bars

The synthetic fluid inclusion technique was used to determine the partitioning behavior (distribution constant) of Na and K between coexisting liquid and vapor phases in the H2O-NaCl-KCl system at 500-1000 bars, 600-800 °C, and for fluids with initial Na/K molar ratios of 3:1, 2:1, 1:1, 1:2 and 1:3. Compositions of coexisting liquid and vapor phases in the binary H2O-KCl system were also determined. Distribution constants for sodium (DNaL/V) and potassium (DKL/V) were calculated from salinities of the synthetic fluid inclusions determined by microthermometry. For a given temperature, pressure and molar Na/K ratio, the distribution constants for Na and K between liquid and vapor are essentially equal, with only a slight preference for Na to partition into the vapor phase and for K to partition into the liquid phase. As such, fluid phase immiscibility and the subsequent physical separation of the liquid and vapor phases owing to buoyancy differences are unlikely to lead to fluids with variable Na/K in different parts of magmatic-hydrothermal systems. The liquid-vapor partitioning behavior obtained here is consistent with previously determined Na/K partitioning between granitic melts and exsolved magmatic aqueous fluids, and with compositions of hydrothermal fluids and associated feldspars in potassic alteration zones of porphyry copper deposits.