AN EXPERIMENTAL INVESTIGATION OF THE EFFECT OF CO2 ON NACL SOLUBILITY IN HYDROTHERMAL SOLUTIONS USING SYNTHETIC FLUID INCLUSIONS

The purpose of this paper is to evaluate the temperature dependence of the effect of CO2 on NaCl solubility in hydrothermal solutions. Many geochemists have reported sodium chloride solubility in water. However, the effect of CO2 has rarely been considered, even though most natural hydrothermal fluids have a composition of H2O-NaCl-CO2 system. Recently, Schmidt et al. (1995) have suggested that 5 mol % CO2 lower NaCl solubility by 1 weight percent around 320°C, but its temperature dependence have not been considered. This study may contribute to improve the equations of state for the system H2O-NaCl-CO2, and describe hydrothermal system more precisely. Synthetic fluid inclusion proposed by Sterner and Bodnar (1984) was chosen for experimental technique. Fluid inclusions were synthesized in pre-fractured quartz core. Experimental solution contains 30-40 wt % NaCl and 5 mol % CO2. Silver oxalate (Ag2C2O4) was used as a CO2 source. The core, silica powder, and starting materials were loaded into a gold capsule and held for 7-14 days. After experiment, the core was sectioned, and halite dissolution temperatures (TmNaCl) of synthesized inclusions were measured using heating-freezing stage. Measured TmNaCl gave solubility of NaCl. A total of NaCl solubility experiments was successfully completed. A solubility curve of halite for 160-320°C was obtained in CO2-bearing brine. It shows that halite solubility in CO2-bearing water was about one percent lower than that in CO2-free water (Sterner et al., 1988). This study determined TmNaCl as 331.4±6.6°C for 40 wt % NaCl and 5 mol % CO2- bearing aqueous fluid. Our data is in close agreement with that of Schmidt et al. (1995). Schmidt and Bodnar (2000) have suggested that halite becomes about 1 wt % less soluble in 10 mol % CO2-bearing water than that of 5 mol %. Our result indicates that solubility of NaCl goes down further as an increase in CO2 concentration.