Experimental investigation of talc solubility in H 2O-MgCl 2-NaCl-HCl fluids in the range 500-700°C, 2 kb
The equilibrium solubility of the talc-quartz mineral assemblage in H 2O-MgCl 2-NaCl-HCl fluids at 2 kb and temperatures in the range 500-700°C has been determined using rapid-quench hydrothermal techniques. Nacl concentrations from 0.0 m to approximately 6 m were utilized. The Ag-AgCl acid buffer technique was used to control the activity of the aqueous HCl o species in the fluid at elevated temperature and pressure. At 500°C, the concentration of Mg measured in the fluid after quench decreased in a systematic, nonlinear fashion with increasing concentrations of NaCl, from 2.8 m Mg in NaCl-free solutions to 1.5 m in 5.1 m NaCl. In contrast, at 600 and 700°C Mg molalities increased with increasing NaCl molalities. At 600°C, the concentration of Mg increased from 0.072 to 0.12 m as the concentration of NaCl increased from 0.0 to 6.1 m. At 700°C, Mg molality increased from an NaCl-free value of 0.0078 m, to 0.031 m in 6.3 m NaCl. The observed solubilities were compared to concentrations of aqueous species calculated by solving the series of mass-action and mass-balance equations that describe the activities and molalities of the relevant species in the fluid. It is concluded that at 500°C, MgCl o2, MgCl +, and Mg 2+ are the only Mgspecies necessary to account for the observed behavior. The calculations suggest that the decrease in Mg results from both the increasing concentrations of Cl - as NaCl concentration increases, and the changing dielectric properties of the fluid. At 600 and 700°C, the presence of Mg-species in addition to MgCl 20, MgCl +, and Mg 2+ may be required to describe the increased solubility observed in the experiments. Use of the slope-quantification method for identifying aqueous species (e.g., SEWARD, 1976) indicates that higher order Mg-Cl species or Na-Mg-Cl species are possible, but the precision of the technique is not sufficient to uniquely identify the species.