Synthetic fluid inclusions: XII. The system H 2ONaCl. Experimental determination of the halite liquidus and isochores for a 40 wt% NaCl solution

Abstract

The slopes of the liquidus and lines of constant liquid-vapor homogenization temperature (iso-Th) in P- T space for a 40 wt% NaCl bulk composition in the H 2O-NaCl system were determined using synthetic fluid inclusions. Inclusions were synthesized in the one-phase field at 350°–800°C and 1–6 kbar, and the temperatures of liquid-vapor homogenization and halite dissolution were determined on a heating/cooling stage. The pressure along the liquidus corresponding to a measured halite dissolution temperature [Tm (halite)] was determined from the intersection of the inclusion iso-Th line in the one-phase field with the measured Tm (halite). The P- T position of the iso-Th line in the one-phase field was determined from the measured liquid-vapor homogenization temperature [Th ( L- V)] of a given inclusion and the known experimental formation temperature and pressure for that same inclusion. The slope of the 40 wt% halite liquidus determined from synthetic fluid inclusions is −5.5°C/kbar. This value is in good agreement with an extrapolation of the data of Gunter et al. (1983) and cooling data of Koster van Groos (1991) to lower salinities, but shows considerable disagreement with the extrapolated heating data of Koster van Groos (1991). Over the salinity range 27–100 wt% NaCl, the slope of the halite liquidus predicted by the results from this study combined with data of Milne (1978), Gunter et al. (1983), and cooling data of Koster Van Groos (1991) is given by dT dP (°C/kbar) = − 38.38 + 0.90S − 0.0029S 2 , where “ S” is the salinity in wt% NaCl. Microthermometric data combined with the known temperatures and pressures of formation were used to construct lines of constant liquid-vapor homogenization temperature (iso-Th) for fluid inclusions with a salinity of 40 wt% NaCl. The slopes of the iso-Th lines for inclusions which homogenize by vapor-bubble disappearance decrease smoothly with increasing Th, from 14.1 bars/°C for inclusions which homogenize at 323°C to 5.52 bars/°C for inclusions with homogenization temperatures of 650°C. For inclusions which homogenize by halite dissolution (Th < 323°C), the slopes of the iso-Th lines within the one-phase liquid field decrease with increasing Th ( L- V) from 21.5 bars/°C (150°C) to 14.1 bars/ °C (323°C). The average slopes of the iso-Th lines within the liquid + halite field are much steeper than those in the one phase field, and decrease from 24.8 bars/°C at 150°C to 20.3 bars/°C at 300°C.