EXPERIMENTAL FORMATION AND RAMAN MICROPROBE ANALYSIS OF NA2SO4-BEARING SYNTHETIC AQUEOUS FLUID INCLUSIONS IN HALITE
The experimental procedure consisted of dissolving Na2SO4 in distilled water to concentrations of 5×10-5 to 0.1 molal, heating the solution to 90° C, and adding halite to the point of saturation. Three sets of solutions of variable Na2SO4 concentration were made at average pH values of 1.5, 4.5, and 6. Speciation calculations carried out using the Geochemist's Workbench software showed that the dominant species in solution would be SO42-, Na2SO40, and HSO4-. To strain out any undissolved halite crystals, the solution was poured through filter paper into a second vessel and allowed to cool for no more than 10 minutes to minimize evaporative concentration. Within this time interval, numerous macroscopic halite crystals precipitated that consistently contained abundant fluid inclusions.
Fluid inclusion composition analyses were carried out using a JY Horiba LabRam HR laser Raman microprobe at Virginia Polytechnic Institute & State University. The Raman v1a1 band for sulfate at 982 cm-1 was quantifiable at concentrations as low as 0.01 molal. The ratio of the height of the signal intensity of this band to the height of the signal intensity of the v2a1 band for water produced strong linear correlations with total dissolved sulfate concentration. These synthetic fluid inclusion standards therefore will be valuable in future studies where sulfate concentration in natural NaCl-rich fluid inclusions needs to be quantified.