EXPERIMENTAL FORMATION AND RAMAN MICROPROBE ANALYSIS OF NA₂SO₄-BEARING SYNTHETIC AQUEOUS FLUID INCLUSIONS IN HALITE

Sulfur is a major constituent of many hydrothermal mineral deposits. As a result, the form and concentration of sulfur in hydrothermal fluids play a crucial role in how these deposits originate but unfortunately are commonly not well known. Raman spectroscopy is a powerful tool for identifying aqueous sulfur species in samples of hydrothermal fluids that have been trapped as fluid inclusions in minerals. In addition, when referenced to standard solutions of known composition, the relative intensities of peaks in the Raman spectra of a fluid inclusion analysis can be used to determine absolute concentrations of species in the fluid inclusion. The present study represents an attempt to generate standard solutions of variable but known sulfate concentration that have been trapped as synthetic fluid inclusions in halite that would be useful for calibrating Raman sulfate spectra in natural NaCl-rich fluids.

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 SO₄^2-, 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.