PRELIMINARY EXPERIMENTAL DETERMINATION OF HYDROTHERMAL EXCHANGE OF MULTIPLE S ISOTOPES AT 350°C AND 500 BARS: PYRITE AND NATIVE SULFUR REACTION IN ACIDIC NaCl FLUIDS

Non-traditional stable isotopes and multiple sulfur isotopes are increasingly being applied to understanding hydrothermal and other natural processes, but little or no experimental isotope exchange data exist for either δ³³S or δ⁵⁶Fe fractionation. In particular, ³³S and Fe isotope studies of seafloor hydrothermal vent systems have shown significant fractionation and, for ³³S, slight deviations from mass dependent fractionation have been attributed to differences in chemical or biological processes. Experimental data for ³³S and Fe fractionation at hydrothermal conditions are needed to improve interpretations of processes in these complex systems.

Hydrothermal experiments have been carried out with pyrite and elemental sulfur as starting solid materials in 350°C NaCl fluid. This system was chosen because it produces acidic fluids (pH₂₅ = 1.5-2.0) with H₂S/SO₄ ratios of about 2, and Fe concentration of 2-4.5 mmol/kg so that all S species, dissolved Fe, and solid phases can be isotopically characterized. Under these conditions relatively rapid sulfur isotope exchange is expected over 6-14 day duration of the experiments. Fe isotope fractionation data will be reported separately.

Sulfur isotopic systematics (δ³³S and δ³⁴S) of pyrite, S, SO₄, and H₂S have been determined by isotope ratio mass spectrometry of SF₆ in the stable isotope laboratory at MIT. Results at 350°C indicate that isotopic equilibrium was attained between aqueous sulfate and sulfide after 13.5 days as indicated by δ³⁴S_SO4-H2S = 17.14 permil compared to previous experimental fractionation data of 17.2 permil. Similarly, δ³³S indicates equilibrium sulfate-sulfide fractionation of 8.70 permil and ln(δ³³S+1)/ln(δ³⁴S+1) of 0.514±0.01, which equals the expected mass-dependent fractionation of 0.515. In contrast, sulfur isotope compositions for bulk pyrite changed little due to exchange reactions, but a relatively coarse grain size (≤64µm) was used and we are currently investigating surficial equilibration on grain rims and the possible affects of armoring by native S.