| Summary: | Multiple sulfur isotope systematics of geothermal fluids at Krafla NE Iceland were studied in order to determine the source and reactions of sulfur in the systems. Fluid temperatures ranged from 192 to 437°C with liquid water, vapor and superheated vapor being present in the reservoir. Dissolved sulfide (ΣS-II) and SO4 predominated in the water phase with trace concentrations of S2O3 whereas H2S was the only species observed in the vapor phase. The reconstructed sulfur isotope ratios of the reservoir fluids based on samples collected at surface from two-phase well discharges indicate that δ34S and Δ33S of sulfide in the reservoir water and vapor are in the ranges of -1.45 to +1.13 ‰ and -0.017 to -0.001 ‰ whereas the δ34S and Δ33S ranges of sulfate are significantly different or +3.40 to +13.37 ‰ and 0.000 to -0.036 ‰, respectively. Depressurisation boiling upon fluid ascent and progressive fluid-rock interaction and sulfide mineral (pyrite) formation results in the liquid phase becoming progressively isotopically lighter both with respect to δ34S and Δ33S. In contrast, the H2S in the vapor phase and pyrite formed become isotopically heavier. The observed Δ33S and δ34S systematics for geothermal fluids at Krafla suggest that the source of sulfides in the fluids is the basaltic magma, either through degassing or upon dissolution of unaltered basalts. At high temperatures, insignificant SO4 is observed in the fluids but below ~230°C significant concentrations of SO4 are observed, the source considered to be H2S oxidation. Sulfate originated from the meteoric source water of the geothermal fluids is, however, considered to be negligible. The key factors controlling the multiple sulfur isotope systematics of geothermal fluids are the isotope composition of the source material, and isotope fractionation associated with aqueous and vapor speciation and how these change as a function of processes occurring in the system such as boiling, oxidation and fluid-rock interaction. Stöðugar brennisteinssamsætur í ...
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