Multiple sulfur isotope fractionation and mass transfer processes during pyrite precipitation and recrystallization: An experimental study at 300 and 350 °C

Equilibrium multiple sulfur isotope fractionation factors (S-33/S-32 and S-34/S-32) between aqueous SO4, H2S, and coexisting pyrite under hydrothermal conditions were determined experimentally at 300-350 degrees C and 500 bars. Two different experimental techniques were used to determine the fractionation factors and the rate of S isotope exchange between pyrite and constituent aqueous species, H2S and SO4; (1) closed system gold capsule pyrite-H2S exchange experiments and (2) complimentary time-series experiments at 300 and 350 degrees C, 500 bars using flexible gold cell hydrothermal equipment, which allowed monitoring the multiple S isotope composition of dissolved S species during pyrite precipitation and subsequent recrystallization. The three isotope technique was applied to the multiple S isotope data to demonstrate equilibrium S isotope fractionation between pyrite and H2S. Results at 350 degrees C indicate ln(34)alpha(Pyrite/H2S) = -1.9 parts per thousand and ln(33)alpha P-yrite/H2S = -1.0 parts per thousand. The ln(34)alpha(Pyrit/H2S) is not only different in magnitude but also in sign from the commonly used value of 1 parts per thousand from Ohmoto and Rye (1979). This experimental study also demonstrated initial S isotope disequilibrium amongst the aqueous S-species and pyrite during rapid precipitation, despite aqueous speciation indicating pyrite saturation at all stages. Textural, crystallo-graphic, and S isotope interpretations suggest that pyrite formed by means of the FeS pathway. The initial S isotope disequilibrium between formed pyrite and dissolved S-species was effectively erased and approached isotopic equilibrium upon recrystallization during the course of 4297 h. Interpretation of seafloor hydrothermal vent sulfides using the revised equilibrium S-34/S-32 fractionation between pyrite and H2S suggests that pyrite is close to S isotope equilibrium with vent H2S, contrary to previous conclusions. The experimental data reported here broaden the range of pyrite formation mechanisms at seafloor hydrothermal vents, in that mineral formation pathway and equilibration rates need to be considered to account for the well-recognized S isotope variability that often characterizes these systems. Published by Elsevier Ltd.