Multiple sulfur isotope fractionation and mass transfer processes during pyrite precipitation and recrystallization: An experimental study at 300 and 350 °C. (15th September 2015) - Syverson, Drew D.
Abstract: Equilibrium multiple sulfur isotope fractionation factors ( 33 S/ 32 S and 34 S/ 32 S) between aqueous SO4, H2 S, and coexisting pyrite under hydrothermal conditions were determined experimentally at 300–350 °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, H2 S and SO4 ; (1) closed system gold capsule pyrite−H2 S exchange experiments and (2) complimentary time-series experiments at 300 and 350 °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 H2 S. Results at 350 °C indicate ln 34 α Pyrite / H 2 S = −1.9‰ and ln 33 α Pyrite / H 2 S = −1.0‰. The ln 34 α Pyrite / H 2 S is not only different in magnitude but also in sign from the commonly used value of 1‰ 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, crystallographic, 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 34 S/ 32 S fractionation between pyrite and H2 S suggests that pyrite is close to S isotope equilibrium with vent H2 S, 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.