Sulfur isotope partitioning during experimental formation of pyrite via the polysulfide and hydrogen sulfide pathways: implications for the interpretation of sedimentary and hydrothermal pyrite isotope records

We show that the sulfur isotopic composition of sedimentary and hydrothermal pyrite is a good approximation of the average sulfur isotopic composition of the dissolved sulfide sources from which the pyrite formed. Consequently, pyrite sulfur isotope systematics normally provide little evidence of the pyrite-forming mechanism in most natural systems. Stable sulfur isotope partitioning during pyrite (FeS2) synthesis via the polysulfide and H2S pathways was investigated between 80 and 120 °C. Iron monosulfide (FeS) was reacted with hydrogen sulfide (H2S) or tetrasulfide (S42−) in aqueous solution under strictly anoxic conditions. The results provide independent confirmation of the hydrogen sulfide and polysulfide mechanisms. The measured isotopic composition of the synthesized pyrite is compared with (1) isotopic mixing models of the reactant reservoirs and (2) predictions based on the suggested mechanisms for the hydrogen sulfide and polysulfide pathways for pyrite formation. The isotopic composition of the pyrite product is consistent with the result predicted from the reaction mechanisms. Pyrite produced via the H2S pathway has a composition reflecting isotopic contributions from both FeS and H2S reservoirs. Pyrite formed via the polysulfide pathway inherits an isotopic composition dominated by the polysulfide reservoir. In both cases, solubility driven isotope exchange between FeS and aqueous S species contribute to the final pyrite composition. We show that published experimental sulfur isotope data for pyrite formation which apparently support conflicting pyrite-forming pathways, are consistent with pyritization via the polysulfide and H2S pathways. Formation rates of natural pyrite, however, may be too slow compared to solubility exchange for the influence of the reaction pathway on the isotopic composition to be significant.