Iron isotope fractionation during sulfide-promoted reductive dissolution of iron (oxyhydr)oxide minerals

Abstract

Iron isotopes are a valuable tool for evaluating processes that control Fe redox cycling in modern and ancient environmental settings. However, robust evaluation of Fe isotope compositions in natural samples requires that fractionations associated with key (bio)geochemical reactions are well-defined. The reductive dissolution of Fe (oxyhydr)oxide minerals mediated by dissolved sulfide exerts a major influence on solid phase Fe mineralogy and dissolved porewater Fe profiles during early diagenesis of organic-rich sediments, but to date, no studies have investigated Fe isotope fractionations during this process. Here, we report the results of laboratory sulfidation experiments, examining apparent Fe isotope fractionations for a variety of Fe (oxyhydr)oxide minerals. The iron isotope compositions of reaction products were determined for both the reduction-dominated and dissolution-dominated steps of the reaction. The reductive step for lepidocrocite and hematite produced Fe(II) that was up to 0.25 ‰ heavier than the bulk starting mineral. By contrast, the reduction of ferrihydrite produced isotopically light Fe(II), with isotope compositions −0.1 to −0.6 ‰ lower than the initial mineral. Consistent with previous studies of the reductive dissolution of Fe (oxyhydr)oxide minerals via abiological and biological pathways, the lighter isotope was preferentially released from the mineral surface during the dissolution phase for all minerals, with dissolved Fe2+ isotope compositions up to ∼2.0 ‰ lower than the surface bound Fe(II). The magnitude of isotopic fractionation during both of these steps is directly related to rates of reaction, and is thus controlled by factors such as sulfide concentration, mineral concentration, crystal structure, surface area and pH. Our data demonstrate that dissolved Fe2+ with δ56Fe compositions approaching −1.0 ‰ is readily generated during the overall reaction, suggesting that sulfide-promoted reductive dissolution of Fe (oxyhydr)oxide minerals may contribute significantly to the generation of light Fe isotope compositions in anoxic settings.