Experimental partitioning of osmium between pyrite and fluid: Constraints on the mid-ocean ridge hydrothermal flux of osmium to seawater

This study presents experimental, geochemical modeling, and field data focusing on the partitioning of osmium (Os) between pyrite and fluid upon precipitation at conditions representative of mid-ocean ridge hydrothermal environments. Dissolved Os partitions strongly into pyrite upon precipitation at experimental conditions, 350 °C and 50 MPa, with a representative relative Os/Fe partition coefficient, DPyrite-FluidOs/Fe, between 10 and 15. Integrating the experimentally determined DPyrite-FluidOs/Fe into a geochemical model indicates that a significant amount of Os is retained within the subseafloor due to sulfide precipitation induced by mixing of conductively heated seawater with pristine high temperature hydrothermal fluids that are enriched in dissolved metals, such as Os and Fe. Comparison with existing Os concentration and isotopic data of hydrothermal fluids and sulfide minerals from a wide range of hydrothermal systems with the experimental and modeling constraints suggest that modern high temperature hydrothermal systems are a minor source of unradiogenic Os to the modern global ocean dissolved Os budget due to the majority of Os being sequestered into sulfide minerals formed within and on the seafloor.