Geochemistry and iron isotope systematics of hydrothermal plume fall-out at East Pacific Rise 9°50′N

While gross hydrothermal fluxes entering the ocean are known to be significant, much remains unknown about the fate of this material as it disperses through the oceans, and its impact upon ocean biogeochemistry. Mineral precipitation within hydrothermal plumes removes hydrothermally-sourced metals from solution and also acts to scavenge trace elements from the surrounding water column. Here, we investigate the fate of particulate Fe released from high-temperature hydrothermal venting at EPR 9°50′N and its potential impact on local deep-ocean Fe-isotopic and geochemical budgets. We measured the geochemical composition, mineralogy and Fe isotope systematics of hydrothermal plume products in order to determine whether mineral precipitation imposes characteristic Fe-isotope “fingerprints” for hydrothermally sourced Fe in the deep ocean. Our sampling includes sediment trap deployments after the eruptive event of Jan. 2006, allowing the examination of temporal changes of hydrothermal fluxes over a 160 day period. Results show that Fe isotope composition in the high-temperature vent fluids is rather constant over the sampling period 2004–2008, and that secular variations of δ⁵⁶Fe values of plume particles from − 0.03 to − 0.91‰ (relative to IRMM-14 standard) could be explained by local processes leading to variable mixing extents of hydrothermal, biogenic and lithogenic particles. Through geochemical modeling, we have calculated the relative abundances of hydrothermal plume components such as sulfides, Fe oxyhydroxides, organic matter, biogenic and lithogenic phases. We demonstrate that Fe isotope fractionation in the hydrothermal plume occurs during the formation and rapid settling of Fe-sulfides that are characterized by δ⁵⁶Fe values ranging from − 0.73 ± 0.13‰ to − 0.86 ± 0.13‰, which is systematically lower than the end-member hydrothermal fluids (δ⁵⁶Fe = − 0.4‰). This study suggests that both the initial Fe isotope composition of the high-temperature vent fluids and its initial Fe/H₂S ratio (i.e. Fe-sulfide precipitation versus Fe-oxyhydroxide precipitation) should impose characteristic Fe isotope “fingerprints” for hydrothermally derived Fe in the deep ocean.