UNCHARTED SINKS WITHIN THE MARINE RE AND OS CYCLE: ADSORPTION BY HYDROTHERMAL PLUMES

Osmium (Os) and rhenium (Re) are redox-sensitive, siderophilic (iron loving), and respond acutely to environmental changes and global geologic events [1], as reflected by the elemental and isotopic composition of Os and Re preserved in the geologic record. We investigated the geochemical behavior of dissolved Os (dOs) and Re (dRe) during the formation of hydrothermal plumes when seawater mixes with high-temperature fluids venting from Earth’s mid-ocean ridge (MOR) system. The experiment examined the adsorption of dOs and dRe onto hematite (Fe₂O₃), which is ubiquitous to hydrothermal metalliferous sediments and serves as a proxy for Fe-oxy-hydroxide minerals [2] formed within plumes on- and off-axis of MOR hydrothermal environments. We employed a distinct Os- and Re-bearing synthetic seawater solution to quantify the rate and degree of dOs and dRe adsorption onto hematite at pH conditions relevant to modern Earth, ~8.0. Time-series samples of solution were recovered to monitor the change in dOs and dRe concentration as hematite and solution approached adsorption equilibrium. Subsequently, an enriched ¹⁹⁰Os-bearing solution was introduced to test for elemental exchange between adsorbed-Os and dOs. Experimental results revealed a high affinity for hematite to adsorb and effectively remove dOs derived from seawater and vent fluids, but not dRe. We are awaiting results from the dissolved ¹⁹⁰Os-tracer added to the experimental solution that would indicate whether dOs exchanges with adsorbed-Os after hematite-fluid adsorption equilibrium has been reached. Our novel results indicate Fe-oxide minerals forming within metalliferous sediments and hydrothermal plumes can function as a significant sink in the marine Os cycle, and that the magnitude of this Os sink is likely sensitive to changes in ocean chemistry, especially dissolved oxygen, throughout Earth’s history [3].

1. Sharma, M., 2012. Applications of Os and Ir as Biogeochemical Tracers in the Environment. Handbook of Environmental Isotope Geochemistry 1, 205-227.
2. Goldberg, T., Archer, C., Vance, D., Poulton, S.W., 2009. Mo isotope fractionation during adsorption to Fe (oxyhydr)oxides. Geochimica et Cosmochimica Acta 73, 6502-6516.
3. Peucker-Ehrenbrink, B., Ravizza, G., 2000. The marine osmium isotope record. Terra Nova 12, 205-219.