Processes affecting the isotopic composition of dissolved iron in hydrothermal plumes: a case study from the Vanuatu back-arc

Hydrothermal discharge is one of multiple Fe sources contributing to the marine dissolved Fe budget, but its relative importance remains controversial and largely unconstrained. Isotopic characterization of Fe sources and mass balance calculations are considered as a potential tool to unravel the complex interplay of various Fe fluxes, but require a more profound knowledge of the processes that fractionate Fe isotopes in hydrothermal plumes. We present dissolved Fe isotope data from two buoyant plumes and profiles traversing the non-buoyant plume above the Nifonea vent field in the Vanuatu back-arc. Our findings support opposing controls from precipitation of Fe-sulfides and Fe-oxyhydroxides that either enrich the heavy or the light isotopes within the residual dissolved Fe fraction, but also suggest significant influence from size partitioning of oxidized Fe following precipitation as well as potential effects from organic complexation. At low aggregation of precipitated Fe-oxyhydroxides or stabilization of significant Fe(III)_aq dissolved δ⁵⁶Fe is only marginally affected by Fe(II) oxidation, which strengthens the role of Fe-sulfide precipitation during early plume rise in determining the isotope signature of hydrothermal Fe exported to the open ocean. Dissolved δ⁵⁶Fe range from − 0.73 ± 0.05‰ to − 0.16 ± 0.05‰, and suggest that sulfide-rich basalt-hosted hydrothermal systems and slow oxidation kinetics in the Pacific favor export of stabilized dissolved Fe with heavier δ⁵⁶Fe values than the true vent fluids (δ⁵⁶Fe of ∼− 0.5‰).