Dissolved Cu isotope compositions in hydrothermal plumes over back-arc volcanoes in the Northeast Lau Basin, Southwest Pacific Ocean

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Seafloor hydrothermal venting may be an important source of marine Cu and affect the biogeochemical cycling of Cu in the oceans. The distribution of Cu and its isotope compositions (δ65Cu) can provide insight into seafloor hydrothermal processes and their role in the mass balance of global Cu. To date, there are no published Cu isotope data for hydrothermal plumes and very few reports on Cu concentration distributions. This study presents both Cu concentrations and dissolved Cu isotope (δ65Cu) in hydrothermal plumes from back-arc volcanoes in the Northeast Lau Basin. The dissolved Cu (dCu) concentrations range from 2.14 to 5.66 nM most of which are higher than the deep seawater concentrations. However, the concentrations for some plume samples were lower than the deep seawater dCu concentrations due to adsorption onto particles in the plumes. The δ65Cu of hydrothermal plumes vary from 0.25 to 1.05 ‰. As plumes dispersed, the Cu isotopes from high-temperature Mata Fitu and Mata Ua vents shifted towards light values. In contrast, the δ65Cu in plumes from low-temperature East Mata and West Mata vents show increasingly higher values with plume dispersal. Rayleigh distillation models are built based on the adsorption of dCu onto Fe particles and complexation with organic ligands to describe the dCu isotope evolution in hydrothermal plumes. The results suggest that the adsorption and organic complexation may be the likely explanations for the observed dCu isotope compositions in plumes from high- and low-temperature venting, separately, besides the mixing with background seawater. Our measured δ65Cu in three fluid samples (0.08, 0.09 and 0.20 ‰) are lower than that of the characterized sinks (∼0.3 ‰) in the oceans, indicating that hydrothermal fluids might be a source of light Cu isotopes. However, the δ65Cu (0.53 ‰ on average) in plume samples are higher than 0.3 ‰ and these seafloor hydrothermal plumes do not seem to be a source of light Cu isotope of dCu, at least near the venting sites. Our study first reveals the Cu isotope compositions and evolution in hydrothermal plumes and provides new hints about the impact of seafloor hydrothermal venting on the modern oceanic Cu isotope budget.