Coupled Se and S isotope systematics of sulfides: a novel tool to trace magmatic volatile input into submarine hydrothermal systems

The effect of magmatic volatiles on the metal budget of hydrothermal mineralization often remains unclear and requires novel tools to be developed for a more comprehensive understanding. Here, we present coupled Se and S isotope data combined with trace element micro-analysis of hydrothermal sulfides from the PACMANUS hydrothermal system in the Eastern Manus back-arc basin. PACMANUS includes several dacite-hosted vent fields that occur at a water depth of 1640 to 1740 m, and which discharge fluids of distinct temperatures (6 to 360°C). The sample set includes hydrothermal sulfides from chimneys and talus material sampled by ROV, as well as from the sub-seafloor upflow zone recovered by rock-drill and during ODP Leg 193. The sample set therefore features a vertical geochemical record through a hydrothermal system with variable magmatic volatile input that allows to study the Se isotope fractionation processes and its potential application to trace magmatic volatile influx. The mineralization of the chimneys is characterized by an inner chalcopyrite-dominated lining along the central fluid conduits, which transitions into zones of pyrite, marcasite, chalcopyrite, barite, and anhydrite, and finally into a sphalerite, pyrite, and marcasite assemblage in the outermost chimney wall. High-precision δ^82/76Se and δ³⁴S data coupled with Te/Sb, and Te/As ratios in hydrothermal sulfides - a novel proxy for constraining magmatic volatile influx in subduction-related submarine hydrothermal systems [1] - revealed a positive correlation, implying that magmatic volatile influx is recorded by the Se isotope system. In addition, seawater mixing has little effect on the δ^82/76Se composition of hydrothermal sulfides, which makes it superior for tracing metal sources compared to the S isotope system that often yields ambiguous source signatures due to seawater overprinting [2]. We suggest that Se isotopes can trace the variable input of magmatic volatiles and define two endmembers: (1) a seawater-rock dominated endmember with low Te/Sb, positive δ³⁴S, and strongly negative δ^82/76Se values, and (2) a magmatic volatile dominated endmember with high Te/Sb, negative δ³⁴S, and δ^82/76Se values of ~0 ‰. Our integrated approach introduces Se isotopes as a novel tool to elucidate the geochemical evolution of seafloor hydrothermal systems and the processes controlling the mobilization and transport of elements in these environments.

[1] Falkenberg, J. J. et al. (2024), Geochimica et Cosmochimica Acta, 373, 52-67. [2] Grosche, A. et al. (2024), Geochimica et Cosmochimica Acta, 372, 13-27.