Trace element fractionation and precipitation in submarine back-arc hydrothermal systems, Nifonea caldera, New Hebrides subduction zone

Abstract

Hydrothermal activity is abundant at volcanic structures in subduction zones, including those associated with young volcanism in back-arc regions. Fluid boiling is a common process in these environments, but its fractionation and precipitation effects on trace metals and metalloids are still poorly constrained. The submarine back-arc hydrothermal system of Nifonea caldera hosts two recently discovered active vent sites with sulphide-sulphate chimneys showing a diverse mineralogy and chemistry. The focused discharge of fluids with temperatures (up to 368 °C) near the seawater boiling curve at ~ 1860 m water depth and “jets of steam” emitted from the chimney structures suggest fluid boiling. Fluid processes, as well as metal and metalloid deposition vary on a relatively small spatial-scale (<0.5 km2) and coincide with changes in sulphide-sulphate mineralogy and texture between different chimneys with zoning and dendritic intergrowths, indicating temperature gradients and fluid boiling. Boiling-induced precipitation, together with seawater mixing in the sub-seafloor led to a depletion of Zn, Ga, Ge, Ag, Cd, Sb, Au and Pb in the discharging fluids and their precipitates at the main compared to the northern vent site, also resulting in a depleted trace element signature with respect to most other back-arc hydrothermal systems in the Pacific Ocean. A magmatic-hydrothermal signature (high SO42-) in some of the discharging fluids propose a weak magmatic volatile influx to the Nifonea caldera hydrothermal system. However, S isotope data provides no evidence for a magmatic volatile component and rather suggests, in combination with the sulphide-sulphate (Zn, Ge, Se, Ag, Cd, Sb, Ba, Au and Pb) and fluid data (high K) that the Nifonea caldera hydrothermal system is dominantly controlled by a combination of boiling, mixing and water–rock interaction.