Anomalous alkaline sulphate fluids produced in a magmatic hydrothermal system - Savo, Solomon Islands

In magmatic–hydrothermal and associated geothermal systems, acidic magmatic-derived fluids (pH < 3) commonly discharge from springs proximal to the vent of active (degassing) volcanoes and more alkaline (pH > 5) geothermal fluids are typically limited to lateral outflows some distance from the main vent. Here we describe an unusual hydrothermal system associated with Savo volcano, a recently eruptive (1830–1840 AD) trachyte-dominated island arc stratovolcano in the Solomon Islands. Hot springs (∼ 100 °C) near to the volcanic crater discharge alkaline waters instead of the more commonly recognised acidic fluids. The hydrothermal system of Savo dominantly discharges sinter and travertine-forming alkaline sulphate (pH 7–8) waters at hot springs on its upper flanks, in addition to a small number of lower discharge acid sulphate springs (pH 2–7). Alkaline sulphate fluids are dilute, chloride-poor (< 50 mg/l), sulphate- (> 600 mg/l) and silica-rich (> 250 mg/l). They have restricted δ34SSO4 (5.4 ± 1.5‰) and δ18OH2O values (− 4‰; local non-thermal groundwater is − 8‰). Acid sulphate springs discharge low chloride (< 20 mg/l), high sulphate (300–800 mg/l) waters, with variable silica (100–300 mg/l) and distinctly lower δ34SSO4 values (− 0.6 ± 2.5‰) compared to the alkaline sulphate fluids. They also display high δ18OH2O and δDH2O relative to non-thermal groundwater. Geochemical modelling shows that water–rock reaction and dilution in the presence of secondary anhydrite, pyrite and quartz leads to chloride being diluted to low concentrations, whilst maintaining high sulphate and silica concentrations in the fluid. Strontium, oxygen and hydrogen isotopes confirm water–rock reaction and mixing with groundwater as primary controls on the composition of the alkaline sulphate springs. The highly unusual dilute chemistry of all discharges at Savo is a consequence of high regional rainfall, i.e. climatic control, and results from open system mixing at depth between hydrothermal and meteoric waters.