Geochemistry of hot-springs at the SuSu Knolls hydrothermal field, Eastern Manus Basin: Advanced argillic alteration and vent fluid acidity

SuSu Knolls is an area of ongoing magmatic activity and recent volcanism located in the back-arc spreading environment of the Manus Basin in the Bismarck Sea, Papua New Guinea. In 2006, hydrothermal fluids were collected from three areas of submarine hot-spring venting and analyzed for the chemical and isotopic composition of major and trace species. Fluids were characterized by temperatures that varied from 226–325 °C, and formed grey to black smoke as they mixed with bottom seawater. The compositions of seawater derived vent fluids are regulated by the relative contributions of fluid-rock and fluid-sediment interaction, phase separation, and the addition of volatiles from magmatic degassing. In addition to phase separation, leaching of Cl from felsic rocks that compose the lithosphere in back-arc environments may produce Cl concentrations in excess of seawater values.

The measured pH25°C of SuSu Knolls smoker fluids varied from 1.5–3.7, a range that includes values substantially more acidic than typically observed in fluids at mid-ocean ridge spreading centers. Late stage addition of magmatic volatiles in the shallow seafloor is directly responsible for the most acidic fluids (pH25°C values below 2). In contrast, the acidity of vent fluids characterized by pH25°C values between 2 and 3 is not the direct result of the direct addition of magmatically-derived acidic species. Instead, the pH of these fluids likely reflects reaction with rocks that were previously altered by highly acidic magmatic fluids to an advanced argillic alteration assemblage containing quartz-illite-pyrophyllite-anhydrite ± alunite in hydrothermal upflow zones. Fluids that do not react with advanced argillic alteration assemblages during upflow have measured pH25°C values between 3 and 4.