Unique Geochemistry of Submarine Hydrothermal Fluids from Arc‐Back‐Arc Settings of the Western Pacific

Recent studies using research vessels and submersibles have revealed unique chemical properties of hydrothermal fluids from arc-back-arc settings in western basins of the Pacific Ocean. We review the most notable of these subduction-related, tectonically induced geochemical features, as expressed in fluids discharged directly from the seafloor and the hydrothermal plumes that result. In particular, phase separation, interaction with sediments, and material supply from the subducting slab impart characteristic chemical signatures to arc-back-arc fluids. Special emphases are placed on the richness of volatile components and the extreme acidity (pH < 2) so far observed for these fluids. This paper also reviews two advances in observational technology especially useful in the arc-back-arc environment: an in situ chemical analyzer for monitoring temporal variations in fluid chemistry, and a gas-tight fluid sampling system that ensures high-integrity recovery of volatile-rich fluids.