Application of B, Mg, Li, and Sr Isotopes in Acid-Sulfate Vent Fluids and Volcanic Rocks as Tracers for Fluid-Rock Interaction in Back-Arc Hydrothermal Systems

The Manus Basin hosts a broad range of vent fluid compositions typical for arc and back-arc settings, ranging from black smoker to acid-sulfate styles of fluid venting, as well as novel intermediate temperature and composition hybrid smokers. We investigated B, Li, Mg, and Sr concentrations and isotopic compositions of these different fluid types as well as of fresh and altered rocks from the same study area to understand what controls their compositional variability. In particular, the formation of acid-sulfate and hybrid smoker fluids is still poorly understood, and their high Mg concentrations are explained either by dissolution of Mg-bearing minerals in the basement or by mixing between unmodified seawater and magmatic fluids. Mg isotope ratios of the acid-sulfate fluids from the Manus Basin are seawater-like, which supports the idea that acid-sulfate fluids in this study area predominantly form by mixing between unmodified seawater and a Mg-free magmatic fluid. Changes in the B, Li, and Sr isotope ratios relative to seawater indicate water-rock interaction in all acid-sulfate fluids. Further, the combination of delta Li-7 with B concentrations of the same fluids links changes in delta Li-7 to changes in (1) basement alteration, (2) water-to-rock ratios during water-rock interaction, and/or (3) the reaction temperature. These isotope systems, thus, allow tracing of basement composition and acid-sulfate-driven alteration of the back-arc crust and help increase our understanding of hydrothermal fluid-rock interactions and the behavior of fluid-mobile elements.