Boron isotope geochemistry of vent fluids from arc/back-arc seafloor hydrothermal systems in the western Pacific

Boron concentrations and isotopic compositions were determined for vent fluids from 11 sites in arc/back-arc seafloor hydrothermal systems. This new dataset allowed a systematic comparison of boron behavior during water-rock interaction in different geological settings. In sediment-starved hydrothermal systems (e.g., Manus Basin, Izu-Bonin Arc, Mariana Trough), end-member fluids showed large variations in boron concentration (0.53-1.52 mmol/kg) and delta B-11 value (13.5-29.7%), reflecting the interaction of seawater with varied types of rock that occur in arc/back-arc settings. Based on the model calculations, the boron concentration and delta B-11 value of the fluid are better explained by an equilibration between seawater and rock than a mixing model. The Chlorine-depleted fluids from phase-separated hydrothermal systems (e.g., North Fiji Basin) had low boron concentrations (0.44-0.55 mmol/kg) and high delta B-11 values (34.5-36.1%). The small fractionations of boron and boron isotopes during phase separation suggest that these characteristics were acquired during water-rock interaction rather than phase separation and segregation processes. The fluids from sedimenthosted hydrothermal systems (e.g., Okinawa Trough) are characterized by high boron concentrations (3.9-4.8 mmol/kg) and low delta B-11 values (2.5-2.9%). These fluids also showed high Cs/B ratios, indicating interactions with terrigenous sediments. Model calculations demonstrate that the combined evaluation of delta B-11 values and Cs/B ratios are useful for a quantitative estimate of sedimentary contributions in seafloor hydrothermal systems. (C) 2014 Elsevier B.V. All rights reserved.