Boron isotope compositions of fluids and plumes from the Kueishantao hydrothermal field off northeastern Taiwan: Implications for fluid origin and hydrothermal processes

Boron is a common element in vent fluids of seafloor hydrothermal fields, and it has been used to understand the hydrothermal flux and water-rock interaction in hydrothermal systems. We have measured the boron concentration and isotope composition of seawater, andesite, hydrothermal fluid and plume samples from the Kueishantao hydrothermal field. The delta B-11 value of ambient seawater near the field is 40.05 +/- 0.01 parts per thousand, and the boron concentration is 3.81 mg/L Andesite rocks from the hydrothermal field have an average boron content of 153 ppm. The hydrothermal fluids from the yellow spring and white spring span a small range of delta B-11 values, from 3327 +/- 022 to 36.84 +/- 0.11 parts per thousand, and plumes from both springs also cover a small range, from 37.56 +/- 0.01 to 40.37 +/- 0.21 parts per thousand. Hydrothermal fluids from both springs in the Kueishantao hydrothermal field have variable B enrichments relative to seawater between 7 and 21%. They have B concentrations (4.10-4.64 mg/L) that are slightly higher and delta B-11 values (3327-36.84 parts per thousand) that are lower than those of the hydrothermal plumes (3.94-4.17 mg/L, 37.56-40.37 parts per thousand). Hydrothermal fluids and plumes display a very regular array of data points in a delta B-11-B diagram, suggesting that the boron of hydrothermal fluids and plumes is mainly from seawater and that little of it is, from andesite. This implies that the interaction of subseafloor fluid and -andesite at the Kueishantao hydrothermal field is of short duration. In all the fluids, from springs to hydrothermal plumes, the pH values, B concentrations and B isotopic compositions show significant correlations with each other suggesting that the delta B-11/B and pH/B ratios of hydrothermal plumes have stable values over the small distance form vent to plume (<15 m). Thus the B concentrations and B isotopic compositions of hydrothermal plumes can be used to describe the diffusive processes governing the chemical compositions of hydrothermal plumes in the seawater environment. The water/rock ratios, based on the B concentrations and delta B-11 values, are between 1.96 and 3.63. The hydrothermal flux of boron from the yellow spring into the oceans is between 1.17 x 10(5) mol/yr and 132 x 10(5) mol/yr, and from the white spring it is between 6.69 x 10(4) mol/yr and 7.17 x 10(4) mol/yr, assuming that only andesites are present in the reaction zone. (C) 2013 Elsevier B.V. All rights reserved.