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

Abstract

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 δ11B value of ambient seawater near the field is 40.05±0.01‰, and the boron concentration is 3.81mg/L. Andesite rocks from the hydrothermal field have an average boron content of 15.3ppm. The hydrothermal fluids from the yellow spring and white spring span a small range of δ11B values, from 33.27±0.22 to 36.84±0.11‰, and plumes from both springs also cover a small range, from 37.56±0.01 to 40.37±0.21‰.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 (4.10–4.64mg/L) that are slightly higher and δ11B values (33.27–36.84‰) that are lower than those of the hydrothermal plumes (3.94–4.17mg/L, 37.56–40.37‰). Hydrothermal fluids and plumes display a very regular array of data points in a δ11B–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 δ11B/B and pH/B ratios of hydrothermal plumes have stable values over the small distance form vent to plume (<15m). 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 δ11B values, are between 1.96 and 3.63. The hydrothermal flux of boron from the yellow spring into the oceans is between 1.17×105mol/yr and 1.32×105mol/yr, and from the white spring it is between 6.69×104mol/yr and 7.17×104mol/yr, assuming that only andesites are present in the reaction zone.