Chlorine isotopic compositions of deep saline fluids in Ibusuki coastal geothermal region, Japan: using B–Cl isotopes to interpret fluid sources

We report chlorine stable isotopic compositions (δ³⁷Cl, expressed in ‰ relative to the standard mean ocean chloride) as well as δ²H and δ¹⁸O values of deep saline fluids taken at eight drill-holes reaching from 73 to 780 m below sea level in the Ibusuki coastal geothermal region, Japan. Analytical results show that the δ³⁷Cl values narrowly range between −0.26 and +0.21 ‰ with an analytical precision of ±0.06 ‰. Except for one sample, the samples examined are negative in δ³⁷Cl value with varying Cl/B molar ratios from 117 to 1265. A correlation study between the Cl/B molar ratio and the δ³⁷Cl/δ¹¹B ratio indicates a hyperbola-type mixing of at least two Cl sources in the Ibusuki region. One of them depletes in ³⁷Cl with a higher value of Cl/B molar ratio; and the other one enriches in ³⁷Cl with a lower Cl/B molar ratio. The former is chemically identical to that of the deep brine, which is altered seawater through the seawater–hot rock interaction. The latter is chemically similar to gas condensate derived from the high-temperature (890 °C) vent of an island-arc volcano near the Ibusuki region.