title: Chlorine isotopic compositions of deep saline fluids in Ibusuki coastal geothermal region, Japan: using B-Cl isotopes to interpret fluid sources.
abstract: We report chlorine stable isotopic compositions (δ(37)Cl, expressed in ‰ relative to the standard mean ocean chloride) as well as δ(2)H and δ(18)O 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 δ(37)Cl 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 δ(37)Cl value with varying Cl/B molar ratios from 117 to 1265. A correlation study between the Cl/B molar ratio and the δ(37)Cl/δ(11)B ratio indicates a hyperbola-type mixing of at least two Cl sources in the Ibusuki region. One of them depletes in (37)Cl with a higher value of Cl/B molar ratio; and the other one enriches in (37)Cl 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.
authors.affiliation: a Department of Geochemistry, Faculty of Earth Sciences , University of Utrecht , Utrecht , The Netherlands.
authors.forename: Masaaki
authors.initials: M
authors.lastname: Musashi
authors.forename: Takao
authors.initials: T
authors.lastname: Oi
authors.forename: Rob
authors.initials: R
authors.lastname: Kreulen
mesh_terms: D002713:Chlorine / Q000032:analysis*; D004784:Environmental Monitoring; D045482:Hot Springs / Q000737:chemistry*; D007554:Isotopes / Q000032:analysis; D007564:Japan; D012623:Seawater / Q000032:analysis*
chemical_list: D007554:Isotopes; D002713:Chlorine
keywords: Ibusuki; boron-11; chlorine-37; coastal geothermal region; deep brine; hydrogen-2; isotope geochemistry; oxygen-18; sea water; subduction process; volatile