Gas geochemistry of geothermal fluids from the Hatchobaru geothermal field, Japan

•Fluid & gas chemistry of the Hatchobaru geothermal field, Kyushu, Japan was studied.•Gas chemistry is likely to reflect the signature of the magmatic heat source.•Carbon isotope systematics yielded an apparent equilibrium temperature of ∼400 °C. We analyzed the chemical and isotopic composition of the geothermal fluids from the Hatchobaru geothermal field in the Kyushu Island, Japan. Fluid chemistry showed similarity with that reported in earlier studies. Chemical geothermometry provided an estimated reservoir temperature of 250–300 °C. Helium and carbon isotopic ratios of the steam is likely to reflect the signature of the magmatic heat source. The apparent equilibrium temperature for carbon isotope exchange between CO2 and CH4 was 375–430 °C. The sulfur isotope data were in accordance with the idea that an acidic Cl-SO4-type fluid was modified from a neutral Cl-type fluid.