Abstract
Chemical and isotopic compositions have been determined for steam and water samples from fumaroles, hot springs and wells in the Onikobe geothermal field. The thermal waters are grouped into acid sulfate type, acid sulfate-chloride type and NaCl type waters. The stable isotopic data indicate that the reservoir fluids extremely enriched in 18O are meteoric in origin and that the NaCl type hot spring waters are mixtures of these fluids and meteoric waters. The acid sulfate type waters are formed in shallow aquifer with oxidation of fumarolic hydrogen sulfide. The sulfate enriched in 34S from acid sulfate-chloride and NaCl type waters originate in the disproportionation of volcanic sulfur dioxide. δ13CCO2 value suggests that a carbon source for C02 in steam and waters at Onikobe is related to volcanic activity. The gas ratio such as CO2/N2, N2/Ar and He/Ar indicates that the Onikobe geothermal fluid is a mixture of volcanic gases and groundwaters and that these gases are dissolved in liquid reservoir fluids at near reservoir temperature.
