Fumarolic gases with temperature ranging from 98°C to 308°C in Kuju-Iwoyama fumarolic area of Kuju Volcano were sampled with minimal atmospheric air contamination for isotopic and chemical analyses. Stable isotope ratios of hydrogen and oxygen (δD and δ¹⁸O) of the steam condensate samples showed that the fumarolic steams were mixtures of magmatic steam and local meteoric water. On the other hand, the He-N₂-Ar contents of the fumarolic gas samples indicated that they consisted of magmatic gas, air and air-saturated water (ASW). From the quantitative estimation by means of the δD and δ¹⁸O values and He/Ar ratios of the fumarolic gases, the proportion of mixing for the three end-members : magmatic fluid_(g), air_(g), ASW₍₁₎ was estimated to be 1 : 0.0005-0.0021: 0.0009-0.0043. This relative proportion and a linear relationship between the δ-values (D/H and¹⁸O/¹⁶O) of steam and He/Ar ratios which are corrected to be free from ASW contribution strongly suggest that the underground air and the underground water can be treated as the vadose air and the vadose water, respectively. It suggests that the fumarolic fluids of Kuju-Iwoyama would be formed by the mixing of the vadose fluids (vadose air and vadose water) and the magmatic fluid into the fumarolic gas conduits within the vadose zone. This mixing model reconciles the contradiction between the isotopic and chemical data of fumarolic steams and gases which are obtained from several active volcanoes (e. g. Satsuma-Iwojima Volcano, Kuju-Iwoyama between 1959 and 1984) ; δD and δ¹⁸O of steam condensates show the typical values of andesitic magmatic steam, whereas He/Ar ratios of fumarolic gases clearly indicate the mixing of air and/or ASW into magmatic fluid.

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In order to establish the geothermal history and fluid chemistry in the Yuzawa-Ogachi geothermal area, Akita Prefecture, northeastern Japan, fluid inclusions in minerals were studied microthermometrically and geochemically. The area is one of the active geothermal areas in Japan, and the Uenotai geothermal power station is in operation. The studied samples were quartz, calcite and zeolite of hydrothermal veins and granites collected from geothermal exploration wells (YO-2, 3, 5, 7 and KU-1). Observation and microthermometrical data on samples from the well YO-3 reveal that four types of fluid inclusions occur : (1) liquid CO₂-bearing inclusions in pegmatite, (2) liquid-rich inclusions of relatively high salinity in granite, (3) inclusions trapping boiling fluid in hydrothermal veins at shallower levels (<800 m) formed at the early stage of the geothermal activity, and (4) inclusions formed from the present-day geothermal fluid at deeper levels (>800 m). Besides them, (5) polyphase inclusions occur in hornfels and granite at the well YO-2. Fluid inclusions of types (1) and (2) might be formed at or after solidification of the Cretaceous granite of the basement in the area, and those of type (5) were at solidification and contact metamorphism accompanied by the Miocene granitic intrusion. Comparison of homogenization temperatures of fluid inclusions with temperature logging data imply that geothermal fluid of 250∼300°C flows at deeper part around the well YO-3 and the geothermal activity is vigorous, although the activity has declined at shallower levels (<800 m). Borehole temperatures exceed 200°C at deeper levels (>1000∼500 m) of the wells YO-2, 5 and 7, but geothermal activity has declined at the wells YO-2 and 5 except YO-7. Gas analyses of the fluid inclusions in quartz of hydrothermal veins from the wells Y0-3 and KU-1 show that the inclusions are mainly composed of H₂O (>99.6 mol%) with small amounts of CO₂ (0.08∼0.38 mol%) and trace amounts of N₂ (0.01∼0.07 mol%), CH₄ (0.001∼0.004 mol%) and Ar (<0.0006 mol%) as non-condensable gases, as well as our previous works on other geothermal systems in Japan. The microthermometric results and previous works depict the following geological history in the area. In Cretaceous, granitic rock was intruded into metamorphic rocks, and then the Onikobe-Yuzawa mylonite zone was formed. In Neogene, the Ogachi caldera was formed and igneous rocks were intruded. In Quaternary to present, volcanic activity occurred, and the current geothermal system was formed about ten thousand years ago. Differences between present and past thermal contours, which are estimated by the microthermometry and previous works, indicate that geothermal fluid flowed from the heat source to west probably at the early stage of the geothermal activity. The flow has been already terminated and changed to northeast and southwest. Paths of the geothermal fluids (highly permeable zones) are radially distributed around the heat source. The gas analyses shows that CO₂/N₂ and CO₂/CH₄ ratios of the fluid inclusions are lower than those of the present-day discharged fluids in the area. The differences in the ratios may be ascribed to degree of degassing, and the fluid inclusions were probably formed by trapping fluids that were weakly influenced by degassing. The gas ratios of fluid inclusions at shallower levels (<800 m) of YO-3, which were formed at the past stage of the geothermal activity, are lower than those at deeper levels (>800 m) formed at the present stage. This shows that the chemical composition of the geothermal fluid has changed with time.

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