水素と酸素の安定同位体比からみた別府温泉における地熱流体の移動過程

抄録

Hydrogen and oxygen isotope ratios of waters, including steam condensate, thermal water, cold groundwater and precipitation in the Beppu geothermal field, were analyzed to study the origin and migration processes of geothermal fluids. In the Beppu area, geothermal fluids, i. e. chloride water and steam, ascend from depths via faults, discharging at relatively high elevations and mix with shallow meteoric waters to form various types of hot-spring waters. Several distinct types of thermal waters flow laterally through fan deposits. By comparing the isotopic compositions of thermal waters with those of cold waters at various elevations, it was found that steam-heated waters flowing in shallow levels from the geothermal area originate from shallow groundwaters in the fan, and that bicarbonate type hot-spring waters flowing in relatively deep levels are derived from waters recharged in the mountainous region behind the fan area. Non-boiling, dilute, chloride-type hot-spring waters, which tend to have higher δ¹⁸O values with higher Cl concentrations, are diluted by meteoric waters recharged at various elevations
The isotope ratios and chloride concentrations in boiling waters show a positive correlation between δ¹⁸O values and chloride concentrations, when corrected for steam loss in the boreholes. This indicates that meteoric waters are mixed with parent geothermal waters (C1 = 1300-1700 ppm). These geothermal waters have a similar range of δD values as the local meteoric waters. They are meteoric waters affected by oxygen isotope exchange with the surrounding rock under high tempera-ture conditions. A a D180 diagram shows that all the liquid waters within the study area lie in the mixing range between the meteoric waters (δD = -56±5‰,δ¹⁸O = -8.6±0.6‰) and the pa-rent geothermal waters (δD = -56±5‰, δ¹⁸O = -6.1±0.6‰).
On the other hand, isotopic compositions of steam condensates lie within a relatively wide range. The underground steam can be separated by a single-step process from the mixtures of the meteoric waters and the parent geothermal waters at various temperatures between 110°C and 300°C. The iso-topic compositions of steam condensates in the vapor-dominated region at higher elavations of the study area correspond to relatively high separation temperatures (230-300°C), and suggest that the steam originates from the deep parent geothermal waters. On the other hand, the steam condensates in shallow liquid-dominated regions at lower elevations correspond isotopically to relatively low tempera-tures (110-240°C), indicating various stages of mixing processes between the cold meteoric waters and the high temperature parent geothermal waters. The isotopic compositions of steam condensates appear to show a reasonable tendency for the fraction of meteoric water in steam to increase as the equilibrium temperature of steam separation decreases.