We performed fluid inclusion study of an investigation well (WD-la; 3, 729 m in total depth) in the Kakkonda geothermal field, Japan. WD-la was drilled in the Quaternary Kakkonda granite from 2, 860 m through 3, 729 m in depth which is the heat source of the Kakkonda geothermal system. The boundary between hydrothermal convection and heat conduction zones was found around 3, 100 m by temperature loggings. We discussed thermal structure in the Kakkonda granite and permeation of meteoric water into the Kakkonda granite. Temperature of the Kakkonda granite shallower than 3, 100 m can be evaluated roughly from homogenization temperature of the liquid-rich inclusion which has a minimum salinity in every depth. Since the Kakkonda granite deeper than 3, 100 m is less permeable at greater depths, meteoric water permeates less into the Kakkonda granite. The evidence is as follows: (a) Minimum salinities of the liquid-rich inclusions shallower than 3, 250 m are close to 0 wt. %, although those deeper than 3, 300 m increase. (b) Homogenization temperature of the liquid-rich inclusion, which has minumum salinity, increases just slightly between 2, 750 m and 3, 250 m, then they increase steeply deeper than 3, 300 m. (c) According to the result of gas analysis for fluid inclusions by laser Raman microprobe spectroscopy, CO
2 and H
2S were not detected shallower than 3, 150 m, although they were detected deeper than 3, 350 m. We also built a geothermal model of the Kakkonda field based on the fluid inclusion study, showing thermal structure and fluid flow in the Kakkonda geothermal system.
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