A geochemical survey of soil gases (He, H
2, Rn, CO
2 and its δ
13C) was carried out in the North Kirishima geothermal field, Japan. As a result, three anomalous zones were identified. All three Zones (A, B and C) have a high Rn content. Furthermore, the CO
2 content in Zone A and H
2 content in Zone C are also high. The carbon isotopic ratios (δ
13C) of CO
2 are : -23 to -22‰from Zone A, -18 to -17‰ from Zone B and -16 to -11‰ from Zone C. The low δ
13 C values of CO
2 from Zone A suggest its organic origin probably from the 200-300 m thick lacustrine sediments underlying the area. On the other hand, the values from Zone C are higher and seem to be affected by volcanic CO
2 (∼ -7 to -2‰). The significant concentrations of H
2 (∼ 410 ppm) and the high δ
13C values of CO
2 from Zone C suggest the existence of a high temperature geothermal fluid underneath the zone. This is supported by the fact that high temperature geothermal fluid was discharged from Well KT-4 in this zone. It is inferred that soil gases in the geothermal area contain some components derived from a deeply underlying geothermal system, and can be very useful in geothermal prospecting.
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