For soil gases from the Doroyu-Kawarage geothermal field in northern Japan, the concentrations of Rn, H
2, CH
4, CO
2 and rare gases (He and Ne) together with isotopic ratios of CO
2 and He (δ
13C of C0
2 and
3He/
4He) were measured. In presently developing geothermal field to the northern part of Doroyu fault, Rn concentrations of soil gases are clearly higher than those to the southern part. On the other hand, the distribution of low δ
13C values (-20∼-24‰) corresponds to that of Sanzugawaformation.This formation contains the organic matter underlying only to the northern part of the fault. However, higher δ
13C values to the northern part, coexist with high concentration of H
2. Soil gas samples from Kawarage and Doroyu geothermal manifestations contain high concentrations of Rn, H
2 and CO
2, and the δ
13C values also are extremely high. Therefore, it is inferred that CO
2 derived from a deep-seated geothermal system are added to those originated from organic matter in the Sanzugawa formation. These data indicate that the δ
13C values of CO
2 can be useful in geothermal prospecting in addition to information provided by H
2 measurement. Soil gases from Kawarage and the vicinity do not give the high Rn concentrations, in spite of the high Hg concentrations. Such a fact may give the information on the alteration by the secondary hydrothermal water without U and Th, which is originated from the vaporization of deep-seated geothermal water. Furthermore, the
3He/
4He and
4He/
20Ne ratios of Kawarage fumarolic gas are 9.5×10
-6 and 180, respectively, indicating that the He is originated from magmatic gas. This fact is consistent with a consideration that the heat source of this geothermal field is the magma of the neighboring Quaternary volcanos such as Mt. Takamatsudake and Mt. Kabuto.
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