Information on Geothermal System Obtained by Chemical and Isotopic Characteristics of Soil and Fumarolic Gases from the Doroyu-Kawarage Geothermal Field, Akita, Japan

Abstract

For soil gases from the Doroyu-Kawarage geothermal field in northern Japan, the concentrations of Rn, H₂, CH₄, CO₂ and rare gases (He and Ne) together with isotopic ratios of CO₂ and He (δ ¹³C of C0₂ 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 H₂. Soil gas samples from Kawarage and Doroyu geothermal manifestations contain high concentrations of Rn, H₂ and CO₂, and the δ ¹³C values also are extremely high. Therefore, it is inferred that CO₂ 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 CO₂ can be useful in geothermal prospecting in addition to information provided by H₂ 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.