Chemical and Isotopic Characteristics of Soil Gases from the North Kirishima Geothermal Field

Abstract

A geochemical survey of soil gases (He, H₂, Rn, CO₂ and its δ¹³C) 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 CO₂ content in Zone A and H₂ content in Zone C are also high. The carbon isotopic ratios (δ¹³C) of CO₂ are : -23 to -22‰from Zone A, -18 to -17‰ from Zone B and -16 to -11‰ from Zone C. The low δ¹³ C values of CO₂ 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 CO₂ (∼ -7 to -2‰). The significant concentrations of H₂ (∼ 410 ppm) and the high δ¹³C values of CO₂ 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.