秋田県玉川温泉における地熱水の成因及び地球化学的経年変動

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In the Tamagawa hot spring area, three different types of geothermal activity are found, which are hot water of acid Cl-SO_4 type at Obuki, fumarole steam containing CO_2 and H_2S but no SO_4 and HC1 and hot water of acid SO_4 type. The chemical composition and hydrogen and oxygen isotopic ratio of these hot water and steam have been observed since 1978. The hydrogen and oxygen isotopic ratios are clearly different among these three types, and it is explained that the Obuki hot water is mixture of volcanic steam and local meteoric water, the fumarole steam boils away from the Obuki hot water at about 150℃ underground, and the hot water of SO_4 type is surface water heated by steam similar as the fumarole steam. Significant variations of chemical and isotopic compositions have been observed on the Obuki hot water during the present observation as well as the observation by others since 1951. The relationship between the Cl content and the isotopic ratios reveal that the variations are caused by change in the mixing ratio of volcanic steam and meteoric water. Soon after the explosion of Mt. Yake from 1948 to 1951, the proportion of volcanic steam increased from 14 to 25%, and then had kept rather constant. In 1977, the proportion started decreasing sharply, and then, in 1992, returned to the state before the explosion, though slight increasing happened again since 1997. As regards the SO_4 content, a remarkable change, which is not due to the change in the mixing ratio of volcanic steam and meteoric water, happened from 1972 to 1991. In practical geochemical monitoring on geothermal water of volcanic gas origin, it is necessary to note that variation in SO_4^<2-> content is not always due to the change in the contribution of volcanic gas.
日本地球化学会の論文
2002-06-10

秋田県玉川温泉における地熱水の成因及び地球化学的経年変動

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