流体地球化学的にみた雲仙地溝の熱水系と温泉の生成過程

Abstract

In this study, a model for hydrothermal system and formation processes of hot springs (Obama, Unzen and Shimabara hot springs) in Unzen Graben of the Shimabara Peninsula was constructed on the basis of a detailed analysis of hydro-geochemical data obtained by Ohsawa et al. (2002) and NEDO (1988). Both Obama and Unzen hot springs at the western part of the graben, which are located at the southwest side of the volcanic center of Unzen Volcano, are generated by one active liquid-dominant hydrothermal fluid of Na-Cl type. Fumarolic area adjoining Unzen hot spring being on the uplands is developed by a steam separated from the hydrothermal fluid at some 300°C, and hot water of H-SO₄ type of Unzen hot spring is formed by mixing of the secondary steam into shallow groundwater at about 150°C. Heat and material source of the liquid-dominant hydrothermal fluid is a high-pressure magma-derived hydrothermal fluid in which chloride is contained in Na-form. The “residual” deep thermal water migrates laterally toward the western coast while conductively cooled, and finally the mixing of seawater into the thermal water results in the formation of boiling or hot water of Obama hot spring at approximately 200°C. Warm water of HCO₃ type of Shimabara hot spring at the eastern part of the graben, which is located at the east side of the volcanic center of Unzen Volcano, is formed at less than loot by mixing of a low-temperature magma-derived C0₂-dominant gas into shallow groundwater of meteoric origin: This low-temperature gas might be derived from a solidified magma relevant to a monogenetic volcano, Mt. Mayu-yama formed 3000 years before present, but the formation mechanism of such gas is still unaccounted for. This hydro-geochemical study cannot pinpoint the precise location of the primary magma chamber of Unzen Vocalno which is still hotly argued, however the conclusion of this study for the aspect of geochemical formation mechanism of hot spring waters agrees to the previous model (Ohta, 1973; 1975) on the whole.