Abstracts of Annual Meeting of the Geochemical Society of Japan

The possibility of exploiting deep-sea mineral resources has gained global attention in recent years. At the same time, concerns have been raised on the potential impacts of these activities. Japan is conducting both exploitation program at JOGMEC and exploration/research program named Jipangu-in-the-Ocean as a trans-ministerial project. Increasing number of researchers not only from JAMSTEC, AIST, and NIES, but also from several universities, companies, and organizations are involved in the project. There are three main topics of research; that is, basic scientific research on the genesis of mineral deposits, development of technologies for seafloor exploration, and, survey and monitoring of ecosystem. This presentation summarizes the role of geochemists in the long-run project.

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Submarine hydrothermal activities have attracted great interests across broad disciplines since their discovery in 1970s. In recent, they are regarded to be potential resources of base, rare and even precious metals. Under such circumstances, at least three types of studies, investigations and explorations are conducted in Okinawa Trough: 1) exploration of ore minerals, 2) continental shelf survey and 3) academic studies. Here the authors briefly summarize recent discovery and achievements of above described projects.

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Li isotope ratios in Okinawa Trough hydrothermal fluids show higher concentrations and lower isotopic ratios than those in MOR hydrothermal fluids. It suggests Okinawa Trough hydrothermal fluids react with sediment and volcanic rocks.

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Molecular hydrogen (H₂) in fumarolic gases shows the hydrogen isotope exchange equilibrium with coexisting H₂O at a temperature more than 400 °C. Recently, we developed a new remote temperature sensing using the characteristics of D/H ratio of H₂ (HIReTS). In this method, the D/H ratio of fumarolic H₂ is obtained remotely from the observation of volcanic plume, and the outlet temperature of the fumaroles is estimated from the D/H ratio, assuming that the hydrogen isotope exchange equilibrium is quenched within volcanic plume during the process of admixture between fumarolic gases and air. In the previous studies applying HIReTS in active volcanoes, such as Aso and Satsuma-Iwo volcano in Japan, the volcanic plumes were taken at the rim of each volcanic crater, at the distance of a few hundred meters from fumaroles. Direct access to such volcanic crater rims, however, is neither practical nor safe in many highly active volcanoes. In general, volcanic plumes spread laterally at the height close to the summit of each volcano where the plumes are ejected. As a result, we must utilize some flying devices, such as aerial vehicles, balloons, or kites, to take the samples of volcanic plume in safety area distant from fumaroles, for the aim of applying HIReTS to highly active volcanoes.

In this study, we used an aerial vehicle to take samples of volcanic plume ejected from highly active Sakurajima volcano in Japan, to determine the outlet temperature of fumaroles remotely by using HIReTS, at the distant of more than 3 km from the fumaroles. Sakurajima volcano has continued its explosive activity at the summit crater since 1955 so that nobody dared to determine the outlet temperature of fumaroles directly.

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