Geochemical Studies on Mg-bearing Carbonates in Ryukyu Islands and Okinawa Trough.

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Geochemical studies on carbonate sediments and related materials collected mainly from submarine hydrothermal sites of Taketomijima-Island and Okinawa Trough back arc basin have been carried out. The research results are summarized as follows.[A] Taketomijima submarine hydrothermal sites:(1) Samples of carbonate sediments, hot spring water and associated gas were collected from the bottom of coral reef of Taketomijima (ca. 22m in depth) by scuba divers.(2) The carbonate sediments are composed of foraminifera test, shell or fragments of coral skeleton. The dark colored secondary carbonates covered the cavity of pores of sediments. The proportion of dolomite and content of heavy metals such as Hg, As, Sb, Ba, Fe, Mn, Cd and U are abnormally high in the dark portion.(3) The chemistry and mineralogy of sediments of Taketomijime hydrothermal site is analogous to that of Kagosima-bay which is considered to be related to the volcanic activity of the adjacent area.(4) Frequent earthquake activ … More ity has been observed from January 23, 1991 until now at Iriomotejima Island, which is located ca. 30 Km west of Taketomijima hot spot. It will be important whether Taketomijima hot spring activity and Iriomote earthquake activity are related each other or not. The water temperature, chemical composition of water and associated gas should be monitored.[B] Okinawa Trough hydrothermal sites :(1) The carbonate sediments are collected from Iheya deep of Okinawa Trough by research submersible 'SINKAI2000' in 1989 and 1990.(2) X-ray diffraction and neutron activation analysis revealed that these carbonates are mainly composed of calcite, rhodechlosite (MnCO3) and kuthonite (CaMn (CO3) 2) and rich in Mn, As, Hg, and Ag.(3) On the other hand, sulphate and sulphide sediments are collectd from Izena deep in 1989. The gamma-ray spectroscopy showed that activity of Ra-226, Ra-228 and their daughter nucleides is extremly high in barium sulphate containing sediments. These sediments are deposited by hydrothermal activity in recent years.(4) Liquid CO2 and solid gas hydrate of CO2 were observed at Izena deep hydrothermal sites. Less