[Abstract]

Kozushima is a volcanic island in the Pacific Ocean located about 170 km southwest of Tokyo. Kozushima is composed of at least 16 rhyolitic monogenetic volcanoes. It has been inferred that the eruption of rhyolites began at least several tens of thousands years ago and was repeated intermittently until 838 A.D. There are only a few eruptions whose ages were well determined, and the precise eruption history is still unsettled. The present study aims to cross check the ages determined by ESR method with those by other methods and to elucidate the overall eruption history of Kozushima. Samples from 17 rhyolitic lavas and one pyroclastic deposits (Chichibuyama pyroclastic surge deposit-A: Cb-A) were collected and analyzed. ESR signals of Al and Ti-Li centers were observed in quartz phenocrysts. The accumulated doses were obtained from the dose responses of these two signals, while natural dose rates were calculated from the uranium, thorium, and potassium concentrations with necessary corrections. ESR ages were obtained by dividing the accumulated doses by dose rates. Regarding Tenjyosan, Kobeyama, Ananoyama, and Hanatate volcanoes (previously reported ages are 838 A.D. to several tens of thousand of years), the determination of accumulated dose was unsuccessful because they are too young. ESR ages for the other volcanoes are as follows: 26 +-1 ka (Al center) and 32 +-3 ka (Ti-Li center) for Cb-A, 27 +-3 ka and 24 +-3 ka for Jyogoyama, 28 +- 4 ka and 26 +-5 ka for Ohsawa, 23 +-2 ka and 31 +-3 ka for Takodoyama, 29 +-1 ka and 30 +-2 ka for Matsuyamahana, 35 +-5 ka and 37 +-2 ka for Membo, 23 +4/-3 ka and 29 +-3 ka for Nachisan, 32 +-1 ka and 48 +-2 ka for 262m yama, 46 +-8 ka and 40 +8/-6 ka for Sawajiriwan (Nagahama), 54 +9/-8 ka and 48 +-6 ka for Awanomikoto, 52 +5/-4 ka and 51 +-4 ka for Nagumiwan (Nagahama), 40 +4/-3 ka and 69 +-3 ka for Hashiruma, 46 +-3 ka and 68 +-5 ka for Sanukayama, and 64 +5/-7 ka and 65 +9/-8 ka for KannonÅæ ura. Regarding Cb-A, the ESR age of Al center is fairly close to the previously reported 14C ages (22-26 cal. ka). However, the discrepancy between Al center and Ti-Li center is present. Despite that the ESR ages for all lavas have at least +-10% errors, the ESR ages are in good agreement with stratigraphy. On the basis of the results of ESR dating and stratigraphy, the overall eruption history in Kozushima is proposed.

Key words:

Kozushima, ESR dating, rhyolite

e-mail:

tadashi@eps.s.u-tokyo.ac.j