[SGL30-P02] Zircon U-Pb dating and Fission track age of a Pliocene Ohta Tephra in the Tokai Group
Keywords:Ohta Tephra, Tokai Group, Zircon, U-Pb dating, Fission track age
The Ohta tephra in the Pliocene Tokai Group is one of representative tephra with widespread depositions in central Japan. Although age determination of the Ohta tephra is important for regional correlation of the Pliocene strata in Japan, the age had not been well-constrained by previous studies, showing a wide age range from 3.3 to 5.24 Ma by zircon fission track (FT) dating . Thus, in this study, zircon U-Pb and FT double dating for single grain was newly applied for the Nakatsugawa I and II volcanic ash layers, which have been correlated to the Ohta tephra .
Two samples from the outcrop in the Nakatsugawa City, Gifu Prefecture were used for the dating. Weighted average U-Pb ages from measured grains except heterogeneous and discordant grains were 3.78 ± 0.10 Ma and 3.90 ± 0.11 Ma. Weighted average FT ages from the grains used for the U-Pb dating were 3.74 ± 0.24 Ma and 4.11 ± 0.27 Ma. Although the closure temperature of zircon is about 900 °C for U-Pb method  and about 240 °C for FT method , the U-Pb ages are included in the range of the error of the FT ages. This indicates that the zircons were crystallized at high temperature and then quenched, and that these ages correspond to the eruption age of the tephra. This result is consistent with the study by Satoguchi et al. (2005)  that estimated the 3.9 Ma as the age of the Ohta tephra based on the paleomagnetic and biostratigraphic correlations with the Miura Group. The zircon U-Pb and FT double dating for the Ohta tephra in this study will contribute to the regional correlation of Pliocene strata in central Japan.
This study was carried out under contract with the Agency of Natural Resources and Energy (ANRE), Ministry of Economy, Trade and Industry (METI) of Japan as part of its R&D program supporting development of technology for geological disposal of high-level radioactive waste.
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