ESR (electron spin resonance) ages were determined for barite crystals extracted from hydrothermal sulfide deposits taken at Daiyon-Yonaguni Knoll field, Hatoma Knoll field, Iheya North Knoll field, Hakurei Site of Izena Hole field, Yoron Hole field of the Okinawa Trough. The ages range from 4.1 to 16,000 years, being consistent with detection of 228 Ra in younger samples and radioactive equilibrium/disequilibrium between radium and daughter nuclei. The variation of the ages within each sample is mostly within the statistical error range. The relative order of the ages is consistent with the result of 226 Pb method, but the determining absolute ages is still an issue. The order of ages of the 5 hydrothermal fields would be arranged, from young to old as follows; Yoron Hole field, Daiyon-Yonaguni Knoll field, Hatoma Knoll field, being nearly equal to Iheya North Knoll field. KeywordsBarite Dating Electron spin resonance Hydrothermal activities IntroductionThere have been many scientific efforts devoted for studies on active sea-floor hydrothermal fields found in Okinawa Trough (Ishibashi et al. Chap. 23). The evolution of these hydrothermal fields is one of the important issues. Dating methods have been employed for hydrothermal sulfide deposits, such as the U-Th disequilibrium method (e.g. You and Bickle 1998) applicable for the age range more than several thousand years, and 226 Pb and 228 Th method (e.g. Noguchi et al. 2011) for the range less than 150 years. The age range of several hundred years is essential to estimate the life time of hydrothermal activities. However, dating methods for the age range are lacking. Okumura et al. (2010) made the first practical application of ESR dating technique to a sample of seafloor hydrothermal barite to obtain ages of 300 and 3,620 years, while Kasuya et al. (1991) first pointed out that barite can be used for ESR dating. Toyoda et al. (2011) determined the optimum ESR condition while Sato et al. (2011) confirmed that the signal is thermally stable enough for an age range of several thousand years.Seafloor massive sulfide deposits are composed of sulfide minerals such as pyrite, sphalerite, chalcopyrite and galena, and also include sulfate minerals such as anhydrite and barite. The sulfate minerals precipitate by mixing of hydrothermal fluid and seawater, while the sulfide minerals precipitate from the hydrothermal fluid mainly by cooling (Hannington et al. 1995). It is generally considered that a combination and/or switch of these two modes of hydrothermal precipitation lead to growth of sulfide structures such as a chimney and mound and that of large hydrothermal T. Fujiwara S. Toyoda (*) A. Uchida Okayama University of Science, Okayama, Japan e-mail: toyoda@dap.ous.ac.jp deposits (e.g. Tivey 2007). Takamasa et al. (2013) determined ESR ages for barite in sulfide deposits in the South Mariana Trough hydrothermal field, and concluded that the ages are consistent with U-Th ages. In this study, the ages of various hydrothermal sulfides in the Okinawa Trough a...

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Electron spin resonance (ESR) dating of barite has been recently developed and is now practically applied to barite extracted from sea-floor hydrothermal deposits. The evolution of the accumulated dose to barite is simulated for an actual sample of barite in sea-floor hydrothermal sulfide deposit to find that the contribution of radioactive nuclei of 228 Ra series can be important for the samples younger than 300 years old. Currently, any date over 50 years should be considered a maximum value when the 228 Ra content is not obtained. The method to estimate the contribution from 228 Ra series has to be developed in future for those in which 228 Ra is not detected. The age limit of ESR dating of barite would be 5000 to 6000 years due to the decay of 226 Ra, which is also found by simulation of the accumulated dose.

The alpha effectiveness value (k-value) for the ESR (Electron Spin Resonance) signal due to SO3- in barite was revised by comparing the dose responses of the signal intensities to gamma rays and to 4 MeV He+ ion doses in natural sea-floor hydrothermal barite samples actually used for dating. Of the values obtained for a synthetic, a natural old, and a natural young samples, the one for the natural young sample is tentatively adopted, which is 0.053 ± 0.006, although further works are still necessary to establish this value.