17:15 〜 18:30
[SGL22-P03] ESR dating of sea-floor hydrothermal barite with the regenerative protocol
キーワード：重晶石、ESR年代測定法
Barite (BaSO4), one of sulfate minerals, is crystallized with a chemical reaction of hydrothermal fluid containing barium ion (Ba+) and seawater with sulfate ion (SO42-) in sea-floor hydrothermal areas. While it had been pointed out that the electron spin resonance (ESR) signal in barite might be used for ESR dating, it was shown that the SO3- signal is practically useful for dating of sea-floor hydrothermal activities. After ESR measurement conditions and thermal stability of the signal were checked, actual systematic dating was shown to be successful in dating hydrothermal activities in the Okinawa Trough. Conventionally, ESR dating employs the additive method, where gamma ray irradiation artificially enhances the signal intensities. The dose response is then extrapolated to the zero ordinate to find the equivalent dose given by natural radiation. In the additive dose protocol, the signal intensity increases significantly in younger samples, while the signal is almost saturated in older samples where the fitted saturating exponential curve would have lower reliability, and it may be questionable whether the equivalent dose is correctly calculated. In the present study, we tried using the regenerative protocol which, in principle, has smaller errors and higher accuracy in obtaining the equivalent dose as it is estimated by interpolating the observed dose response. However, in this case, it is necessary to erase the signal once and the signal is regenerated by irradiation. Then, it is necessary to confirm that the process to erase the signal (usually heating) does not change the property of the signal formation (for example, the efficiency).
Barite crystals extracted from chimney samples at the Iheya site in the Okinawa Trough were used. The extracted barite was heated at temperatures, 350, 400, 450, and 600℃ to erase the signal, and then gamma ray irradiation was performed at Takasaki Research Institute of Japanese Atomic Energy Agency. After that, ESR measurement was performed at room temperature with an ESR spectrometer (JEOL PX-2300) at Okayama University of Science Technology Center.
It was found that the signal intensity in the samples heated at 600 ℃ before irradiation was small than the one in those at and less than 450 ℃. The erasing temperature is one of the important factors in finding the appropriate regenerative dose protocol.
[1] Fujiwara T., Toyoda S., Uchida A., Nishido H. and Ishibashi, J., (2016), Geochronometria, 43, 174-178.
Barite crystals extracted from chimney samples at the Iheya site in the Okinawa Trough were used. The extracted barite was heated at temperatures, 350, 400, 450, and 600℃ to erase the signal, and then gamma ray irradiation was performed at Takasaki Research Institute of Japanese Atomic Energy Agency. After that, ESR measurement was performed at room temperature with an ESR spectrometer (JEOL PX-2300) at Okayama University of Science Technology Center.
It was found that the signal intensity in the samples heated at 600 ℃ before irradiation was small than the one in those at and less than 450 ℃. The erasing temperature is one of the important factors in finding the appropriate regenerative dose protocol.
[1] Fujiwara T., Toyoda S., Uchida A., Nishido H. and Ishibashi, J., (2016), Geochronometria, 43, 174-178.