5:15 PM - 6:45 PM
[SCG48-P38] ESR and radioisotope disequilibrium dating of barite occurring in Higashi-Aogashima sea-floor hydrothermal area
Keywords:dating, barite, ESR, disequilibrium, sea-floor hydrothermal
The ages of deep-sea floor hydrothermal activities provide important data for discussing the potential of the sea-floor mineral resources and the evolution of the biological communities living on these hydrothermal energies. Our research group has, for the first time, practically applied the electron spin resonance (ESR) dating method to sea-floor hydrothermal barite (BaSO4), formed by the reaction of Ba in hydrothermal fluids with sulfate ions in seawater. Since barite contains Ra substituted for Ba, radioactuve disequilibrium ages with radium (226Ra-210Pb and 228Ra-228Th) can also be obtained. By combining these three methods, systematic dating of barite samples collected from hydrothermal areas off the coasts of Okinawa, Izu, and Ogasawara has been conducted.
In 2015, a new field, the Higashi-Aoga-shima Hydrothermal Field, was discovered east of Aoga-shima, one of the Izu Islands. It was found that the collected ore samples from this field contained a high concentration of gold. Research has been ongoing to investigate the concentration mechanism. In June 2023, the Japan Agency for Marine-Earth Science and Technology conducted the KM23-08_09C expedition using the research vessel "Kaimei." We applied, in the present study, these dating methods, the radium disequilibrium and the ESR dating methods, to barite samples collected during the expedition.
Samples with a high content of barite were selected, one sample from the Central Cone site and two samples from the East site, and were analyzed. The ore samples collected by a submersible vessel were cut and further divided into centimeter-sized pieces. After soaking in deionized water to measure mass, the samples were dried to measure mass to determine the water content. Twenty grams of each sample were crushed, and the bulk Ra concentration was determined using a low-background gamma-ray spectrometer. Subsequently, barite was extracted by chemical processing, and gamma-rays of several doses were given. ESR measurements were carried out to measure the signal intensity of the SO3- radicals, and the total absorbed doses due to natural radiation were obtained from the dose-responses.
The extracted barite was againe analyzed using the low-background pure Ge semiconductor gamma-ray spectrometer to obtain the concentrations of radioisotope nuclides (226Ra, 228Ra, 228Th, 210Pb). The dose rate of natural radiation received by the barite was calculated from the amounts of the determined radioisotope nuclides. Taking into account the decay due to the age of the nuclides, the change in the dose rate was calculated, and ESR ages were obtained. The radioisotope disequilibrium ages were also determined.
According to the obtained ESR ages, this hydrothermal field has a history of approximately 1000 years, although it does not coincide with the radioisotope disequilibrium ages. It is speculated that such results occurred due to the recent ongoing accumulation of barite.
In 2015, a new field, the Higashi-Aoga-shima Hydrothermal Field, was discovered east of Aoga-shima, one of the Izu Islands. It was found that the collected ore samples from this field contained a high concentration of gold. Research has been ongoing to investigate the concentration mechanism. In June 2023, the Japan Agency for Marine-Earth Science and Technology conducted the KM23-08_09C expedition using the research vessel "Kaimei." We applied, in the present study, these dating methods, the radium disequilibrium and the ESR dating methods, to barite samples collected during the expedition.
Samples with a high content of barite were selected, one sample from the Central Cone site and two samples from the East site, and were analyzed. The ore samples collected by a submersible vessel were cut and further divided into centimeter-sized pieces. After soaking in deionized water to measure mass, the samples were dried to measure mass to determine the water content. Twenty grams of each sample were crushed, and the bulk Ra concentration was determined using a low-background gamma-ray spectrometer. Subsequently, barite was extracted by chemical processing, and gamma-rays of several doses were given. ESR measurements were carried out to measure the signal intensity of the SO3- radicals, and the total absorbed doses due to natural radiation were obtained from the dose-responses.
The extracted barite was againe analyzed using the low-background pure Ge semiconductor gamma-ray spectrometer to obtain the concentrations of radioisotope nuclides (226Ra, 228Ra, 228Th, 210Pb). The dose rate of natural radiation received by the barite was calculated from the amounts of the determined radioisotope nuclides. Taking into account the decay due to the age of the nuclides, the change in the dose rate was calculated, and ESR ages were obtained. The radioisotope disequilibrium ages were also determined.
According to the obtained ESR ages, this hydrothermal field has a history of approximately 1000 years, although it does not coincide with the radioisotope disequilibrium ages. It is speculated that such results occurred due to the recent ongoing accumulation of barite.