5:15 PM - 7:15 PM
[SGL22-P04] ESR dating of sea-floor hydrothermal barite - initial concentration of 228Ra/226Ra ratio and neutron activation analysis of Ba
Keywords:ESR dating, barite, sea-floor hydrothermal area, Ra isotopic ratio
Electron spin resonance (ESR) dating is based on the phenomenon that unpaired electrons are generated by natural radiation and accumulate in minerals over geological timescales. Our research group has, for the first time, successfully applied the ESR dating method to seafloor hydrothermal barite (BaSO4). The ages of seafloor hydrothermal activities provide crucial data for discussing the potential of seafloor mineral resources and the evolution of biological communities sustained by hydrothermal activities.
There have been studies aimed at improving the ESR dating method for hydrothermal barite. In the present study, we further refined the method by introducing two factors:
(1) the initial 228Ra/226Ra ratio incorporated into barite
(2) barite content in the bulk sample, estimated by neutron activation analysis.
The primary natural radiation source in barite is Ra, which substitutes for Ba in the crystal structure. While 226Ra has a half-life of 1600 years, the half-life of 228Ra is much shorter, at 5.75 years, and is therefore unobservable in samples older than approximately 50 years.
In past studies, the contributions of 228Ra and its daughter nuclei to the radiation dose in barite have been neglected when 228Ra was not detected in the sample. However, a previous study indicated that this contribution cannot be ignored for samples younger than 300 years in certain cases. In the present study, the present-day 228Ra/226Ra ratio in hydrothermal fluid was measured using a commercial Mn fiber, which absorbs radium from water. We assumed that this measured ratio corresponds to the initial ratio incorporated into the barite at the time of its formation. Based on this assumption, we estimated the contribution of 228Ra to the radiation dose and corrected the age accordingly.
The barite content in the bulk sample is a parameter necessary for estimating the dose rate to barite. Previously, this value was obtained by dividing the weight of the chemically extracted barite by the initial bulk sample weight. However, there is no guarantee that the extracted material is pure barite, and some barite may be lost during the extraction process.
In the present study, we introduced neutron activation analysis to measure the Ba concentration in the bulk sample, allowing for a more accurate estimation of the barite content.
In 2015, a new hydrothermal field, the Higashi-Aogashima Knoll Caldeta hydrothermal Field, was discovered east of Aoga-shima, one of the Izu Islands. Ore samples collected from this field were found to contain a high concentration of gold, prompting ongoing research into the concentration mechanism.
The JAMSTEC conducted research cruises KM23-08_09C in June-July 2023, as well as KM24-09 in August–September 2024, using the research vessel Kaimei. In the present study, we applied the improved ESR dating method to a total of ten samples: three from the Central Cone site and seven from the East site, collected during these expeditions.
The bulk Ra concentration was determined using a low-background gamma-ray spectrometer. Subsequently, barite was extracted through chemical processing, and gamma-ray doses of several levels were applied. ESR measurements were then performed to measure the signal intensity of SO3- radical and the total absorbed dose from natural radiation was obtained from the dose response.
Mn fiber, placed in a stainless steel cage, was exposed to hydrothermal fluid for 15 minutes to 2 hours during the dive. After rinsing and drying, the fiber was measured using a low-background, pure Ge gamma-ray spectrometer.
Neutron activation analysis was conducted using a pure Ge gamma-ray spectrometer on bulk samples after 30 minutes of neutron irradiation in a research reactor at Institute of Integrated Radiation and Nuclear Science, Kyoto University.
The present-day 228Ra/226Ra ratio was determined to be 0.20 ± 0.02. Assuming this value represents the initial ratio incorporated into the barite, the ESR ages were corrected accordingly. Ages initially estimated to be about 300 years without this consideration became approximately 30 years younger after correction. As results, ESR ages ranging from 0 to 2100 years were obtained for the present samples.
There have been studies aimed at improving the ESR dating method for hydrothermal barite. In the present study, we further refined the method by introducing two factors:
(1) the initial 228Ra/226Ra ratio incorporated into barite
(2) barite content in the bulk sample, estimated by neutron activation analysis.
The primary natural radiation source in barite is Ra, which substitutes for Ba in the crystal structure. While 226Ra has a half-life of 1600 years, the half-life of 228Ra is much shorter, at 5.75 years, and is therefore unobservable in samples older than approximately 50 years.
In past studies, the contributions of 228Ra and its daughter nuclei to the radiation dose in barite have been neglected when 228Ra was not detected in the sample. However, a previous study indicated that this contribution cannot be ignored for samples younger than 300 years in certain cases. In the present study, the present-day 228Ra/226Ra ratio in hydrothermal fluid was measured using a commercial Mn fiber, which absorbs radium from water. We assumed that this measured ratio corresponds to the initial ratio incorporated into the barite at the time of its formation. Based on this assumption, we estimated the contribution of 228Ra to the radiation dose and corrected the age accordingly.
The barite content in the bulk sample is a parameter necessary for estimating the dose rate to barite. Previously, this value was obtained by dividing the weight of the chemically extracted barite by the initial bulk sample weight. However, there is no guarantee that the extracted material is pure barite, and some barite may be lost during the extraction process.
In the present study, we introduced neutron activation analysis to measure the Ba concentration in the bulk sample, allowing for a more accurate estimation of the barite content.
In 2015, a new hydrothermal field, the Higashi-Aogashima Knoll Caldeta hydrothermal Field, was discovered east of Aoga-shima, one of the Izu Islands. Ore samples collected from this field were found to contain a high concentration of gold, prompting ongoing research into the concentration mechanism.
The JAMSTEC conducted research cruises KM23-08_09C in June-July 2023, as well as KM24-09 in August–September 2024, using the research vessel Kaimei. In the present study, we applied the improved ESR dating method to a total of ten samples: three from the Central Cone site and seven from the East site, collected during these expeditions.
The bulk Ra concentration was determined using a low-background gamma-ray spectrometer. Subsequently, barite was extracted through chemical processing, and gamma-ray doses of several levels were applied. ESR measurements were then performed to measure the signal intensity of SO3- radical and the total absorbed dose from natural radiation was obtained from the dose response.
Mn fiber, placed in a stainless steel cage, was exposed to hydrothermal fluid for 15 minutes to 2 hours during the dive. After rinsing and drying, the fiber was measured using a low-background, pure Ge gamma-ray spectrometer.
Neutron activation analysis was conducted using a pure Ge gamma-ray spectrometer on bulk samples after 30 minutes of neutron irradiation in a research reactor at Institute of Integrated Radiation and Nuclear Science, Kyoto University.
The present-day 228Ra/226Ra ratio was determined to be 0.20 ± 0.02. Assuming this value represents the initial ratio incorporated into the barite, the ESR ages were corrected accordingly. Ages initially estimated to be about 300 years without this consideration became approximately 30 years younger after correction. As results, ESR ages ranging from 0 to 2100 years were obtained for the present samples.