Ai Uchida

ESR dating of barite in sulphide deposits formed by the sea-floor hydrothermal activities

Barite is a mineral newly found to be practically useful for electron spin resonance (ESR) dating of sulphide deposits formed by the sea-floor hydrothermal activities. The recent studies for the properties of the ESR dating signal in barite are summarised in the present paper as well as the formulas for corrections for accurate dose-rate estimation are developed including the dose-rate conversion factors, shape correction for gamma-ray dose and decay of (226)Ra. Although development of the techniques for ESR dating of barite has been completed, further comparative studies with other dating techniques such as U-Th and (226)Ra-(210)Pb dating are necessary for the technique to be widely used.

Dating of Hydrothermal Mineralization in Active Hydrothermal Fields in the Southern Mariana Trough

Ages of sulfide and sulfate mineralized samples collected from active hydrothermal fields in the Southern Mariana Trough were determined. In addition to samples collected from active and inactive chimneys, and sulfide breccia during dive expeditions, massive sulfide ores obtained by shallow drilling were studied. We applied 230Th/234U radioactive disequilibrium dating technique to sulfide minerals, as the collected mineralized samples were dominated by marcasite, pyrite and sphalerite. In addition, electron spin resonance (ESR) dating was applied to a few barite-rich samples, for comparison purpose. A laser step heating 39Ar-40Ar dating of the basement volcanic rock samples was also attempted.

ESR Dating of Barite in Sea-Floor Hydrothermal Sulfide Deposits in the Okinawa Trough

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 228Ra 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 226Ra-210Pb 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.

The alpha effectiveness of the dating ESR signal in barite: Possible dependence with age

Abstract 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.

ESR dating of sea-floor hydrothermal barite: Contribution of 228Ra to the accumulated dose

Abstract 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 228Ra 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 228Ra content is not obtained. The method to estimate the contribution from 228Ra series has to be developed in future for those in which 228Ra is not detected. The age limit of ESR dating of barite would be 5000 to 6000 years due to the decay of 226Ra, which is also found by simulation of the accumulated dose.

Immediate Change of Radiation Doses from Hydrothermal Deposits

The radiation from the sulfide deposits taken at Okinawa Trough was measured on-board by a NaI gamma ray spectrometer immediately after the retrieval of the sample onto the research vessel. Gamma ray peaks were identified as those from daughter nuclei of 226Ra and 228Ra. The slight change with time of the radiation dose from those deposits is probably due to the geometric arrangement in measurements. As results, no change in radiation dose was observed in the time range up to 90 h after retrieval of the samples from the submarine vehicle.

226Ra-210Pb and 228Ra-228Th Dating of Barite in Submarine Hydrothermal Sulfide Deposits Collected at the Okinawa Trough and the Southern Mariana Trough

The 226Ra-210Pb and 228Ra-228Th ages were obtained for barite crystals in hydrothermal sulfide deposits taken at the Okinawa Trough and the Southern Mariana Trough. After calibrating the measurement systems with standard samples with pitchblende, it was confirmed that the U and Th concentrations obtained for GSJ samples are consistent with literature values. It was shown that radon does not escape from barite crystals extracted from hydrothermal sulfide deposits, which indicates that 226Ra-210Pb dating method works for these barite crystals. Most of the 226Ra-210Pb and 228Ra-228Th ages are younger than ESR and U-Th ages, where this inconsistency would be explained by the mixture of the barite crystals with younger and older ages, formed by several hydrothermal events.

OSL dating of sea floor sediments at the okinawa trough

In estimating the ages of sea floor hydrothermal deposits, the age of the sediments overlying it would give the youngest limit if the sedimentation age is correct. The OSL (optically stimulated luminescence) method was applied to two sediment cores taken by an acrylic corer from the seafloor in the Okinawa Trough. The ages, obtained by the polymineral fine grain method, did not correlate with the stratigraphic sequence within the core, implying either insufficient bleaching of the sediments at the time of the deposition or mixing of sediments of different ages. The polymineral fine grain OSL dating method did not work to date the present sediment samples at the sea floor in the Okinawa Trough.