抄録
An age determination and the chemical composition of hydrothermal barite, collected from an Okinawa Trough hydrothermal field, were studied. Initially contaminated 210Pb in sulfide and other minerals were leached by a chemical treatment. After the treatment, the radioactivity and chemical composition were determined by gamma spectrometry and neutron activation analysis, respectively. The following results were obtained: 1. An accurate and reproducible age was obtained by a chemical treatment of barite samples. 2. A matrix effect of barite samples on gamma-ray measurement was significantly observed, and was calibrated with mixed NaCl-BaSO4 references. 3. The precipitation age, calculated by the 210Pb/226Ra method after correction was concordant with the 228Th/228Ra and 228Ra/226Ra methods. 4. The Sr/Ba weight ratios were in the range of 0.007∼0.053, which seems to have positive relationship with the temperature of the hydrothermal vent fluid. 5. A chronological study of barite in sample No. 974R1 showed that Sr/Ba weight ratio seems to be high in older samples, and tends to decrease in young samples, which would suggest a possible alteration of the hydrothermal activity in this field.
