Abstract
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.
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