^<210>Pb/^<226>Ra法による熱水性重晶石の生成年代測定(<特集>若手研究者の初論文特集)

概要

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An age determination and the chemical composition of hydrothermal barite, collected from an Okinawa Trough hydrothermal field, were studied. Initially contaminated ^<210>Pb 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-BaSO_4 references. 3. The precipitation age, calculated by the ^<210>Pb/^<226>Ra method after correction was concordant with the ^<228>Th/^<228>Ra and ^<228>Ra/^<226>Ra 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.
2004-09-05