Influence of Th-rich mineral phases on U–Th radioactive disequilibrium ages of sulfide deposits from the Okinawa Trough

Abstract

Many samples of hydrothermal sulfide minerals from the Okinawa Trough yield 234U–230Th radioactive disequilibrium ages that are inconsistent with the ages of coexistent barite obtained using electron spin resonance (ESR) method. These results contrast strongly against results obtained for the Southern Mariana Trough, for which the U–Th radioactive disequilibrium ages are roughly consistent with the ESR ages. Sulfide minerals from the Okinawa Trough show large variation in U and Th abundances, suggesting open-system behavior. We infer that the interaction of hydrothermal fluids with thick terrigenous sediment caused large-scale migration of U and Th in the Okinawa Trough. The (234U/230Th) – (232Th/230Th) plots for the sulfide samples from the Hatoma Knoll and the Izena Hole show two mixing trends for each site. The presence of Th-rich phase with high (230Th/234U) ratio seems to cause incorrect age estimates of the sulfide deposits.