Abstract
Abstract Activity ratios among 238U-230Th-226Ra in the products from Izu arc volcanoes, Japan, were observed in order to estimate the time scale of magmatic processes and the magma generation for Izu arc volcanism. Activity ratios of 238U/230Th and 226Ra/230Th in the basaltic and andesitic products from Izu arc volcanoes were greater than unity, being enriched in 238U and 226Ra relative to 230Th. The 226Ra/230Th activity ratio versus 238U/230Th activity ratio diagram for these products showed positive correlation, suggesting that the 238U-230Th-226Ra disequilibria occurred during the magma genesis by the additions of U- and Ra-rich fluids derived from the subducting slab by dehydration to the mantle wedge. The 230Th-226Ra radioactive disequilibria observed in the basaltic and andesitic products imply a short period of time (<8000 years) between the magma genesis and the eruption. The majority of rhyolitic products was considered to be almost in equilibrium of 238U=230Th=226Ra. The observation that 238U-230Th-226Ra for the rhyolite are in radioactive equilibrium suggested that the rhyolitic magma from Izu arc was generated in the partial melting of the earth crust heated by the basaltic magma of high temperature.
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