Abstract
Observation was made with rhyolitic volcanic rocks erupted from Kozu-shima, Niijima and Higashi-Izu monogenetic volcano group for activity ratios of 230Th/238U and 226Ra/230Th. While the activity ratios of basaltic products from Izu arc volcanoes showed disequilibria of 230Th/238U<1 and 226Ra/230Th>1, the majority of rhyolitic products was considered to be almost in equilibrium of 238U=230Th=226Ra. The observation that 238U-230Th and 230Th-226Ra for the rhyolite are in radioactive equilibrium is consistent with the recent petrological suggestions that the rhyolitic magma from Izu arc was generated in the partial melting of the crust, where 238U-230Th and 230Th-226Ra are in equilibrium, by the basaltic magma of high temperature. Some of the rhyolite samples showed the enrichment of 230Th over 238U, indicating that the young marine sediments with 230Th>238U may possibly take part in the generation of the rhyolitic magmas.
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