Abstract
Time scale of magmatic processes in the 1986 eruptive activity of Izu-Oshima volcano, Japan, was estimated by the 238U-230Th-226Ra disequilibria observed in the 1986 lava flows. The activity ratios of 226Ra/230Th in the products were larger than unity, being enriched in 226Ra relative to 230Th. The 226Ra/230Th activity ratio versus 238U/230Th activity ratio and Ba/Th concentration ratio diagrams for the 1986 products showed positive correlations, suggesting that the 238U-230Th-226Ra disequilibria occurred during the magma genesis of Izu-Oshima volcano by the additions of U- and Ra-(Ba-)rich fluids derived from the subducting slab by dehydration to the mantle wedge. The 230Th-226Ra disequilibria in the 1986 products suggested that the time scale from the magma genesis to the eruption, including the melting of the mantle wedge, magma storage and magmatic differentiation from basalt to basaltic andesite, was less than 8000years.
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