²³⁸U-²³⁰Th Radioactive Disequilibria in the Volcanic Products from Izu Arc Volcanoes, Japan

Abstract

The timescale of magmatic processes of Izu arc volcanoes, Japan, was estimated by the ²³⁸U - ²³⁰Th disequilibria in the volcanic products from the volcanoes. The majority of the ²³⁰Th/²³⁸U activity ratios of the products were less than unity, being enriched in ²³⁸U relative to ²³⁰Th. The (²³⁰Th/²³²Th) - (²³⁸U/²³²Th) diagram for younger Fuji and Izu-Oshima volcanoes formed a whole rock isochrons, and the ages were l×10⁴ and 2×10⁴ years, respectively. The (²³⁰Th/²³²Th) - (²³⁸U/²³²Th) data set for younger Fuji volcano formed a cluster on the diagram, while those of Izu-Oshima formed another cluster apparently apart from each other, suggesting that the concentration of U and Th may possibly be un-uniform in the mantle beneath Izu arc.