²³⁸U-²³⁰Th-²²⁶Ra Radioactive Disequilibria in the Products from 1707 Eruption of Fuji Volcano, Japan

Abstract

Time scale of magmatic processes in the 1707 eruptive activity of Fuji volcano, Japan, was estimated by the ²³⁸U-²³⁰Th-²²⁶Ra disequilibria observed in the 1707 volcanic products. The activity ratios of ²²⁶Ra/²³⁰Th in the products were larger than unity, being enriched in ²²⁶Ra relative to ²³⁰Th. The decay-corrected ²²⁶Ra/²³⁰Th activity ratio to the time of the eruption versus ²³⁸U/²³⁰Th activity ratio diagram for the 1707 volcanic products showed a positive correlation, suggesting that the ²³⁸U-²³⁰Th-²²⁶Ra disequilibria occurred during the magma genesis of Fuji volcano. The ²³⁰Th-²²⁶Ra disequilibria in the 1707 volcanic 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 andesite, was less than 8000 years.