238U-230Th radioactive disequilibrium in the northern Izu arc: (230Th/232Th) in the sub-arc mantle

Major and trace element abundances, Sr and Nd isotopic ratios, and 238U-230Th radioactive disequilibria have been analyzed for samples from five volcanoes, Oshima, Miyakejima, Niijima, Teishi Knoll and Fuji, located in the northern Izu arc to investigate across-arc and regional variations of chemical compositions of mantle and fluids expelled from a subducting slab. The abundances and abundance ratios of both fluid mobile and immobile trace elements show across-arc variations. All samples but one have radioactive disequilibria, with (238U/230Th) greater than unity (herein, a ratio in parentheses denotes the activity ratio). The observed disequilibrium is similar to those reported for other arc systems, such as Mariana, Tonga, Kermadec, Chile. It originates from higher mobility of uranium relative to thorium during dewatering of the subducting slab. The degree of radioactive disequilibrium of (238U/230Th) decreases with depth to the Wadati-Benioff zone at each volcano. The radioactivity ratios of (230Th/232Th) of the samples from Miyakejima, however, deviate from an across-arc trend formed by Oshima, Niijima, Teishi Knoll and Fuji. The combined data of (230Th/232Th)-(238U/232Th) of the samples from the Izu and the Mariana arc revealed a well defined array in an isochron diagram with the exceptions of the samples from Miyakejima and Alamagan. This finding requires (230Th/232Th) of the sub-arc mantle beneath the arcs of Izu and Mariana except the magma sources of the two islands of Miyakejima and Alamagan was around 1.1 and relatively homogeneous before the final addition of fluid from slab. The homogeneous (230Th/232Th) necessitates input of Th with homogeneous (230Th/232Th) contributed from sediment melt, which dominates the budget of Th in the sub-arc mantle. It also suggests that influence of fluid addition before the final one had not caused a variation in Th isotopic compositions of the sub-arc mantle.