Major and trace element abundances, Sr and Nd isotopic ratios, and 238 U-230 Th 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 ( 238 U/ 230 Th) 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. Th) 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.
The evolution of the magma plumbing system of Miyakejima volcano, an island arc basaltic volcano located on the volcanic front of the Izu arc, Japan, was investigated by combining (i) melting experiments of hydrous basaltic magma, (ii) analysis of melt inclusions in the volcanic rocks, and (iii) thermodynamic modeling using the thermodynamic program (MELTS) to understand the temporal evolution of the magma plumbing system. Basalts of the Ofunato Stage (10,000–4,000 year BP) show limited compositional variation without evidence of magma mixing, and seem to be parental to volcanic rocks of later stages. Hydrous melting experiments were conducted using the Ofunato Scoria (OFS), which represents the most undifferentiated basaltic magma during the past 10,000 years. The magma chamber conditions required to crystallize the phenocryst assemblage of the OFS were constrained to ~150 MPa, ~1100 °C, and ~3 wt% H2O in melt. The constrained H2O content is consistent with H2O content of olivine‐hosted melt inclusions in the OFS. Here we propose that a simple magma plumbing system existed beneath Miyakejima volcano during the Ofunato Stage, comprising a single magma chamber. The MELTS calculations show that andesites of the Tsubota Stage (4,000–2,500 year BP; representing the later part of the Ofunato Stage after a period of dormancy of 3,000 years), can be derived from the Ofunato basalts, provided that H2O content in magma is <1 wt%. The results of modeling indicate that a new magma chamber formed at a shallower depth after the Ofunato Stage, in which the differentiation of degassed basaltic magmas occurred.
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