Beryllium isotopic systematics in island arc volcanic rocks from northeast Japan: Implications for the incorporation of oceanic sediments into island arc magmas

We present beryllium (Be) isotopic ratios (10Be/9Be) for 55 volcanic rocks from 21 Quaternary volcanoes in the northeast Japan arc, including the Hokkaido and central Japan areas (the NEJH). Although Be isotopic ratios in these rocks are much lower than in other arcs reported in the literature, they are much higher than in control samples (ocean island basalt (OIB) and Tertiary basalts) that have no recent addition of 10Be. Hence, our results indicate the incorporation of a component derived from subducted sediments in the Japan arc magmas.Two kinds of Be isotopic variations were observed in the NEJH: 1) variations among lava flows of different ages in the same volcano; and 2) variations among volcanoes in an along-arc or cross-arc direction. In the first case, there are no apparent differences in Be isotopic ratios for lavas of the same age, but relatively large variations among lavas of different ages, and the degree of variation depends on the regional characteristics, indicating that regional differences in near-surface conditions may affect these Be isotopic variations. In the second case, the progressive decrease in Be isotopic ratios in a cross-arc direction suggests the continuous incorporation of subducted sediment, even in the back-arc region. The regional along-arc Be isotopic variations reflect not only the incorporation of sediments into the arc magma, but also regional tectonic and/or geological and geochemical processes.This is the first systematic study of Be isotopic systems in the NE Japan arc, and we provide direct evidence for the incorporation of sedimentary components from the subducting slab into the arc magma, including in the back-arc region at 300 km from the volcanic front, where the Wadati-Benioff Zone is at a depth of 290 km.