Water content in arc basaltic magma in the Northeast Japan and Izu arcs: An estimate from Ca/Na partitioning between plagioclase and melt

Abstract

The variation in water content of arc basaltic magmas in the Northeast Japan arc and the Izu arc was estimated using a simple plagioclase phenocryst hygrometer. In order to construct a plagioclase phenocryst hygrometer optimized for arc basalt magmas, we have conducted high-pressure melting experiments of relatively primitive basalt from the Miyakejima volcano, a frontal-arc volcano in the Izu arc. As a result of the experiments, we found that the Ca/Na partition coefficient between plagioclase and hydrous basaltic melt increases linearly with an increase in H2O content in the melts. We then selected from literature geochemical data sets of relatively primitive basaltic rocks with no evidence of magma mixing and the most frequent Ca-rich plagioclase phenocrysts from 15 basaltic arc volcanoes including both frontal-arc and rear-arc volcanoes. In the 15 volcanoes studied, plagioclase phenocrysts of high anorthite content (An∈>∈90) were commonly observed, whereas plagioclase phenocrysts in rear arc volcanoes usually had a lower anorthite content (90∈>∈An∈>∈80). In all volcanoes studied, the estimated H2O content of basaltic magma was at least 3 wt.% H2O or higher. The magmas of volcanoes located on the volcanic front have about 5 wt.% H2O in magma whereas those from the rear-arc side are slightly lower in H2O content.