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 hydrous 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 compiled published 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.