Uptaking of plagioclase xenocrist into H2O-rich rear-arc basaltic magma

Abstract

Hydrogen concentration of plagioclase xenocrysts in scoriae from the Sannnome-gata volcano, a rear-arc volcano in the northeast Japan arc, was analyzed to understand uptaking process of plagioclase xenocryst (An₃₀) into high-MgO (>8 wt.%) basaltic melt before the eruption. Hydrogen concentration of the outer core of plagioclase is >200 wt. ppm H₂O, while that of inner core is 60 wt. ppm H₂O. This observation suggests that plagioclase xenocryst was in contact with H₂O-rich basaltic melt for a couple of hours before the eruption and that the primary magma of the rear-arc volcano is H₂O-rich as well as that of frontal-arc volcano.