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

Abstract:

Kuritani et al. (2013, Mineral. Petrol.) and Kuritani et al. (2014, Contrib. Mineral. Petrol.) estimated genetic conditions of primary arc magmas beneath the Iwate volcano (a frontal arc volcano in the northeast Japan arc) and the Sannome-gata volcano (a rear-arc volcano in the northeast Japan arc) based on analyses of volcanic rocks and numerical simulation. They estimated that H₂O concentrations of primary melts are 4-5 wt.% beneath the Iwate volcano and 6-7 wt.% beneath the Sannnome-gata volcano, respectively. Their arguments mean that primary melts beneath frontal-arc volcanoes and rear-arc volcanoes are both H₂O-rich, yet there has been no direct evidence to support their arguments at the Sannnome-gata volcano because volcanic rocks are either almost aphyric and/or almost no melt inclusions were found.

Hydrogen concentration in nominally anhydrous minerals serves as a hygrometer of arc basaltic melts (e.g., Hamada et al. 2013, Earth Planet. Sci. Lett.). In this study, hydrogen concentration of plagioclase as a crustal xenocryst was analyzed to estimate H₂O concentration of basaltic melt coexisted with plagioclase before the eruption.

Plagioclase xenocrists were separated from crushed scoria which erupted from the Sannome-gata volcano 20,000-24,000 years ago. Composition of the plagioclase core is homogeneous and ranges from An₃₀ through An₃₅. The rim is 150 to 200-μm-thick dusty zone whose composition is around An₆₀, suggesting that the rim crystallized rapidly from degassed basaltic melt. The profiles of infrared absorption area per unit thickness across the plagioclase core were obtained using Fourier Transform InfraRed spectrometer (FTIR). The inner core contains hydrogen of about 60 wt. ppm H2O, and hydrogen concentration elevates at outer core. Hydrogen concentration at the outermost core of plagioclase is >200 wt. ppm H₂O, suggesting that plagioclase xenocrists were taken by hydrous melt (H2O>5 wt.%; Hamada et al. 2014, Earth Planet. Sci. Lett.) and hydrogen diffused into plagioclase before the eruption. These analytical results confirms that rear-arc primary magmas are H2O-rich as well as frontal-arc primary magmas at the northeast Japan arc.