ABSTRACT

In order to quantify H₂O content and chemical composition of island arc low-K tholeiite magma that crystallized Ca-rich plagioclase, melt inclusions of a typical island arc tholeiite from Izu-Oshima volcano (34°N 44', 139°E 24') were analyzed. Composition of studied plagioclase ranges widely from An₈₃ to An₉₅. Composition of studied melt inclusions also shows wide variation, which suggests that the melt inclusions represent various stages of crystallization differentiation at Izu-Oshima volcano. Ca/Na ratios of plagioclase-hosted melt inclusions are comparable with compositions of aphyric lava, which preclude an exotic origin for the Ca-rich plagioclase. Analyzed H₂O content of the melt inclusions ranges from 0.2 to 2.4 wt.% (0.2 to 1.4 wt.% for plagioclase-hosted melt inclusions and 0.8 to 2.4 wt.% for olivine-hosted melt inclusions). Ca/Na partition coefficient between plagioclase and hydrous basaltic melt, K_D(Ca-Na)^plag-melt, is empirically calibrated based on experimental data as ln K_D(Ca-Na)^plag-melt = 4100/T(K) -800* P(GPa)/T(K) + 2.2* ln (Al₂O₃^melt(wt.%)/Sio₂^melt(wt.%)) + 0.33* √ H₂O^melt(wt.%). Based on the equilibria between host plagioclase and melt inclusion and taking effect of overgrowth into consideration, 3 to 6 wt.% H₂O in melt is required. The lower H₂O content of the analyzed melt inclusions is probably due to the leakage of volatiles through the host crystal during decompression, eruption and quench. Variation in estimated H₂O content in the melt at the time of crystallization of plagioclase (3 to 6 wt.%) can be due to polybaric crystallization from H₂O-saturated melt.