H2O-rich island arc low-K tholeiite magma inferred from Ca-rich plagioclase-melt inclusion equilibria

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, KD(Ca-Na)plag-melt, is empirically calibrated based on experimental data as ln KD(Ca-Na)plag-melt = 4100/T(K) -800* P(GPa)/T(K) + 2.2* ln (Al2O3melt(wt.%)/Sio2melt(wt.%)) + 0.33* √ H2Omelt(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.