Volatile components in island arc magma indicated by mineral and melt inclusions in anorthite megacrysts: case study from Ogi peninsula, Sado Island

Abstract

Ca-rich plagioclase (An > 90 %) larger than 10 mm, which is called anorthite megacryst, occurs commonly from basalt - andesite in Japanese Island arc. We investigated anorthite megacrysts from Ogi Peninsula, Sado island and found melt inclusions rich in volatile components such as Cl and S. Electron microprobe analysis show that SiO₂ and Cl in these melt inclusions range 48 - 56% and 0.01 - 0.12%, respectively. Our results indicate that anorthite megacrysts from Ogi Peninsula grew in volatile-rich environment caused by release of Cl and S from subducting slab.