かんらん岩捕獲岩中のシリシックガラス：組成と成因

Abstract

We review petrological and chemical characteristics of silicic glasses and associated crystalline phases in mantle derived peridotite xenoliths reported from various volcanic rocks at the oceanic island, continental rift, and island arc settings. Most of silicic glasses in peridotite xenoliths reported from oceanic islands and continental rifts are rich in alkalis and silica-undersaturated (SiO2 = 57.4 wt%, Qz-(Ne + Lu) = − 4.45 wt% on average) and accompanied with secondary clinopyroxene and olivine. In the oceanic island and continental rift settings, almost all silicic glasses reported were primarily silica-undersaturated but these compositions were converted into silica-oversaturated by reacting with orthopyroxene. On the other hand, almost all glasses in the sub-arc peridotite xenoliths examined are silica-oversaturated (SiO2 = 63.0 wt%, Qz-(Ne + Lu) =16.6 wt% on average) and accompanied with secondary orthopyroxene, indicating their primary silica-oversaturated characters. Silica-oversaturated melts such as silicic glasses in the sub-arc peridotites can be produced either by hydrous partial melting of mantle peridotite (e.g., Hirose, 1997) or by melting or dehydration of basaltic slab materials (e.g., Prouteau et al., 2001). Silicic but silica-undersaturated melts such as silicic glasses, which are included in peridotite xenoliths from oceanic islands and continental rifts, can be produced either by small-degree partial melting of mantle peridotite (e.g., Falloon et al., 1997) or by reaction between carbonatitic melt and mantle peridotite (e.g., Green and Wallace, 1988).