Fluid Inclusions trapped in Xenoliths from the Lower Crust/upperMantle Beneath Jeju Island (I); A Preliminary Study

This paper describes the textural relations of mantle xenoliths and fluid inclusions in mantlederived rocks found in alkaline basalts from Jeju Island which contain abundant ultramafic, felsic, and cumulate xenoliths. Most of the ultramafic xenoliths are spinel-lherzolites, composed of olivine, orthopyroxene, clinopyroxene and spinel. The felsic xenoliths considered as partially molten buchites consist of quartz and plagioclase with black veinlets, which are the product of ultrahigh-temperature metamorphism of lower crustal materials. The cumulate xenoliths, clinopyroxene-rich or clinopyroxene megacrysts, are also present. Textural examination of these xenoliths reveals that the xenoliths are typically coarse grained with metamorphic characteristics, testifying to a complex history of evolution of the lower crust/upper mantle source region. The ultramafic xenoliths contain protogranular, Porphyroclastic and equigranular textures with annealing features, indicating the presence of shear regime in upper mantle of the Island. The preferential associations of spinel and olivine with large orthopyroxenes suggest a previous high temperature equilibrium in the high-Al field and the original rock-type was a Al-rich OrthOPyrOXene-bearing peridotite without garnet. Three types of fluid inclusions trapped in mantle-derived xenoliths include C0,-rich fluid (Type I), multiphase silicate melt (glass I: devitrified crystals d= one or more daughter crystals + one or more vapor bubbles) (Type II), and sulfide (melt) inclusions (Type III). COi-rich inclusions are the most abundant volatile species in mantle xenoliths, supporting the presence of a separate C0,-rich phase. These C0,-nch inclusions are spatially associated with silicate and sulfide melts, suggesting immiscibility between them. Most multiphase silicate melt inclusions contain considerable amount of silicic glass, reflecting the formation of silicic melts in the lower crust/upper mantle, Combining fluid and melt inclusion data with conventional petrological and geochemical information will help to constrain the fluid regime, fluid-melt-mineral interaction processes in the mantle of the Korean Peninsula and pressure-temperature history of the host xenoliths in future studies