Slab-derived fluids play an important role in heat and material transfer in subduction zones. Dehydration and decarbonation reactions of minerals in the subducting slab have been investigated using phase equilibria and modeling of fluid flow. Nevertheless, direct observations of the fluid chemistry and pressure-temperature conditions of fluids are few. This report describes CO2-bearing saline fluid inclusions in spinel-harzburgite xenoliths collected from the 1991 Pinatubo pumice deposits. The fluid inclusions are filled with saline solutions with 5.1 ± 1.0% (wt) NaCl-equivalent magnesite crystals, CO2-bearing vapor bubbles, and a talc and/or chrysotile layer on the walls. The xenoliths contain tremolite amphibole, which is stable in temperatures lower than 830 °C at the uppermost mantle. The Pinatubo volcano is located at the volcanic front of the Luzon arc associated with subduction of warm oceanic plate. The present observation suggests hydration of forearc mantle and the uppermost mantle by slab-derived CO2-bearing saline fluids. Dehydration and decarbonation take place, and seawater-like saline fluids migrate from the subducting slab to the mantle wedge. The presence of saline fluids is important because they can dissolve more metals than pure H2O and affect the chemical evolution of the mantle wedge.

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Noble gas, Nd and Sr isotopic ratios and major and trace element compositions were determined for ultramafic xenoliths and their host Cenozoic alkali basalts from Baegdusan, Baegryongdo, Jogokri, Jejudo in the Korean peninsula, and Long Quan, close to the Baegdusan in northeastern China, to characterize the lithospheric mantle and the source of alkali basaltic magmatism beneath the active continental margin of the southeastern part of the Eurasian plate. The xenolith samples yield significantly variable 3 He/ 4 He ratios ranging from <0.2 to 16.8 R A , with most samples (3.5-7.9 R A ) lower than the MORB value (~8 R A ). Among them, high 3 He/ 4 He ratios obtained by melting the samples are considered to reflect cosmogenic contribution. The 40 Ar/ 36 Ar ratios are much lower than the MORB values. Enriched Nd-Sr isotopic compositions in xenoliths and host basalts from the Baegdusan and Baegryongdo areas suggest assimilation of EMII lithosphere and/or continental crust. Widely ranging trace element concentrations in the xenoliths and highly saturated incompatible elements in the host alkali basalts are observed. K-Ar age data show that Cenozoic alkali volcanism in the Korean peninsula occurred intermittently, ranging in age from 21 Ma through 11.5-5.0 Ma to 0.1 Ma, and becoming gradually younger toward the south of the peninsula. Our geochemical and isotopic data suggest the presence of heterogeneously metasomatized enriched lithospheric mantle generated at an ancient subduction zone within the continental margin of the southeastern end of the Eurasian plate. Degree of enrichments by the metasomatism is discussed based on the observed 3 He/ 4 He ratios in the xenolithic olivines.Keywords: noble gas isotopes, ultramafic xenoliths, Nd-Sr isotopes, alkali basalts, metasomatized mantle wedge volcanism (Park and Park, 1996); differing degrees of partial melting of an EMI type lithospheric mantle source (Hsu et al., 2000); and melting of a depleted subcontinental lithospheric mantle source (Kim et al., 2002). In addition to these models, evidence of continental rift zone magmatism has been found in the Boun area, where a peridotite characterized by high Na-clinopyroxene has been reported by Arai et al. (2001). Recently, Sumino et al. (2000 found evidence of mantle plume type He in mantle xenoliths from Takashima, Japan, which is only ca. 200 km from the Jejudo area of the Korean peninsula (Fig. 1).Most lherzolites found in the peninsula are compositionally close to the subcontinental mantle peridotite with equilibrium temperatures and pressures estimated to be 820-1210°C and 11-28 kb, respectively

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