Studied waters belong to warm (T=30–50°C), alkaline (pH=8.9–9.3), low mineralized (TDS<235 mg/l) Na–HCO3 or Na–SO4–HCO3 thermal waters with high content of SiO2 (up to 81 mg/l) and F (up to 3.9 mg/l), occur on modern volcano–tectonic rejuvenated areas of Eastern Sikhote–Alin orogenic belt. Low 3He concentration as well as N2/O2 and N2/Ar ratios exclude influence of deep mantle fluid. New rare earth element data constrain our understanding of water–rock interaction occurring in the water source region. Meteoric origin of waters is proved by stable isotope values varying from −71‰ to −136.1‰ and from −10.8‰ to −18.8‰ for δ2H and δ18O respectively. REE patterns reflect high pH, resultfing from water–rock interaction and oxidative conditions. Calculations of deep aquifer temperature using Na–K and quartz geothermometers show 116.8–131.1° C and 82.2–125.8°C respectively. Presence of deep faults both with abnormal thermal gradient (∼45–50 K/km) define unique geochemical shape of thermal waters of Sikhote–Alin, area, where no present volcanic activity is registered.

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Ophiolites are divided into lherzolite-type (L-type) and harzburgite-type (H-type) by the lithology of their mantle peridotites. Rare depleted harzburgite-type (DH-type) is distinguished from the normal H-type by the more refractory nature of its mantle peridotite and the occurrence of orthopyroxene-type cumulate rocks including iron-rich harzburgite and orthopyroxenite. The Shelting (Sakhalin) and Krasnaya (Koryak Mountains) ophiolites in Far East Russia, which have both depleted harzburgite and orthopyroxene-type cumulate rocks, belong to this newly defined DH-type. The ophiolites in SW Japan-Primorye, NE Japan-Sakhalin, and the Koryak Mountains in the northwestern Pacific margin have diverse ophiolite types ranging from L- to DH-types. The wide petrological diversity, the common occurrence of DH-type, and the presence of thick crustal sections in these ophiolites suggest regionally inhomogeneous, commonly very high degrees of mantle melting over subduction zones, as in the modern Mariana forearc environment. The ophiolites of Japan and Far East Russia range in age from Early Palaeozoic to Cenozoic and are tectonically underlain by younger blueschists and accretionary complexes. The spatial association of these ophiolites with blueschists is analogous to the ophiolite-blueschist assemblages recovered from the Mariana forearc. This association might have formed in a period of non-accretion at an oceanic subduction zone that was followed by voluminous accretion of sediments, facilitating subsequent uplift of the ophiolites and blueschists.

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