Peridotite xenoliths from Avacha volcano, southern Kamchatka, are mostly clinopyroxene-poor (< 2 vol.%) harzburgite, and have been metasomatized by SiO
2-rich melt/fluid. The metasomatism is characterized by the formation of secondary orthopyroxene replacing olivine. We collected extremely fine-grained peridotite, composed of fine (< 100 µm) minerals (sometimes extremely fine, less than 10 µm), as well as coarse-grained ones. The degree of metasomatism, i.e., the amount of secondary orthopyroxene replacing olivine, of the fine-grained peridotites is usually higher than the coarse-grained ones. Among the fine-grained peridotites, we found one peridotite xenolith (sample #159) containing domains composed of extremely high-Ni (up to 5.3 wt% of NiO) olivine, associated with secondary orthopyroxene, high-Ni MSS (monosulfide solid solutions), and Fe-Ni sepiolite. MSS do not show any exsolution and are chemically homogeneous. The Ni enrichment shows outward diffuse within individual circular domains (< 1 mm across), which are disseminated in the whole thin section. The orthopyroxene (Mg# [= Mg/(Mg + total Fe) atomic ratio]; 0.93) interstitial to the high-NiO olivine and chromian spinel (Cr# [= Cr/(Cr+Al) atomic ratio]; 0.53) also have high NiO contents, up to 1.1 wt% and 1.8 wt%, respectively. This result denies that the formation of high-Ni olivine due to metasomatic formation of low-Ni orthopyroxene, and strongly suggests that Ni addition had occurred after formation of the secondary orthopyroxene. Nickel-rich sepiolite (NiO = 21.6 wt%) is found the center of the Ni-rich domain. We interpreted that the sepiolite is an alteration mineral from some Ni-rich agent, probably Ni-rich sulfide melt, that caused the Ni enrichment of olivine and other minerals. We propose that the sulfide melt play a role as an important carrier of Ni to metasomatize mantle peridotite beneath Avacha volcano. Sulfur is supplied to the sub-arc mantle from subducting slab.
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