Geochemistry of metasomatized peridotites above subducting slab: A case study of hydrous metaperidotites from Happo-O'ne complex, central Japan

Abstract

Hydrous peridotites from the Happo-O'ne area in central Japan, which form a serpentinite mélange in the high-P/T Renge metamorphic belt, vary from depleted lherzolites to harzburgites with subordinate dunite that contains primary chromian spinel with Cr# = 0.72 on average. Systematic variations of major and trace elements of bulk-rock compositions possibly resulted from various degrees of partial melting experienced by their protoliths (15% to <30% fractional melting). Some hydrous peridotites from the tremolite zone are representatives of the mantle peridotite facies with Ti-rich chromian spinel (up to 5.7 wt% TiO2), and these peridotites have the same compositions as other peridotites except for enrichment in Na2O, Ba, and Ti. Their bulk rocks show U-shaped PM (primitive mantle)-normalized REE (rare earth elements) patterns (0.05-0.5 times PM) and are enriched with Cs, Pb, Sr, Ba, and Rb (0.2-20 times PM), indicating metasomatism by fluids derived from the subducting slab. The hydrous peridotites contain low amounts of Zr, Hf, Ta, and Nb (HFSEs), which possibly indicate that metasomatizing melts are not involved. These results are confirmed by in-situ analysis of trem-olites that show U-shaped REE patterns (0.1-3 times PM) and are highly enriched with fluid-mobile elements (B, Li, Cs, Sr, and Pb; 1-100 times PM) and Sc relative to HFSEs.