2026/07/27 by Tatsuhiko Kawamoto, Hajime Taniuchi, Keisuke Kurihara +4
paper · doi:10.1093/petrology/egag061
Abstract Interlayered peridotites and mafic rocks in orogenic peridotite bodies provide evidence for melt–mantle interactions. A 1.2 m-thick interlayered spinel peridotite, plagioclase lherzolite, and mafic rocks is studied in the Horoman peridotite complex, Japan. Major and trace elements compositions of whole-rock and minerals provide constraints on the mechanisms responsible for such interlayered structure. The spinel peridotite and the mafic rocks have the highest and lowest MgO contents, and have the lowest and highest incompatible element contents, respectively. The plagioclase lherzolite is characterized by millimeter-scale alternating laminae of plagioclase-free and plagioclase-clinopyroxene-bearing peridotite. Its composition is intermediate between that of spinel peridotite and mafic rocks. The whole-rock and mineral compositions of the mafic rocks indicate that they formed from N-MORB-like melts. A linear compositional trend of major elements except for FeO*, MnO, P2O5 and Cr on MgO variation diagrams indicate that various proportions of N-MORB-like melts were added to a massive peridotite to form each layer. The FeO* versus MgO diagram shows that the composition varies along iso-Mg# (100 × molar Mg/(Mg + Fe)) lines, which shows olivine crystallization from the injected melts in the open system. This olivine accumulation has been suggested to elucidate whole-rock FeO*–MgO variations in abyssal peridotite. At boundaries between spinel peridotite and mafic rocks, coarse clinopyroxene crystals exhibit Ti-enriched rims, which provides evidence of chemical disequilibrium during initial melt–mantle interaction. As mantle diapirs upwell beneath mid-oceanic ridges, they experience repeated injections of melts with variable volumes and react with them. Chemical gradation in mafic rock layers from boundaries against spinel peridotite is observed in Mg# of mafic minerals in a few cm-scale. Diffusive equilibration between the mafic rocks layer and the spinel peridotite with respect to Mg-Fe equilibrium can be achieved only in several mm-scale. The plagioclase lherzolite layers formed through such mm-scale equilibrium between the peridotite and the melts with respect to Mg-Fe exchange; in contrast, plagioclase and clinopyroxene components were not smeared out and the interlayered structure remained. The observed chemically interlayered structure in the Horoman peridotite records melt injection and subsequent olivine accumulation processes underneath a mid-oceanic ridge.