B-type olivine fabrics developed during progressive retrogression above subducting slab in the mantle wedge

B-type olivine fabrics occur pervasively within highly depleted dunites in a small Imono peridotite body located in the subduction-type Sanbagawa metamorphic belt in the southwest Japan arc, with various microstructures from relatively coarse granular texture to fine-grained intensely sheared texture. The Mg/(Mg + Fe) atomic ratios (Fo number) of olivine within these dunites are constantly around 0.9 and Cr/(Cr + Al) atomic ratios (Cr number) of chromian spinels are constantly around 0.9. It suggesting that they could have evolved through high depleted magma. This provides strong thermal constraints on the formation of the highly depleted dunites that require hot, hydrous shallow mantle (>1250C at <30km depth) in the mantle wedge. Before these peridotites were finally entrained by the Sanbagawa metamorphic belt during progressive retrogression, they could flow downward along the subducting slab up to 2.8 GPa (>80km) at temperatures of 750-800C. Therefore, B-type olivine fabrics should be developed during downward flow along the subducting slab, possibly in relation with dehydration from the slab. Such occurrences provide a new insight of mantle flow in the fore-arc mantle wedge, where hydrated asthenospheric mantle flow is subsequently altered by retrogressive deformation before/during exhumation with metamorphosed slab materials. The small magnitude of S-wave splitting can be explained by the seismic properties of the B-type peridotites for an approximately 10-km thick anisotropic layer subparallel to the subducting slab, indicating that the B-type layer could be one of the dominant sources of seismic anisotropy in the fore-arc region.