The earliest mantle fabrics formed during subduction zone infancy

•Harzburgites from the Izu-Ogasawara Trench preserve mantle fabrics formed during the inception of subduction.•E-type olivine patterns results from the slab-derived fluid into forearc mantle during the inception of subduction.•The structures such as E-type olivine patterns assist us in understanding the development of the mantle wedge from the immature to mature subduction zone.

Harzburgites obtained from the oldest crust-mantle section in the Philippine Sea plate (∼52 Ma) along the landward slope of the southern Izu-Ogasawara Trench, preserve mantle fabrics formed during the infancy of the subduction zone; that is during the initial stages of Pacific plate subduction beneath the Philippine Sea plate. The harzburgites have relatively fresh primary minerals despite of their heavy serpentinizations, and show inequigranular interlobate textures, and crystal preferred orientation patterns in olivine (001)[100] and Opx (100)[001]. The harzburgites have the characteristics of residual peridotites, whereas the dunites, obtained from the same location as the harzburgites, provide evidence for the earliest stages of arc volcanism during the inception of subduction. We propose that the (001)[100] olivine patterns began forming in immature fore-arc mantle with an increase in slab-derived hydrous fluids during the initial stages of subduction in in situ oceanic island arc.