Trench-parallel anisotropy developed in the serpentinized forearc mantle: An example from the Ohmachi Seamount, Izu-Bonin frontal arc

Seismic anisotropy provides clues to understand the flow patterns in the upper mantle. Shear-wave splitting measurements reveal that the directions of the fast polarization axis at most subduction zones are oriented perpendicular to the flow direction. Several subduction systems show large delay times (up to 2 sec) that are incompatible with an origin from crystal-preferred orientation (CPO) of olivine in the anhydrous mantle wedge. On the other hand, it is well understood that water liberated by dehydration reactions within a subducting slab may induce serpentinization in the overlying forearc mantle. Accordingly, serpentine is one of the main constituent minerals in the hydrated mantle wedge: however, the relation between seismic anisotropy and serpentine CPO fabrics is not well understood. During the JAMSTEC YK08-05 cruise, various types of serpentinites were recovered from the Ohmachi Seamount in the Izu-Bonin frontal arc. We report their occurrences and metamorphic petrology, and reveal CPO pattern evolution of the serpentinites. Most olivines and pyroxenes have been directly replaced by antigorite, a high-temperature species. Some antigorite samples show a typical interpenetrating texture, characterized by randomly-oriented antigorite blades. Such antigorite blades are apparently crosscut by schistose antigorite blades showing a shape-preferred orientation. The interpenetrating and schistose antigorites have roughly the same compositions, showing coupled Al-substitution in tetrahedral and octahedral sites. Although the interpenetrating antigorite blades shows a random CPO, the schistose antigorite blades are characterized by a strong alignment of [001] axes normal to the foliation, [010] axes parallel to the lineation, and [100] axes normal to the lineation and within the plane of the foliation. This implies that the CPO in antigorite schist was produced by a progressive simple shear induced by the movement of a subducting slab. We can also suggest that in subduction zones the slowest [001] axis of serpentine (antigorite) is oriented normal to the dip of a subducting slab. This indicates a trench-parallel anisotropy in this region, as the [100] and [010] axes of serpentine have similar seismic velocities.