title: Seismic evidence for flow in the hydrated mantle wedge of the Ryukyu subduction zone.
abstract: It is widely accepted that water-rich serpentinite domains are commonly present in the mantle above shallow subducting slabs and play key roles in controlling the geochemical cycling and physical properties of subduction zones. Thermal and petrological models show the dominant serpentine mineral is antigorite. However, there is no good consensus on the amount, distribution and alignment of this mineral. Seismic velocities are commonly used to identify antigorite-rich domains, but antigorite is highly-anisotropic and depending on the seismic ray path, its properties can be very difficult to distinguish from non-hydrated olivine-rich mantle. Here, we utilize this anisotropy and show how an analysis of seismic anisotropy that incorporates measured ray path geometries in the Ryukyu arc can constrain the distribution, orientation and amount of antigorite. We find more than 54% of the wedge must consist of antigorite and the alignment must change from vertically aligned to parallel to the slab. This orientation change suggests convective flow in the hydrated forearc mantle. Shear wave splitting analysis in other subduction zones indicates large-scale serpentinization and forearc mantle convection are likely to be more widespread than generally recognized. The view that the forearc mantle of cold subduction zones is dry needs to be reassessed.
authors.affiliation: Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan.|School of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK.|Graduate School of Environmental Studies, Tohoku University, Sendai 980-8579, Japan.
authors.forename: Takayoshi
authors.initials: T
authors.lastname: Nagaya
authors.affiliation: School of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK.|School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK.
authors.forename: Andrew M
authors.initials: AM
authors.lastname: Walker
authors.affiliation: School of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK.
authors.forename: James
authors.initials: J
authors.lastname: Wookey
authors.affiliation: Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan.
authors.forename: Simon R
authors.initials: SR
authors.lastname: Wallis
authors.affiliation: Graduate School of Science, Osaka Prefecture University, Sakai 599-8531, Japan.
authors.forename: Kazuhiko
authors.initials: K
authors.lastname: Ishii
authors.affiliation: School of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK.
authors.forename: J-Michael
authors.initials: JM
authors.lastname: Kendall