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.
Highlights
Antigorite-bearing serpentinite is a key agent for the recycling of H2O, carbon, sulfur and other elements from the surface environment into the Earth’s mantle[1,2,3,4]
We apply this model to the well-documented seismic events in the Ryukyu forearc and show how knowledge of the ray path geometry can be combined with S-wave splitting to constrain the structure of the shallow mantle and infer the distribution and proportion of antigorite
We suggest that subduction zones that exhibit a similar pattern of both local-S and SK(K)S observations as found in the Ryukyu arc are likely to have widespread antigorite-dominant shallow mantle that is undergoing convection
Summary
Antigorite-bearing serpentinite is a key agent for the recycling of H2O, carbon, sulfur and other elements from the surface environment into the Earth’s mantle[1,2,3,4]. Some of the best seismic evidence for the presence of antigorite and its distribution in subduction zone mantle wedge comes from the Ryukyu convergent margin[8] (Fig. 2) In this region the large delay times associated with S-wave splitting are much greater than those which might be produced by crustal cracks or olivine-rich mantle[8,29] alone. This has led workers to propose the presence of a 10-kilometer scale thick antigorite-rich shear zone parallel to the subduction boundary[8] with a foliation defined by strongly aligned grains of antigorite. The observed delay times are, similar for both intra-slab and teleseismic events[30]
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