Deep low-velocity layer in the Tonga slab

Abstract:

The subducted oceanic lithosphere can carry a large amount of water into the deep mantle. The dehydration reactions of these hydrous minerals in the slab release a considerable amount of water into the overriding mantle wedge during subduction processes. It is generally believed that the water released during these dehydration events at 50–150 km depths primarily controls the melting and magmatic activities beneath the arc volcanos. However, it remains uncertain how much water can be preserved and delivered to the deep mantle beyond 150 km depth, especially in the cold subduction zones. Thus, it is crucial to provide seismic constraints on the depth extent of the hydrous minerals in a slab. Previous seismic tomography studies in northeast Japan have found a low-velocity layer in the slab crust extending to ~100 km depth, which is attributed to hydrous minerals and potential free fluids. The Tonga subduction zone is characterized by the fastest convergence rate and the coldest thermal condition. We manually examine all P-to-S waves converted at the Tonga slab surface because this phase is very sensitive to the detailed structure of the slab crust and mantle. And we conduct seismic tomography with a particular focus on the velocity structure of the slab interior. New results show a low-velocity layer extending to more than ~180 km depth in the Tonga slab. This deep low-velocity layer suggests that the Tonga slab brings hydrous minerals to a depth greater than previously thought. Our new images provide better constraints on the hydration state of the Tonga slab and help improve our understanding of the water cycle in the deep mantle.