Variable seismic structure near the 660 km discontinuity associated with stagnant slabs and geochemical implications

This study reports a compilation of reflectivity synthetic modeling for the structure of the upper mantle transition zone with high velocity anomalies (HVAʹs), associated with the northwestern Pacific subduction zones. Here, the employed method of reflectivity synthetics effectively determines the structure of flattened HVA, i.e. stagnant slab, as triplicated regional body waves are very sensitive to the velocity discontinuities in the transition zone. Results show a distribution of HVAʹs with or without a depression of the “660 km” discontinuity depth, which indicates a possible variation of geochemical properties at the bottom of the upper mantle. A hypothesis is proposed for this implication, i.e. that the structural variation may represent the contrast between a hydrous garnet-rich layer (subducted crust) and bulk peridotite associated with a stagnant slab, which is supported by the results of recent laboratory experiments. The garnet-rich layer can flow and descend faster than bulk peridotite as hydrous garnet is weaker and denser than peridotite in the transition zone. Given that the Clapeyron slope for hydrous garnet–perovskite is positive at ∼660 km, two zones of HVA with, and without a depression in the discontinuity depth may exist next to each other at the bottom of the transition zone. This variation of the discontinuity depths coincides with the segmentation identified at deeper depths (>629 km) in a P-wave travel-time tomography model although the resolution of the tomography inversion is limited to elaborate the discontinuity structure.