Compositional heterogeneities in the mid-mantle revealed by seismic discontinuities and reflectors

Abstract:

We investigate seismic discontinuities and reflectors in the mid-mantle by analyzing SS precursors recorded at global seismic stations. Our results confirm the global existence of the 520-km discontinuity. Although its depth variations are generally correlated with temperature in the mid-mantle, they cannot be fully explained by the Clapeyron slope of the wadsleyite-ringwoodite phase transition, suggesting both thermal and compositional heterogeneities in the mantle transition zone. A second discontinuity at ~560-km depth, previously interpreted as splitting of the 520-km discontinuity, is most commonly detected in cold subduction zones and hot mantle regions. The depth separation between the 520- and 560-km discontinuities varies from ~80 km in cold regions to ~40 km in hot areas. Because the only known transition in major minerals at this depth in the mantle transition zone is the formation of Ca-pv, the existence of the 560-km discontinuity may imply localized high calcium concentrations in the mid-mantle, possibly related to the recycling of oceanic crust.

We also discover a megameter-scale seismic reflector at about 810-km depth west of the Sea of Okhotsk. It can only be explained as a portion of thickened oceanic crust subducted to the uppermost lower mantle. We propose that this thick crust represents the oceanic plateau, or at least its major part, that was created by massive decompression melting during the initiation stage of the Hawaiian hotspot and then subducted about 20 Ma ago. Combined with plate reconstruction models, our discovery provides temporal and spatial constraints on the early history of the Hawaiian plume.