Variable distribution of subducted oceanic crust beneath subduction regions of the lowermost mantle

Abstract

Subducted oceanic crust (SOC) is one of the major sources of chemical heterogeneity in Earth's mantle. Previous geodynamic modeling studies have shown that SOC in the lowermost mantle could segregate from subducted slabs and accumulate on the core-mantle boundary (CMB). However, previous models are often simplified in a variety of ways that hinder a comprehensive understanding of the dynamics of SOC in the lowermost mantle. Here, I perform 3D high-resolution geodynamic models with single-sided subduction to study the dynamics of SOC in the lowermost mantle, with a focus on regions beneath subduction zones outside the large-low velocity provinces (LLVPs). I find that the ability of a thin (∼10 km) SOC to segregate from the slab is greatly controlled by the morphology of the slab. The segregation of SOC is much more efficient when the slab in the lowermost mantle is folded backward and extends beneath the subducting plate which allows the SOC to directly face or contact the CMB, than when the slab extends beneath the overriding plate with the SOC at the shallowest portion of the slab. The changes of slab morphology cause strong temporal and spatial variations in the distribution of SOC (1) at the CMB, (2) within 50 km above the CMB, and (3) at ∼50–300 km above the CMB, which, together with its variable degrees of mixing with other mantle materials, may explain the variations of seismic velocities, density, size, shape, and distribution of seismic anomalies in the lowermost mantle outside the LLVPs.