Modeling Ultralow Velocity Zones as a Thin Chemically Distinct Dense Layer at the Core-Mantle Boundary

Abstract

Seismically detected ultralow velocity zones (ULVZs) at the the core-mantle boundary reflect the dynamical state and geological evolution of the silicate-metal frontier of Earth's deep interior. However, modeling the dynamical context of ULVZs is hampered by challenges, such as the necessity of fine-scale resolution and the accurate treatment of large viscosity contrasts. Here we extend the treatment of ULVZs using a lubrication theory approach and apply it to numerical and analytical models relevant for mantle convection in the core-mantle boundary region. A generic model of a thin and dense low-viscosity ULVZ layer embedded between an overlying convecting viscous mantle and an underlying inviscid core can explain several features that are consistent with seismic inferences, such as the absence of ULVZs in some regions and a tabular shape where they are concentrated. The model explains how the topography of a ULVZ layer tends to saturate and flatten as it becomes thicker, due to a nonlinear feedback between viscous aggregation beneath upwelling mantle currents and gravitational spreading/relaxation. Implementation of the ULVZ equation in thermal convection models indicates that ULVZs are preferentially gathered beneath long-lived plumes and may not exist beneath newly formed plume roots where there is no source of layer material. The presence/absence of ULVZs and their detailed shapes may provide important insights into the dynamical state and convective instability of the lowermost mantle thermal boundary layer.