Implications of Partially Molten Ultralow-Velocity Zones for Outermost Core Stratification

The low shear modulus and high temperature of ULVZ implies they may be partly molten. To maintain a dynamo, Earth's ancient core was much hotter than at present and the region will have been more extensively melted, forming a dense basal magma ocean (BMO) in Earth's earlier history. The presence of a BMO will have enhanced core-mantle chemical interactions relative to a solid lowermost mantle, however, recent experimental constraints imply that silicates with Fe#=[Fe]/([Fe]+[Mg])~0.1 or 0.2 cannot be in equilibrium with a core containing only 10% light elements: in this case, the core is under-saturated in O and possibly Si. At the same time, ULVZ sit atop the core and their density implies a Fe# much greater than the ambient mantle, which further exacerbates the magnitude of disequilibrium between core and mantle. This apparent paradox is readily avoided because light elements such as O dissolved into the top of Earth's core are many orders of magnitude too buoyant to be entrained and mixed into the bulk of the outer core, and a stably stratified layer must inevitably have formed at the top of the core that shields the interior and accounts for gradients in chemical potential. Radial transport of light elements in this case is limited to diffusion only, and various evolution scenarios provide new tests for the notion that ULVZ are partially molten, since such a feature appears to be an inevitable outcome of this hypothesis. Such a layer carries observational consequences for the seismic structure of the outermost core and the secular variation of the magnetic field, and while these hypothesis tests are compatible with present data more tests will be needed in the future.