Abstract

Seismic and nutational data hint strongly at a layer comprised of silicate sediments several km thick confined to the top of the liquid outer core, directly beneath the core-mantle boundary and equating with observed ultra low velocity zones (ULVZs)1. Its origin is thought to be due to high pressure chemical reactions that take place between Fe-rich silicate in the lower mantle and liquid iron in the underlying outer core. We speculate that the Fe silicate is dominated by the newly discovered post-perovskite phase (ppv). Initial numerical investigations show that viscous compaction in the sediment layer will act to expel interstitial core metal liquid and reduce an initial 50% porosity to a residual value of < 0.1 on timescales of the order 80-100 Ma1. Using a modified form of Biot's equations2, we show that deformation of a poro-viscoelastic sediment pile will respond by drawing up Fe liquid metal into the layer from below at a rate proportional to the shear stress rate. Instead of a static, isolated residual porosity distribution in the most compacted upper regions of sediment, we envisage a more dynamic environment where fresh core liquid is emplaced periodically into the slowly accumulating pile. Estimates of the potential magnitude of the instability, including pressure changes, local fluid flow rates and timescales compare favourably with fluid motions in the convecting outer core. A key difference between both models, which are in fact complementary, relates to the rheology and microscale deformation behaviour of the assumed ppv-dominated sediment. The small grain size of the suspension is close to the limit of dilatant behaviour (c. 10-6 m) in granular materials. More information is required on the chemical and physical behaviour of the post-perovskite phase at lengthscales characteristic of geophysically interesting colloidal suspensions under high P-T conditions.