Abstract

Seismological and other geophysical evidence suggest that small scale heterogeneity and anisotropy on a scale of c. 10 km at the base of D'' may be due to the presence of a melt phase of undetermined composition, while geochemical evidence suggests some plume-related magmas may comprise 1-5% core derived material. Motivated by these observations, we present a mechanical model for the extraction of core melt upwards across the CMB into the mantle side region of D'' and subsequent interaction with the post-perovskite (PPV) phase transition. A strong requirement of the model is that the D'' region behaves as a poro-viscoelastic material on timescales comparable with the characteristic Maxwell relaxation time. Upwelling of outer core fluid can in principle be driven by a number of external deformation mechanisms including stresses associated with the new phase transition, loading by cold downwellings, or instabilities in the rotating outer core due to a `bumpy' CMB. Using new {\it ab-initio} estimates of the PPV elastic constants, we show that shear-enhanced dilation of a poro-viscoelastic D'' matrix has the potential to drive local fluid flow in the elastic limit on a timescale of 1-100 years. If loading rates locally exceed c. 10$^{-10}$ s$^{-1}$, calculated core metal flow rates are of the order 10$^{-4}$ ms$^{-1}$, far in excess of previous estimates based on static percolation or capillary flow. Provided this minimum required loading rate is maintained, core liquid metal could in principle be transported several 10's of km upwards into D'' on geologically short timescales, resulting in local rapid changes in electrical conductivity. Given the strong assumed dependence of the PPV phase transition on composition, periodic localised excursions of infiltrating Fe rich liquid metal from