Shear-induced material transfer across the core-mantle boundary aided by the post-perovskite phase transition

We present a novel mechanical model for the extraction of outer core material upwards across the CMB into the mantle side region of W and subsequent interaction with the post-perovskite (ppv) phase transition. A strong requirement of the model is that the W region behaves as a poro-viscoelastic granular material with dilatant properties. Using new ab-initio estimates of the ppv shear modulus, we show how shear-enhanced dilation promoted by downwelling mantle sets up an instability that drives local fluid flow. If loading rates locally exceed C. 10(-12) s(-1), calculated core metal upwelling rates are > 10(-4) m/s, far in excess of previous estimates based on static percolation or capillary flow. Associated mass flux rates are sufficient to deliver 0.5 % outer core mass to D in < 10(6) yr, provided the minimum required loading rate is maintained. Core metal transported upwards into D may cause local rapid changes in electrical and thermal conductivity and rheology that if preserved, may account for some of the observed small wavelength heterogeneties (e.g. PKP scattering) there.