Earth Planets Space, Vol. 57 (No. 5), pp. 459-464, 2005LETTER
Nick Petford1,Dave Yuen2, Tracy Rushmer3, John Brodholt4, and Stephen Stackhouse4
1Center for Earth and Environmental Science Research, Kingston University, Surrey, KT1 2EE, UK
2Minnesota Supercomputer Institute, University of Minnesota, Minneapolis, MN55455, USA
3Department of Geology, University of Vermont, Burlington, Vermont, 04505, USA
4Department of Earth Sciences, University College London, WC1E 6BT, UK
(Received December 3, 2004; Revised April 1, 2005; Accepted April 19, 2005)
We present a novel mechanical model for the extraction of outer core material 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 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.
Key words: Post-perovskite, dilatancy, D", core metal transport, strain rate, deformation.