Abstract

Our understanding of the core-mantle boundary (CMB) region has improved over the past several years due to better seismic resolution, the discovery of the post-perovskite phase and the acknowledgement that the region is likely highly chemically diverse. One of the outstanding problems that continues to grow is in regard to how CMB heterogeneity is developed through time and to what extent there is material transfer from the fluid outer core across the CMB and into D". We have explored the role of severe loading deformation at the core mantle boundary, for example from slab interaction with the PPV phase transition. We find that for a particular class of material response (viscoelastic dilatancy), deformation can create strain gradients that allow transport of fluid against gravity such that more dense outer core fluid can be drawn across the CMB and infiltrate the base of the lowermost mantle. Where loading rates locally exceed c. 10^{-12}s-1, calculated core metal flow rates are far in excess of previous estimates based on static percolation or capillary flow. These numerical results are intriguing and we have explored the mechanism of shear-enhanced dilation via experiment. Preliminary results from simple shear experiments conducted at 3 GPa and 1200°C on FeS (20%) in an olivine matrix shows local migration of FeS to edges where the matrix butts up against impermeable olivine pistons. Migration has also occurred upwards, against gravity. Our preferred segregation mechanism is that local pressure gradients are generated by a transient granular dilatancy, driven by shear. Local and periodic fluxing of Fe-rich liquid metal from the outer core into the lowermost mantle may help explain some aspects of CMB compositional variation (enrichment in Fe) and cause temporal fluctuations in the position of the post-perovskite phase transition.