A diffusion mechanism for core–mantle interaction

It is thought that mixing of Earth's outer core material back into the mantle following core formation may be responsible for the proportion of elements observed in upper mantle rocks, but the mechanism by which this might occur remains unknown. Leslie Hayden and Bruce Watson report experimental results from a study of grain-boundary diffusion of siderophile ('iron-loving') elements through polycrystalline MgO. Siderophile elements are good indicators of a core contribution to the mantle due to their extreme enrichment in the core. They find diffusivities high enough to mark out grain-boundary diffusion as a potentially significant mechanism in the transport of siderophile elements across the core–mantle boundary over geologically significant distances (tens of kilometres) over the age of the Earth. Experimental results from a study of grain-boundary diffusion of siderophile ('iron-loving') elements through polycrystalline MgO are reported. The diffusivities computed are high enough to allow transport of a number of siderophile elements over geologically significant length scales (tens of kilometres) over the age of the Earth. Understanding the geochemical behaviour of the siderophile elements — those tending to form alloys with iron in natural environments — is important in the search for a deep-mantle chemical ‘fingerprint’ in upper mantle rocks, and also in the evaluation of models of large-scale differentiation of the Earth and terrestrial planets. These elements are highly concentrated in the core relative to the silicate mantle, but their concentrations in upper mantle rocks are higher than predicted by most core-formation models1,2. It has been suggested that mixing of outer-core material back into the mantle following core formation may be responsible for the siderophile element ratios observed in upper mantle rocks3. Such re-mixing has been attributed to an unspecified metal–silicate interaction in the reactive D″ layer just above the core–mantle boundary4. The siderophile elements are excellent candidates as indicators of an outer-core contribution to the mantle, but the nature and existence of possible core–mantle interactions is controversial5. In light of the recent findings that grain-boundary diffusion of oxygen through a dry intergranular medium may be effective over geologically significant length scales6 and that grain boundaries can be primary storage sites for incompatible lithophile elements7, the question arises as to whether siderophile elements might exhibit similar (or greater) grain-boundary mobility. Here we report experimental results from a study of grain-boundary diffusion of siderophile elements through polycrystalline MgO that were obtained by quantifying the extent of alloy formation between initially pure metals separated by ∼1 mm of polycrystalline MgO. Grain-boundary diffusion resulted in significant alloying of sink and source particles, enabling calculation of grain-boundary fluxes. Our computed diffusivities were high enough to allow transport of a number of siderophile elements over geologically significant length scales (tens of kilometres) over the age of the Earth. This finding establishes grain-boundary diffusion as a potential fast pathway for chemical communication between the core and mantle.