Metal-silicate segregation in deforming dunitic rocks

We report the results of an investigation of the microstructural evolution of rocks composed of a silicate plus a molten metal sulfide phase deformed plastically in simple shear. Deformation experiments on samples of San Carlos olivine (Fo₉₀) + 3, 5, or 9 vol% iron sulfide reveal the segregation of iron sulfide melt from the solid polycrystalline olivine matrix into regions enriched in metallic melt separated by regions depleted in the metallic melt. Previously published experimental studies on core-composition metals in silicate rocks, performed under hydrostatic pressure conditions, suggested that metallic melts could not segregate from silicates by porous flow below a critical melt fraction of ∼0.05. In contrast, we demonstrate that metallic melts can segregate from solid silicates by grain boundary percolation assisted by deformation, down to a pinch-off melt fraction of ∼0.01. The segregated metallic melt can accumulate into bands containing melt fractions exceeding 0.3. Using a model network of melt-rich sheets with melt fractions of 0.3, calculated Earth-scale permeabilities are as high as 10 m. Segregation velocities for these sulfide melts can be up to 150 km/yr, fast enough for formation of Earth's core to proceed by deformation-induced segregation well within the geochemically determined timescale of 30 million years. Therefore plastic deformation can facilitate removal of large enough quantities of metallic liquid from a solid silicate mantle to satisfy the geochemical constraints of the inefficient core formation theory without invoking the presence of a magma ocean.