Deformation-induced metal melt networks in silicates: Implications for core-mantle interactions in planetary bodies

Previous analyses of texturally equilibrated, hydrostatically annealed olivine rocks containing metal and metal-sulfide melts have determined that these melts do not form interconnected networks at low volume fractions. To characterize the change in connectivity of metal melt in response to deformation, we used optical microscopy and electron microprobe analyses to investigate the microstructures of samples of olivine + gold sheared to shear strains of > 200%. Deformation causes the development of a strong melt-preferred orientation 13-15° to the shear plane and antithetic to the shear direction. This melt-preferred orientation consists of previously isolated pockets of gold melt that have connected into networks extending for many grain lengths, as well as linear “trails” of isolated pockets. Melt was observed to have migrated 125-200 μm in response to pressure gradients induced by serrations in the pistons in contact with the sample, proving that finite permeability exists during deformation well below the critical melt fraction for connectivity. Consideration of the balance between differential and interfacial stresses on melt pockets indicates that melt channels formed in response to deformation can drain rocks to a very small melt fraction in a direction subparallel to the shear direction, stranding less melt than drainage under hydrostatic conditions. Therefore, core formation by percolative flow of non-wetting melt in actively deforming planets and planetesimals may be accomplished more efficiently than previously believed. In addition, core material entrained in the mantle at the core-mantle boundary will be strongly aligned sub-horizontally in regions of horizontal shear and have little to no permeability in the vertical direction, providing a mechanism to trap core material in Dʺ.