We report the influence of shear deformation on the microstructure of water-saturated olivine and clinopyroxene aggregates. Prior to deformation, the aqueous fluid was isolated in pockets along grain corners in the olivine-water aggregates, while it was interconnected by a network of grain-edge tubules in the clinopyroxene-water aggregate. During deformation of both types of aggregates, the aqueous fluid phase segregates into grain boundaries at an angle of ∼16° to the shear direction, inclined in a sense antithetic to the applied shear. Fluid-rich planes formed by such dynamic wetting of grain boundaries lead to the formation of high permeability paths in the matrix. In a deforming mantle, such high permeability paths will enhance the efficiency of aqueous fluid extraction from subducting slabs.