An indirect method based on the effect of water on plastic deformation mechanisms in quartz is used to estimate water diffusion penetration depths at high pressure. Diffusivities calculated by this method are orders of magnitude greater than those determined at low pressure, and high-pressure activation energy for water diffusion is three to five times greater than low-pressure values. This implies very temperature-sensitive hydrolytic weakening at deep crustal and upper mantle depths, potentially leading to a sharp mechanical decoupling of the lithosphere.