The volume diffusion coefficient of water in ice VI was determined in the pressure-temperature range of 1.3 – 1.9 GPa and 300 – 320 K by in situ isotope tracer diffusion experiments. We determined the activation energy of the volume diffusion to be 61.9 ± 9.5 kJ/mol. The viscosity of polycrystalline ice VI under diffusion creep was estimated from the diffusion coefficients based on the theory of the diffusion creep. From a compilation of viscosity values in the current diffusion creep regime and the viscosity previously determined by plastic deformation experiments of ice VI in a high stress regime (Durham et al. 1996), here we provide the relationships between viscosity, stress, and the average grain size of polycrystalline ice VI. The most plausible deformation mechanism of the layers of ice VI underneath the internal oceans in large icy bodies was inferred from the viscosity-stress-average grain size relationship. We also discuss the critical thickness of the ice VI layer, which determines the onset of thermal convection
