The radiation-enhanced diffusion of implanted Au markers in amorphous Si has been measured in the temperature range 77-693 K. Samples were irradiated with 2.5 MeV Ar ions. The diffusion coefficients show three well-defined regions. For temperatures <400 K, diffusion is athermal and due to ballistic mixing. For temperatures in the range 400-700 K diffusion is Arrhenius-type with an activation energy of 0.37 eV and is considerably enhanced over the normal thermal diffusion. The defects that cause the enhanced diffusion come from nuclear energy loss processes. Thermal diffusion, with an activation energy of 1.42 eV, dominates at temperatures greater than 750 K.