Abstract. Transposed temperature drop calorimetry at 1000 ~ was performed on natural zircons (ZrSiO4) from Sri Lanka that were partially to completely metamict due to e-decay event damage (0.06 to 11.7x l02s e-decay events/mg). The enthalpy of annealing at room temperature (AH ..... 1) varies sigmoidally as a function of radiation dose. AH ..... l reaches a saturation plateau at radiation doses greater than 5 x 1015 a-decay events/mg. The annealing of several samples to a crystalline structure with broadened diffraction peaks does not significantly affect the enthalpy of annealing. The large magnitude of the enthalpy of annealing plateau, -59 __ 3 kJ/mol, suggests that the damage to the structure is pervasive on the scale of ~ngstroms, consistent with the loss of mid-range order characteristic of a glass. The energetics are consistent with, but do not require, chemical heterogeneity caused by micro-domains of amorphous SiO2-rich and ZrO2-rich regions in the metamict state.