Long-term stability of alpha particle damage in natural zircon

We report the first discovery of radiation damage haloes generated by alpha particles in zircon. Proterozoic zircon crystals from a potassium-rich leucogranite from the Adirondack Mountains, New York State, have interior regions that are generally low in actinide elements (UO2 + ThO2 ≤ 0.02 wt.%) but show a remarkable pattern of heterogeneous metamictisation. The degree of radiation damage in these regions is not uniformly low, as would be expected if it corresponded to the observed actinide distribution patterns and age of the crystals. Rather, radiation damage is significantly increased in the outermost micro-areas of the low-actinide regions. The additional radiation damage is assigned to the impact of alpha particles emitted from closely neighboured, high-actinide regions (UO2 + ThO2 0.1-0.4 wt.%) in these zircon crystals. The phenomenon that within heterogeneous zircon crystals, highly alpha-emitting regions are surrounded by haloes of enhanced radiation damage with radii ≥ 10 μm, is the analogue of the common alpha damage haloes in minerals such as biotite, chlorite, and cordierite. The existence of alpha damage haloes in zircon implies that point defects (e.g., Frenkel defect pairs), as generated by alpha particles at the end of their trajectories, may have high long-term stability. This seems to contradict prevailing theories according to which point defects in zircon are annealed continuously. We suggest that even though most of the structural damage in partially metamict zircon is generated through recoils of heavy daughter nuclei upon emission of an alpha particle, defects generated by the alpha particles themselves contribute notably to the total radiation damage. Therefore, alpha damage cannot be neglected in the discussion and modelling of radiation damage phenomena in minerals and ceramic waste forms.