Mapping radiation-damage annealing in zircon

Radiation damage accumulates in the zircon lattice due to α-disintegration of trace levels of U, Th, and their α-emitting daughters. Upon heating, the lattice damage anneals in two stages by elimination of point defects and crystallization of amorphous domains. Raman spectroscopy is the method of choice to track lattice repair over the two stages due to changes in Raman positions and bandwidths (FWHM) of different bands during annealing. In annealing experiments with controlled time and temperature conditions, the process is usually monitored by Raman measurements after each experimental run. Raman mapping of partially annealed zircon proves to be even more effective than point measurements: (1) the zoning in actinide concentrations enables the comparison of annealing in zones with different initial damage; (2) spatial effects of annealing such as the fading of zoning can be taken into account; (3) tracking changes in the Raman signal from each spot in the map enables to acquire a large amount of annealing data.

While earlier studies produced Raman maps of zircon annealed at high temperatures, we focused on low-temperature stage I annealing. We carried out isothermal annealing experiments on four polished Plešovice zircon grains (1.5 – 2 mm diameter), at temperatures between 300 and 610 °C, with durations between 5 minutes and 70 days, mapping out four Raman bands after each annealing run. Our results show the progressive evolution of the Raman parameters with annealing time and temperature and allow the comparison to natural samples that underwent annealing during their geological history.