Short communication: Crystalline features of fission tracks in monazite: Evidence from swift heavy ion irradiation and transmission electron microscopy

Abstract. To further understand monazite fission-track thermochronometry, three experiments were conducted. 1) Cretaceous monazite-(Ce) was irradiated with 80 and 200 MeV Xe ions using a tandem accelerator. Transmission electron microscopy (TEM) was then used to define crystalline structures of spontaneous fission tracks in monazite. 2) Zircon powder was irradiated with 80 MeV Xe ions. TEM was subsequently used and the results were compared with those obtained from monazite. 3) Quaternary monazite-(Ce) in a resin mount was irradiated with 80 MeV Xe ions. Next, chemical treatment (etching) was performed to ascertain etchability of the ion tracks in monazite. These irradiation conditions correspond to the energies of heavily charged particles of spontaneous fission and the entire spontaneous fission event. This experiment simulated the damage process associated with spontaneous fission. First, from the TEM images of ion-irradiated Cretaceous monazite, ion track damage could be visualized as a low-density columnar region where the crystal lattice was maintained, and no sign of amorphization for both energies was observed. These results suggest that point defects accumulate around the ion path in monazite, in contrast to the amorphous features in zircon under the same irradiation conditions. Secondly, these etching experiments on ion-irradiated Quaternary monazite indicate that even non-amorphous domains can be selectively etched, which is consistent with previous studies on etching ion tracks in non-amorphous materials, as well as natural fission tracks and alternative ion tracks on monazite. These observations suggest that etchability of fission tracks in monazite is likely attributable to accumulated point defects rather than to amorphous regions. Hence, estimation of ultra-low closure temperature in the monazite fission track system should be derived from the formation of point defects rather than from amorphous regions, since the former can be easily annealed compared to the latter.