Irradiation effects in monazite–(Ce) and zircon: Raman and photoluminescence study of Au-irradiated FIB foils

Lamellae of 1.5 µm thickness, prepared from well-crystallised monazite–(Ce) and zircon samples using the focused-ion-beam technique, were subjected to triple irradiation with 1 MeV Au(+) ions (15.6% of the respective total fluence), 4 MeV Au(2+) ions (21.9%) and 10 MeV Au(3+) ions (62.5%). Total irradiation fluences were varied in the range 4.5 × 10(12) – 1.2 × 10(14) ions/cm(2). The highest fluence resulted in amorphisation of both minerals; all other irradiations (i.e. up to 4.5 × 10(13) ions/cm(2)) resulted in moderate to severe damage. Lamellae were subjected to Raman and laser-induced photoluminescence analysis, in order to provide a means of quantifying irradiation effects using these two micro-spectroscopy techniques. Based on extensive Monte Carlo calculations and subsequent defect-density estimates, irradiation-induced spectroscopic changes are compared with those of naturally self-irradiated samples. The finding that ion irradiation of monazite–(Ce) may cause severe damage or even amorphisation, is in apparent contrast to the general observation that naturally self-irradiated monazite–(Ce) does not become metamict (i.e. irradiation-amorphised), in spite of high self-irradiation doses. This is predominantly assigned to the continuous low-temperature damage annealing undergone by this mineral; other possible causes are discussed. According to cautious estimates, monazite–(Ce) samples of Mesoproterozoic to Cretaceous ages have stored only about 1% of the total damage experienced. In contrast, damage in ion-irradiated and naturally self-irradiated zircon is on the same order; reasons for the observed slight differences are discussed. We may assess that in zircon, alpha decays create significantly less than 10(3) Frenkel-type defect pairs per event, which is much lower than previous estimates. Amorphisation occurs at defect densities of about 0.10 dpa (displacements per lattice atom). ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1007/s00269-018-0975-9) contains supplementary material, which is available to authorized users.