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

• Published in: Physics and chemistry of minerals Vol. 45; no. 9; pp. 855 - 871
• Main Authors: Nasdala, Lutz, Akhmadaliev, Shavkat, Artac, Andreas, Chanmuang N., Chutimun, Habler, Gerlinde, Lenz, Christoph
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01-10-2018; Springer Nature B.V
• Subjects: Alpha decay; Continuous annealing; Cretaceous; Crystallography and Scattering Methods; Decay rate; Defects; Displacements (lattice); Earth and Environmental Science; Earth Sciences; Estimates; Fluence; Foils; Geochemistry; Ion beams; Ion irradiation; Ions; Irradiation; Low temperature; Mineral Resources; Mineralogy; Monazite; Monte Carlo simulation; Original Paper; Photoluminescence; Radiation damage; Zircon; Heavy-ion irradiation; Raman spectroscopy; Radiation damage; Focused ion beam; Photoluminescence
• ISSN: 0342-1791; 1432-2021
• DOI: 10.1007/s00269-018-0975-9

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).