Annealing radiation damage and the recovery of cathodoluminescence

Abstract

International audienceThe structural recovery upon heat treatment of a highly metamict, actinide-rich zircon (U~6000 ppm) has been studied in detail using a range of techniques including X-ray powder diffraction, Raman spectroscopy, SHRIMP ion probe, electron microprobe, transmission electron microscopy and cathodoluminescence analysis. The structural regeneration of the amorphous starting material depends on random nucleation. It starts between 800 and 950°C when amorphous ZrSiO4 decomposes to form crystalline ZrO2 and amorphous SiO2. At around 1100°C, well-crystallised ZrSiO4 grows at the expense of the oxides. U has been retained in the newly grown zircon whereas Pb was evaporated during the heat treatment. This process is in marked opposition to the reconstitution of moderately metamict minerals, which experience a gradual recovery controlled by the epitaxial growth at the crystalline–amorphous boundaries. Both of these recovery processes are not the direct inverse of metamictisation. The structural regeneration was found to be connected with a significant increase in the emission of CL. In all cases (annealing heavily damaged zircon and moderately damaged zircon and monazite), we observe that the final, wellcrystallised annealing products emit more intense CL than their radiation-damaged starting minerals, although having almost identical elemental composition. Our observations are taken as evidence that the CL is not only determined by the chemical composition of the sample but is also strongly controlled by structural parameters such as crystallinity or the presence of defect centres

Zircon ages in granulite facies rocks: decoupling from geochemistry above 850 °C?

“…Given the low U concentration (< 1000 ppm) in granulite facies zircon and relatively short accumulation time (< 300 Ma), metamictisation and annealing of metamict domains are not among the possible disturbance mechanisms. In fact, no textures characteristic of metamict zircon-such as suppressed CL (Nasdala et al 2002)have been observed in any of our samples. The average α-doses calculated for zircons from our study (see Table S1 Online Resource 4) are 0.4 × 10 15 α/mg, clearly below the first percolation point (Stage 1: 2.2 × 10 15 α/ mg; Pidgeon 2014) of radiation damage.…”

“…Nasdala et al 2001;Ewing et al 2003), annealing of radiation damage (e.g. Nasdala et al 2002), fluid alteration (e.g. Geisler et al 2002;Vonlanthen et al 2012;Seydoux-Guillaume et al 2015), crystal plastic deformation (e.g.…”

Cathodoluminescence of synthetic zircon implanted by He⁺ ion

“…The CL emissions in a UV-blue region decrease with an increase in dose density, whereas yellow emission first increase but then is reduced by increasing radiation dose, explaining their CL color well. Nasdala et al (2002) reported that CL spectra of highly metamictstate N17 zircon annealed at 1100°C to 1400°C have a similar UV-blue emission, attributed to these intrinsic centers, with intensities higher than that of untreated ones. In this case, a restitution crystalline state was achieved by the recovery from the metamict state by an annealing.…”

“…2), that is too weak to be recognized in its color image. Yellow emission has been extensively reported in various types of natural zircon, and assigned to radiation-induced defects by the disintegration of U and Th or to impurity activation of (UO 2 ) +2 (Götze et al, 1999;Nasdala et al, 2002;Gaft et al, 2002;Tsuchiya et al, 2015). According to Tsuchiya et al (2014), young natural zircon extracted from the Takidani granodiorite aged at ~1.4 Ma does not have an obvious emission bands in a yellow region possibly due to very low radiation damage in its structure.…”

“…Furthermore, a process of metamictization in radioactive minerals (e.g., zircon) has been estimated by luminescence methods for ionimplanted samples as a simulation of radiation-induced damage on minerals (e.g., Weber et al, 1994;Lian et al, 2003;Ewing et al, 2003;Finch et al, 2004;Nasdala et al, 2011). Annealing effects on the CL derived from radiation-induced defect in zircon have been investigated in detail by Raman and CL spectroscopies, and CL color imaging (e.g., Nasdala et al, 2002;Tsuchiya et al, 2015).…”