Annealing radiation damage and the recovery of cathodoluminescence

Cathodoluminescence (CL) microscopy and spectroscopy are luminescence techniques with widespread applications in geosciences. Many rock-forming and accessory minerals show CL characteristics, which can be successfully used in geoscientific research. One of the most spectacular applications is the visualization of growth textures and other internal structures that are not discernable with other analytical techniques. In addition, information from CL imaging and spectroscopy can be used for the reconstruction of processes of mineral formation and alteration to provide information about the real structure of minerals and materials, and to prove the presence and type of lattice incorporation of several trace elements. In the present article, an overview about CL properties of selected minerals is given, and several examples of applications discussed. The presented data illustrate that best results are achieved when luminescence studies are performed under standardized conditions and combined with other analytical techniques with high sensitivity and high spatial resolution.

/pdf/application-of-cathodoluminescence-microscopy-and-2ox0gremrp.pdf

Application of Cathodoluminescence Microscopy and Spectroscopy in Geosciences

Apatite samples from rare-metal mineralization were investigated by a combination of cathodoluminescence (CL) microscopy and spectroscopy, microchemical analysis and trace element analysis. Internal structures revealed by CL can be related to variations in the crystal chemistry and may sometimes reflect changes in the composition of the mineralizing fluids.

Apatite from mineralization related to alkaline rocks and carbonatites (Type 1) typically exhibits relatively homogeneous blue and lilac/violet CL colours due to activation by trace quantities of rare earth element ions (Ce3+, Eu2+, Sm3+, Dy3+ and Nd3+). These results correlate with determined trace element abundances, which show strong light rare earth element ( LREE ) enrichment for this type of apatite. However, a simple quantitative correlation between emission intensities of REE 3+/2+ and analysed element concentrations was not found.

Apatite from P-rich altered granites, greisens, pegmatites and veins from Sn-W deposits (Type 2) shows strong Mn2+-activated yellow-greenish CL, partially with distinct oscillatory zoning. Variations in the intensity of the Mn2+-activated CL emission can be related either to varying Mn/Fe ratios (quenching of Mn activated CL by Fe) or to self-quenching effects in zones with high Mn contents (>2.0 wt.%). The REE distribution patterns of apatite reflect the specific geological position of each sample and may serve as a “tracer” for the REE behaviour within the ore system. Although the REE contents are sometimes as high as several hundred parts per million, the spectral CL measurements do not exhibit typical REE emission lines because of dominance of the Mn emission. In these samples, REE -activated luminescence is only detectable by time-resolved laser-induced luminescence spectroscopy.

Both types of apatite (Type 1 in the core and Type 2 in the rim) were found in single crystals from the Be deposit Ermakovka (Transbaikalia). This finding proves the existence of two stages of mineralization within this deposit.

Cathodoluminescence (CL) behaviour and crystal chemistry of apatite from rare-metal deposits

The effects of crystal-plasticity on the U-Th-Pb system in zircon is studied by quantitative microstructural and microchemical analysis of a large zircon grain collected from pyroxenite of the Lewisian Complex, Scotland. Electron backscatter diffraction (EBSD) mapping reveals a c.18° variation in crystallographic orientation that comprises both a gradual change in orientation and a series of discrete low-angle (<4°) boundaries. These microstructural data are consistent with crystal-plastic deformation of zircon associated with the formation and migration of dislocations. A heterogeneous pattern of dark cathodoluminescence, with the darkest domains coinciding with low-angle boundaries, mimics the deformation microstructure identified by EBSD. Geochemical data collected using the Sensitive High Resolution Ion MicroProbe (SHRIMP) shows a positive correlation between concentrations of the elements U, Th and Pb (ranging from 20–60 ppm, 30–110 ppm, and 14–36 ppm, respectively) and Th/U ratio (1.13 – 1.8) with the deformation microstructure. The highest measured concentrations and Th/U coincide with low-angle boundaries. This enrichment is interpreted to reflect enhanced bulk diffusion of U and Th due to the formation and migration of high-diffusivity dislocations. 207Pb/206Pb ages for individual analyses show no significant variation across the grain, and define a concordant, combined mean age of 2451 ± 14 Ma. This indicates that the grain was deformed shortly after initial crystallization, most probably during retrograde Inverian metamorphism at amphibolite facies conditions. The elevated Th over U and consistent 207Pb/206Pb ages indicates that deformation most likely occurred in the presence of a late-stage magmatic fluid that drove an increase in the Th/U during deformation. The relative enrichment of Th over U implies that Th/U ratio may not always be a robust indicator of crystallization environment. This study provides the first evidence of deformation-related modification of the U-Th system in zircon and has fundamental implications for the application and interpretation of zircon trace element data.

/pdf/enhanced-diffusion-of-uranium-and-thorium-linked-to-crystal-1i39znyj51.pdf

Enhanced diffusion of Uranium and Thorium linked to crystal plasticity in zircon

Relationship among titanium, rare earth elements, U–Pb ages and deformation microstructures in zircon: Implications for Ti-in-zircon thermometry

As the geochemical behavior of the rare earth elements (REE) is of increasing interest in geology (see the reviews edited by Lipin and McKay 1989 and Jones et al 1996), the main REE minerals in most igneous, metamorphic and sedimentary rocks are now being studied in detail REE are major or trace constituents in many minerals (Burt 1989) Besides the specific REE mineral assemblages which occur in alkaline, peralkaline and carbonatitic rocks (Vlassov 1966; Burt 1989; Mariano 1989; Larsen 1996; Taylor and Pollard 1996; Wall and Mariano 1996; Khomyakov 1996), more common accessory minerals such as zircon, apatite, anhydrite, carbonates and fluorites are also REE carriers and play an important role in petrologic processes The major application of the REE studies is the melt mineral partition coefficient, used to model igneous petrogenetic processes This will depend on the compatibility of the REE in major minerals occurring in late differentiated stages such as apatite and zircon (McKay 1989) Most of the REE are well known to be luminescence activators (Pringsheim 1949; Levrenz 1950; Monod-Herzen 1966; Diecke 1968; Marfunin 1979; Marshall 1988, Waychunas 1988) In order to interpret the cathodoluminescence (CL) emissions of natural REE bearing minerals, it is essential to compare their CL spectra to those of synthetic minerals

Guy Rémond

Papers

Systematic Cathodoluminescence Spectral Analysis of Synthetic Doped Minerals: Anhydrite, Apatite, Calcite, Fluorite, Scheelite and Zircon