Combined cathodoluminescence spectroscopy, electron microprobe and laser ablation ICP mass spectrometry analysis: an attempt to correlate luminescence and chemical composition of monazite

Quantitative electron microprobe analyses (EPMA), cathodoluminescence (CL) panchromatic images and monochromatic spectra, as well as laser ablation – inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of minor and trace elements were performed on selected monazite crystals from Dora Maira Massif, Western Alps (Italy). Backscattered electron (BSE) images suggest no patchy or chemical zoning or strong disomogeneity. The EPMA data indicate that the crystals are Ce-monazite with a mostly homogeneous composition in light rare earth elements (LREE). Low (far below 1 wt%) and variable Th, Ca and Y contents were detected. The LA-ICP-MS data indicate that all heavy rare earth elements (HREE) occur as minor or trace elements (from thousand to few ppm). Interesting Si, Sr, U, and Pb concentrations are also detected. Th, Y and Ca values are comparable to EPMA results. The use of panchromatic and monochromatic CL images with CL spectra in combination with microanalytical techniques suggest that the luminescence of monazite is mostly due to four bands centered at 316, 339, 634, and 683 nm, partially overlapping and not clearly related to impurity activators or to microchemical differences. However, we can attempt to attribute the first two bands to Gd3+ and Ce3+, respectively. In addition, several minor peaks centered at 384, 411, 485, 498, 562, and 596 nm also occur in between. These minor peaks may be tentatively assigned to Tb3+, Dy3+, and Sm3+.