Mounting for zircon and its importance for isotopic analysis using Laser Ablation Multi Collector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICP-MS)

Zircon is a mineral known as zirconium silicate (ZrSiO₄) that can be found in sedimentary, igneous, and metamorphic rocks. In isotope geology, it plays an important role as it offers valuable information about the history of the different geological processes of the Earth. This is possible, as Uranium is easily incorporated into the zircon structure and its decay allows us to know its age. Thus, zircon becomes the main mineral used as a geochronometer by the U-Pb method in mass spectrometer analysis (ICP-MS), since all the lead (Pb) trapped in its crystalline structure come from radioactive decay.

However, for an analysis to be carried out efficiently, meticulous and detailed preparation of a mount containing the crystals of interest is necessary. Firstly, to obtain the zircon fraction, it is necessary to extract them from the rock sample, in which the crushing, milling, and sieving process and subsequent separation and concentration on the Wilfley Table are carried out. After that, the fraction with the heavy minerals is concentrated by heavy-liquid separation, and purified by magnetic selection using the Frantz® isodynamic separator. The zircon will be concentrated in the non-magnetic fraction. These procedures enable to produce a concentrate typically with more than 95% purity. Before the mounting procedure, a careful hand-picking crystal selection takes place under a binocular stereomicroscope. The decision of which grains will be selected dated depends of the objective of each analysis and take into account physical aspects of the grains, such as transparency, absence of fractures and/or inclusions, size, and color, among others. Once the selection is made, the grains are glued to double-sided tape adhered to a glass plate. The distribution is made in quadrants, considering that: for analysis of detrital crystals, 4 quadrants of 7x7 grains are made, and for igneous or metamorphic crystals, 2 quadrants 7x8. Next, the grains are completely covered by epoxy resin homogenized and poured into a 2.45 diameter Teflon mold, previously adhered to the tape. The curing process lasts up to 24 hours and the oven temperature remains at 40°C. Once this process is completed, the mount is then removed, cut by a semi-automated mini-lathe to a height of approximately 0.5 cm, sanded and polished. These last two steps are extremely delicate, as sandpaper can damage the zircons and even take the grains out of the mount, causing grain loss. Sanding takes place in a semi-automatic polishing machine using silicon carbide sandpaper and aims to remove the glue left by the double-sided tape. Polishing, on the other hand, has the function of removing the slots left by sanding and make a uniform fat surface. Finally, after taking transmitted-light photos, the mount is sent for imaging by Scanning Electron Microscopy (SEM), to obtain cathodoluminescence images and cleaning with Mili-Q® water (type I ultrapure water) for analysis in the LA-MC-ICP-MS. Therefore, it can be stated that follow all these steps is essential for the proper functioning of the analysis.