Phosphate geochronometer minerals: Crystal chemistry and radiation disorder, methodological issues of their microprobe non-isotope U–Th–Pbtot dating

Abstract

Research subject. Phosphate mineral geochronometers – the international reference sample of Trebilcock monazite from pegmatites with the age of 272 ± 2 Ma, as well as samples of monazite from pegmatites of the Shartash massif and monazite, cheralite and xenotime from leucogranite of the Peshcherninsky stock and diorite of the Khomutinsky massif, Middle Urals. Methods. The composition of minerals was studied using CAMECA SX100 microprobe; Raman spectra were obtained using LabRAM HR800 Evolution confocal spectrometer. Research aim. Study of the internal texture of the grains of phosphate minerals on the basis of their elemental and spectroscopic mapping; analysis of the mineral crystal chemistry and estimation of auto-irradiation doses; microprobe non-isotopic U–Th–Pbtot dating of phosphate minerals; development of the appropriate algorithm for using analytical techniques. Results. It has been shown that the studied monazites belong to the cerium variety with ThO2 content from 1.1 to 17.2; UO2 – from 0 to 0.8; PbO – from 0.01 to 0.23 wt % (detection limits 160, 230, and 110 ppm). When analyzing the PbO content, the background line was interpolated into models of linear background (Trebilcock monazite, monazite and cheralite of the Peshcherninsky stock) and exponential background (monazite of the Shartash massif). It has been shown that for monazite, both huttonite and cheralite types of isomorphism are realized; the non-stoichiometric parameter of its composition β = (Si + Ca)/(Th + U + Pb + S) lies in the range of 0.95–1.05, which indicates the preservation of the U–Th–Pb-system. The analysis of BSE-images, intensity distribution maps of the Th Mα and Pb Mα RE lines, compositional point analyses and the results of spectroscopic mapping of the parameters of the ν1(PO4) vibrational mode testify to high homogeneity of Trebilcock monazite and pronounced zoning of the Ural monazites. It has been shown that the parameters of the ν1(PO4) vibrational mode in monazites are determined by the superposition of two factors, i.e. chemical and radiation disorder. The data on U, Th, and Pb content for different zones of monazite grains were used to perform non-isotopic U–Th–Pbtot dating: weighted average age values for the zones were obtained, and isochron plotting was made on the ThO2* vs. PbO diagram. The datings obtained based on the Trebilcock sample are in satisfactory agreement with the literature. Conclusions. The dating of monazite from leucogranite of the Peshcherninsky stock and the Shartash massif are in agreement with the U-Pb isotopic dating of zircon. The physical and chemical characteristics of cheralite, xenotime, and zircon in samples from the Peshcherninsky stock were analyzed. The U–Th–Pbtot dating of cheralite, xenotime, and zircon was attempted. The described algorithm and analytical methods were used at the Geoanalitik Common Use Center for microprobe non-isotopic dating of phosphate minerals.