Chemical composition and age of the monazite from granitic pegmatites of the Beloyarskaya vein (Zenkovsky massif, Middle Urals)
The relevance of the work is due to the need to improve the method of chemical dating as applied to high-thorium accessory minerals, which are difficult to date by isotope research methods. The purpose of the work is to study the chemical composition of the accessory monazite from granitic pegmatites of the Beloyarskaya vein (Zenkovsky massif, Middle Urals) and to determine its age. Research methodology. Quantitative analysis of the monazite chemical composition was performed on a CAMECA SX 100 X-ray electron probe microanalyzer. Measurement conditions: accelerating voltage 15 kV, current 250 nA, electron beam diameter 2 m. The pressure in the sample chamber is 2 • 10<sup>-4</sup> Pa. The spectra were obtained on tilted wave spectrometers, the intensity was measured using analytical lines: Th Ma, U Mb, Pb Ma, Y La, Si Ka, Ca Ka, P Ka, Ce La, La La, Pr Lb, Nd La, Sm Lb, Dy La, Gd Lb. The calculation of age was carried out according to well-known methods of foreign authors in addition to the authors’ own developments. Results. The monazite chemical composition allows it to be attributed to the cerium variety, the content of radiogenic components varies greatly (in wt. %): ThO<sub>2</sub> - 7.88-12.43; UO<sub>2</sub> - 0.12-0.44; PbO - 0.09-0.15. Of the impurities, significant concentrations of SiO<sub>2</sub> (up to 2.4 wt. %), Y<sub>2</sub>O<sub>3</sub> (up to 2.8 wt. %) and CaO (up to 0.9 wt. %) are noted. It turns out that huttonite (Th<sup>4</sup>+(U<sup>4+</sup>) + Si<sup>4+</sup> REE<sup>3+</sup> + P<sup>5+</sup>) and cheralite (Th<sup>4</sup>+(U<sup>4+</sup>) + Ca<sup>2</sup>+(Sr<sup>2+</sup>, Ba<sup>2+</sup>, Pb<sup>2+</sup>) 2REE<sup>3+</sup>) types of isomorphism are realized in the monazite. A decent lead content and quite good crystallinity of cerium phosphate make it possible to use this mineral as a geochronometer mineral. Conclusions. New data on the chemical composition of the monazite-(Ce) have been obtained, and the Late Permian age of granitic pegmatites of the Beloyarskaya vein has been established by microprobe dating. The U-Th-Pb point ages of the monazite together give a weighted average age of 255 ± 12 Ma (MSWD = 0.10) and an isochrone of 257 ± 24 Ma (MSWD = 0.45). The obtained age values for the monazite from the Beloyarskaya vein are in good agreement with the dating of granitic pegmatites of the Adui granite massif, and it is quite possible that the vein bodies of the Zenkovsky massif may belong to the Adui pegmatite field.