Monazite from lithium-bearing pegmatites of the Lipovskoye vein field, Middle Urals (composition and chemical dating)

Abstract

The relevance of the research is due to the need to improve the method of chemical dating as applied to high-thorium accessory minerals, which are difficult to study by isotope research methods. Purpose of the research is to study the chemical composition of monazite from lithium-bearing granite pegmatites of the Lipovskoye vein field and to determine their age. Research methodology. The quantitative analysis of the chemical composition of monazite was performed on a CAMECA SX 100 electron probe microanalyzer (IGG UB RAS, Ekaterinburg). Measurement conditions: accelerating voltage 15 kV, current strength 250 nA, electron beam diameter 2 μm. The pressure in the sample chamber is 2 ⋅ 10–4 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. Standard samples: ThO2 , UO2 , Pb2 P2 O7 , diopside, synthetic rare-earth phosphates. The intensity measurement time at the peak for Th is 180 s, U is 100 s, and Pb is 500 s (240 s on one and simultaneously 260 s on another spectrometer), for Y and Si 20 s each, for the other elements 10 s; on the background – two times less. The detection limits for Th, U, and Pb in monazite are 290, 350, and 64 ppm, respectively. The oxygen content was determined under the assumption of the stoichiometry of the composition. Results. It has been established that monazite belongs to the cerium variety and is characterized by high contents of thorium (ThO2 up to 23.6 wt. %) and uranium (UO2 up to 2.5 wt. %). At the same time, cheralite-type isomorphism is realized in phosphate. In a closed Th–U–Pb-system (β = 0.92–0.97), according to the results of chemical dating (according to 20 analyses), monazite-(Ce) shows a weighted average age of 243 ± 7 Ma. When plotting the dependence (ThO2 + UO2 eq) – PbO, the points fall on one isochrone. Calculation of the age from the slope of the isochron gave a dating of 242 ± 17 Ma, MSWD = 0.21, probability = 1.00. Conclusion. It has been established that accessory monazite from lithium-bearing granitic pegmatites of the Lipovskoye vein field (Middle Urals) is of Triassic age. Apparently, this dating shows the time of the secondary transformation of lithium-bearing granitic pegmatites, which are often tectonized, and in some places even boudinaged.