Xenotime-(Y) from granitic pegmatites of the Monatsitovaya mine, Aduisky massif, Middle Urals (chemical composition and age)

Abstract

The relevance of the work is due to the need to improve the chemical dating method as applied to high-uranium and high-thorium accessory minerals, which are difficult to study using isotopic research methods. The purpose of the work is to study the chemical composition of accessory xenotime from granite pegmatites of the Monatsitovaya mine (western part of the Aduisky massif) and determine its age. Research methodology. A quantitative analysis of the chemical composition of monazite was performed on a CAMECA SX 100 electron probe microanalyzer (IGG Ural Branch RAS, Ekaterinburg). Measurement conditions: accelerating voltage 15 kV, current 250 nA, electron beam diameter 2 μm. The pressure in the sample chamber is 2 × 10–4 Pa. The spectra were obtained on inclined wave spectrometers, the intensity measurements were carried out along the analytical lines: Y Lα, Si Kα, (TAP crystal analyzer), U Mβ, Pb Mα, Ca Kα, Th Mα, P Kα (PET), Yb Lα, Dy Lα, Er Lα, Gd Lα, Lu Lα, Sm Lβ (LiF). The overlap of peaks of spectral lines was taken into account, which is extremely important in the quantitative determination of lead content (the Y Lγ2,3 и Th Mζ1,2 lines are superimposed on the analytical Pb Mα line). The pulse accumulation time at the peaks of analytical lines was twice as long as for the background and was 60 s for Th, 40 s for U and Pb, and 10 s for other elements. When conducting research, linear experimental design was used to vary the time of measuring the intensities of the Mβ line of uranium, Mα lines of thorium and lead. The calculated detection limits are 345 ppm for U, 283 ppm for Th, 65 ppm for Pb, 205 ppm for Y. Results. It has been established that xenotime belongs to the yttrium variety and is characterized by an increased content of uranium (UO2 up to 8.8 wt. %) and thorium (ThO2 up to 4.5 wt. %). According to the results of chemical dating (based on 32 analyses), xenotime-(Y) shows a weighted average age of 276 ± 7 Ma (MSWD = 0.54). When constructing the dependence (ThO2 +UO2 eq) – PbO, the points fall on one isochron. Calculating the age using the isochron slope angle gave a dating of 276 ± 12 million years (MSWD = 0.94). Conclusions. The obtained age values for the xenotime are in good agreement with the dating of the pegmatoid granites of the Krutikhinsky massif, which is located on the western contact of the Aduisky granite massif and relatively close to the Monatsitovaya mine. It is quite possible that some pegmatites in the western part of the Aduisky massif were formed as a result of the formation of the Krutikhinsky massif.