Abstract
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. Purpose of the work: study of the chemical composition of accessory monazite from granitoids of the crystalline basement of the South Yamal and determination of its age. Research methodology: quantitative analysis of the chemical composition of monazite was carried out using X-ray spectral electron probe microanalyzer CAMECA SX 100 (electron beam diameter from 1 μm, BSE, SE, Cat modes, determination of elements from beryllium to uranium). The spectra were obtained with the help of inclined 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 age calculation was carried out according to the well-known methods of foreign authors in addition to some developments of the author. Results. The chemical composition of monazite makes it possible to classify it as a cerium variety, the content of radiogenic components varies greatly (in wt.%): ThO2 – 5.37–16.31, UO2 – 0.40–0.81, PbO – 0.08–0,19. There are significant concentrations of SiO2 (up to 3.5 wt.%), Y2 O3 (up to 1.8 wt.%) and CaO (up to 1.2 wt.%). It turns out that monazite implements hattonite (Th4+(U4+)+Si4+ → REE3++P5+) and cheralite (Th4+(U4+)+Ca2+(Sr2+,Ba2+,Pb2+) → 2REE3+) isomorphism types. The decent content of lead and high crystallinity of the substance makes it possible to use this mineral as a geochronometer mineral. Conclusions. New data on the chemical composition of monazite have been obtained, and the late Permian age of granitoids has been determined by microprobe dating. The values of the point U–Th–Pb ages of monazite together give a weighted average age of 256 ± 10 Ma (MSWD = 0.15) and an isochron of 254 ± 19 Ma (MSWD = 0.28), which almost ideally coincides with the results of isotopic U–Pb zircon dating from the same rock, 254 ± 3 Ma.
Highlights
Research on geology of the Arctic has recently become important in connection with the oil and gas potential of this vast and still very poorly studied territory, as well as its upcoming division between neighbouring countries in the Arctic region
Monazite is a widespread accessory mineral of granitoids but it is almost not studied mainly due to the well-functioning system for determining the age of accessory zircons using isotope mass spectrometers, and zircon itself is quite easy to isolate from granites
Methods of research A quantitative analysis of the chemical composition of monazite was carried out using the CAMECA SX 100 electron probe microanalyzer (Institute of Geology and Geochemistry of the Ural Branch of the Russian Academy of Sciences, Ekaterinburg)
Summary
Research on geology of the Arctic has recently become important in connection with the oil and gas potential of this vast and still very poorly studied territory, as well as its upcoming division between neighbouring countries in the Arctic region. The Yamal Peninsula is the main gas province of Russia, and one of the few places where the crystalline basement (albeit with great difficulty) is still accessible for direct study. Crystalline basements of oil and gas provinces remain one of the only promising provinces but insufficiently studied. It is the granitoid of the basements that are most promising for oil and gas exploration [1]. Monazite is a widespread accessory mineral of granitoids but it is almost not studied mainly due to the well-functioning system for determining the age of accessory zircons using isotope mass spectrometers, and zircon itself is quite easy to isolate from granites. A similar study was carried out for accessory monazite from granites of the crystalline basement of the Verkhnerechenskaya oil prospecting area (Yamal Peninsula)
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