Chemical composition and age of monazite-(Ce) in granitoids of the crystalline basement from the South Yamal

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.

Seeking the Sources of Dust: Geochemical and Magnetic Studies on “Cryodust” in Glacial Cores from Southern Spitsbergen (Svalbard, Norway)

Natural mineral particulate matter deposited from aerosols and trapped in glaciers—herein defined as “cryodust”—may be an excellent indicator of atmospheric circulation, if terrestrial sources of dust can be identified. In this study, we analyzed the composition of cryodust in shallow ice cores taken from five glaciers in Southern Spitsbergen (Svalbard Archipelago, Northern Norway). The chemical composition, magnetic properties and radiogenic ages of individual grains were measured, where possible, to provide indicators of source areas. To identify mineral and rock fragments, solid particulates were examined by Scanning Electron Microscope fitted with a backscattered electron and Energy Dispersive Spectroscopic detectors. An Electron MicroProbe was employed for the U-Th-Pb chemical dating of monazite grains. Magnetic measurements comprised analyses of magnetic susceptibility (κ) vs. temperature (T) variations and determination of magnetic hysteresis parameters. Monazite ages span 445–423 Ma, consistent with mineral growth during the Caledonian orogeny. Caledonian rocks are exposed in the Nordaustlandet area of North-Eastern Svalbard, and this is the most probable source for monazite grains. Magnetic analyses show a predominance of ferrous (FeII) over ferric (FeIII) phases, consistent with a lack of input from subtropical sources. The results from both methods are consistent with local sources of dust from exposures in the Svalbard archipelago.

Uranium Mineralogical and Chemical Features of the Na-Metasomatic Type Uranium Deposit in the Longshoushan Metallogenic Belt, Northwestern China

The Longshoushan Metallogenic Belt (northwestern China) is known for its word-class Jinchuan Ni-Cu sulfide (Pt) deposit and is also an important uranium metallogenic belt. The Jiling uranium deposit in this belt is a typical Na-metasomatic uranium deposit, which rarely occurs in China. Mineralization in the Jiling uranium deposit is hosted in granitoids that have suffered a Na-metasomatic alteration. There are three kinds of uranium minerals, including uraninite, pitchblende, and coffinite in the Jiling uranium deposit. Pitchblende is the predominant uranium mineral. Integrating the mineralogy and geochemistry of uranium minerals, and in situ electron microprobe analyzer (EMPA) U-Th-Pb chemical dating, we aimed to unravel the age and nature of the mineralization, to decipher the characteristics of the hydrothermal alteration and the U mineralization process. Based on the microtextural features and compositional variations, primary uraninite was altered to uraninite A and B, and fresh pitchblende was altered to pitchblende A and B. The best-preserved uraninite crystals displayed a euhedral-shape with high Pb and low SiO2, CaO, FeO, and Al2O3 contents, and was interpreted as primary uraninite. The EMPA U-Th-Pb chemical ages revealed that uraninite may have formed at 435.9 ± 3.3 Ma. High ThO2 + ΣREE2O3 + Y2O3 contents illustrated that the best preserved uraninite crystallized at a high temperature. Altered pitchblende A showed a relatively brighter gray color in backscattered electron (BSE) images and with a lower SiO2 content than B. Three analysis spots of the fresh pitchblende showed low contents of ΣSiO2 + CaO, indicating no obvious alteration. EMPA U-Th-Pb chemical dating gave a mean chemical age of 361 Ma. The low Th + ΣREE2O3 contents indicated that this pitchblende formed at a relatively low temperature. According to the different characteristics of occurrence and chemical composition, the coffinite in the Jiling uranium deposit can be divided into coffinite A and B, respectively. The compositional variation of the fresh and altered uraninite and pitchblende indicated that both uraninite and pitchblende underwent at least two discrete hydrothermal fluid alterations. The U mineralization was divided into two stages; uraninite was formed at a high temperature and possibly from a magmatic-hydrothermal fluid during ore stage I. Then, pitchblende was formed at a low temperature, during ore stage II. According to the petrographic observations and their chemical compositions, coffinite A and B resulted from the alterations of uraninite and pitchblende, respectively.

Detrital monazite from Upper Jurassic sediments in the central part of the Frolov megadepression, Western Siberia: chemical dating and provenances

Detrital monazite from Upper Jurassic sediments in the central part of the Frolov megadepression, West Siberian megabasin, is studied. Chemical composition of the mineral is studied and data on its age (chemical dating) are presented. Most monazite clasts are characterized by low roundness and their age corresponds to the Lower Paleozoic. Upper Jurassic sediments were likely derived not only from the local Early Paleozoic rock complexes (altaides), which make up the pre-Jurassic basement of the Frolov megadepression, but also from rocks located east and south of this megadepression.

ORIGEM E IDADE U-Th-Pb DO DISTRITO PEGMATÍTICO DE SANTA MARIA DE ITABIRA (MG) REVELADAS PELA MONAZITA

Imagens de elétrons retro-espalhados e microanálises químicas por microssonda eletrônica foram obtidas a partir de cristais de monazita pertencentes ao Distrito Pegmatítico de Santa Maria de Itabira nas lavras de Morro Escuro, Ponte da Raíz e Euxenita. Essas lavras estão nos municípios de Santa Maria de Itabira, Ferros, Guanhães e Sabinópolis, em Minas Gerais. As amostras coletadas fazem parte da Província Pegmatítica Oriental Brasileira (PPOB), situada em um contexto pós-colisional do Orógeno Araçuaí correspondente ao final do Ciclo Brasiliano. Os cristais mostram-se homogêneos, ou seja, livres de domínios/zoneamentos composicionais e seus teores de U, Th e Pb permitiram a obtenção de idade química média de 474 Ma para o distrito pegmatítico da região estudada. Seus padrões de terras raras normalizados ao condrito mostram anomalia positiva de Sm, elemento fortemente particionado em anfibólio. Esta anomalia sugere que no contexto pós-colisional do Orógeno Araçuaí houve a fusão parcial do biotita-hornblenda gnaisse (Grupo Guanhães), rocha encaixante dos pegmatitos, durante o processo de descompressão regional associado ao colapso do orógeno, gerando um magma granítico hidratado o bastante para permitir o avolumado crescimento de cristais dos pegmatitos desse Distrito Pegmatítico, incluindo a monazita. Este mineral atua, portanto, não só como um geocronômetro, mas também como importante indicador petrogenético dos pegmatitos estudados.Palavras Chave: Monazita; Santa Maria de Itabira; datação química U-Th-Pb, Microssonda Eletrônica; indicador petrogenéticoAbstract:ORIGIN AND U-Th-Pb AGE OF THE PEGMATITIC DISTRICT OF SANTA MARIA DE ITABIRA (MG) REVEALED BY MONAZITE. Backscattered electron images and chemical microanalysis by electron microprobe were obtained from monazite crystals belonging to the Pegmatitic District of Santa Maria de Itabira in the mining of Morro Escuro, Ponte da Raíz and Euxenita. These mining are located in the municipalities of Santa Maria de Itabira, Ferros, Guanhães and Sabinópolis in the State of Minas Gerais, Brazil. The monazite crystals are situated in the Eastern Brazilian Pegmatitic Province (EBPP) in the post-collisional context of the Araçuaí Orogen, which correspond to the end of the Brasiliano event. The samples are compositionally homogeneous, with no internal domains/zoning and their U, Th and Pb contents allowed obtaining an average chemical age of 474 Ma for the Santa Maria de Itabira Pegmatite District. Their chondrite-normalized rare earth element patterns show positive Sm anomalies, an element with very high partition coefficient in amphibole. This anomaly suggests that in the post-collisional context of the Araçuaí Orogen there was partial melting of the host rocks of the Guanhães Group (biotite-hornblende gneiss) during the regional decompression process linked with the collapse of the orogen. This contributed to formation of a granitic melt, which was hydrated enough to allow the large crystal growth of the Itambé pegmatites, including monazite. Therefore, this mineral represents not only a geochronometer, but also a petrogenetic indicator of the studied pegmatites.Keywords: Monazite; Santa Maria de Itabira (MG); U-Th-Pb chemical dating, electron microprobe; petrogenetic indicator