A detailed and comprehensive TEM, EPMA and Raman characterization of high-metamorphic grade monazites and their U-Th-Pb systematics (the Góry Sowie Block, SW Poland)

0559644 - GLÚ 2023 RIV NL eng J - Článek v odborném periodiku
Budzyń, B. - Wirth, R. - Sláma, Jiří - Kozub-Budzyń, G. A. - Rzepa, G. - Schreiber, A.
A detailed and comprehensive TEM, EPMA and Raman characterization of high-metamorphic grade monazites and their U-Th-Pb systematics (the Góry Sowie Block, SW Poland).
Chemical Geology. Roč. 607, September (2022), č. článku 121015. ISSN 0009-2541. E-ISSN 1872-6836
Institucionální podpora: RVO:67985831
Klíčová slova: Monazite stability * Mineral-fluid reactions * Dissolution-reprecipitation processes * Radiation damage * Common Pb contamination * LA-ICPMS U-Th-Pb isotopic dating
Obor OECD: Geology
Impakt faktor: 3.9, rok: 2022
Způsob publikování: Open access
https://www.sciencedirect.com/science/article/pii/S0009254122003096

Eleven monazite grains, two from a migmatitic gneiss and nine from two felsic granulites from the Gory Sowie Block (SW Poland) were studied with transmission electron microscopy (TEM), electron probe microanalysis (EPMA), Raman microspectroscopy and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) U-Th-Pb analysis in order to assess processes affecting U-Th-Pb age record. Two monazite grains from the migmatitic gneiss are patchy zoned in BSE imaging and overgrown by allanite, whereas Raman results indicate moderate radiation damage. Monazite in the corresponding TEM foils shows twins and nanoinclusions of fluorapatite, thorianite, goethite, titanite, chlorite and CaSO4. Furthermore, monazite is partially replaced by secondary monazite, forming ca. 100 nm-thick layers, and calcite along grain boundaries. The submicron alterations had little or no effect on the Pb/U and Pb/Th dates, when compared to earlier age constraints on the metamorphism in the studied region. In contrast, monazite from both granulites is homogeneous in eight investigated TEM foils, contains no solid or fluid nanoinclusions or any signs of fluid-induced alterations, with only one exception of a ca. 140 nm-thick crack filled with monazite. The ²⁰⁶Pb/²³⁸U and marginally older ²⁰⁸Pb/²³²Th mean dates pulled for all data show good coherence. However, the ²⁰⁷Pb/²³⁵U isotopic record is disturbed due the presence of common Pb within the entire monazite grain in one granulite and in the cores of two monazite grains in the second granulite, where the U-Pb data of the rims are not compromised and concordant. Due to lack of TEM evidence for fluid-mediated alterations, the age discordance has to be related to addition of common Pb in the monazite lattice or in the micro-cracks. To summarize, the ²⁰⁸Pb/²³²Th data reveal the most accurate ages, which are consistent with previous geochronological studies in the region. Therefore, the Pb/Th chronometer, which is less compromised by age disturbance compared to Pb/U ages, is recommended for monazite geochronology. Application of the submicron scale investigations using TEM is recommended to evaluate potential presence of the submicron inclusions of Pb-bearing phases or compositional alterations of monazite that can remain unnoticed by using standard microanalytical instruments.
Trvalý link: https://hdl.handle.net/11104/0333457