Nanoscale features revealed by a multiscale characterisation of discordant monazite highlight mobility mechanisms of Th and Pb

• Published in: Contributions to Mineralogy and Petrology
• Main Authors: Turuani, M. J., Seydoux-Guillaume, A. -M., Laurent, A. T., Reddy, S. M., Harley, S. L., Fougerouse, D., Saxey, D., Gouy, S., de Parseval, P., Reynaud, S., Rickard, W.
• Other Authors: Laboratoire de Géologie de Lyon - Terre, Planètes, Environnement (LGL-TPE), École normale supérieure de Lyon (ENS de Lyon)-Université Claude Bernard Lyon 1 (UCBL), Université de Lyon-Université de Lyon-Institut national des sciences de l'Univers (INSU - CNRS)-Université Jean Monnet - Saint-Étienne (UJM)-Centre National de la Recherche Scientifique (CNRS), Géosciences Environnement Toulouse (GET), Institut de Recherche pour le Développement (IRD)-Université Toulouse III - Paul Sabatier (UT3), Université de Toulouse (UT)-Université de Toulouse (UT)-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire Midi-Pyrénées (OMP), Université de Toulouse (UT)-Université de Toulouse (UT)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales Toulouse (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France-Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales Toulouse (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France-Centre National de la Recherche Scientifique (CNRS), Observatoire Midi-Pyrénées (OMP), Université de Toulouse (UT)-Université de Toulouse (UT)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales Toulouse (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France, Laboratoire Hubert Curien (LHC), Institut d'Optique Graduate School (IOGS)-Université Jean Monnet - Saint-Étienne (UJM)-Centre National de la Recherche Scientifique (CNRS)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2023
• Subjects: Monazite; Napier complex (Antarctica); Th and Pb mobility mechanisms; Pb-bearing nanophases; Clusters; Atom probe tomography and TEM; Earth Science; [SDU]Sciences of the Universe [physics]; Napier; Antarc*; Antarctica
• Online Access: https://insu.hal.science/insu-04198442; https://doi.org/10.1007/s00410-023-02015-x

International audience Understanding radionuclides mass transfer mechanisms in monazite (LREEPO 4 ) and the resulting features, from the micro- to the nanoscale, is critical to its use as a robust U-Th-Pb geochronometer. A detailed multiscale characterisation of discordant monazite grains from a granulite which records a polymetamorphic history explores the mechanisms of Th and Pb mobility in crystals. Some monazite grains display Th-rich linear features (0.1-1 µm thick) forming a regular network throughout the grain. They are interpreted as resulting from fluid ingress along crystallographically controlled pathways. Nanoscale features termed `clusters' (Ø < 10 nm) are composed of radiogenic Pb (Pb*) ± Si ± Ca and are localised within monazite lattice defects. Their formation results from the competition, over millions of years, of both radiation damage production allowing element mobility (by diffusion) and accumulation in defects and α-healing inducing their trapping. Nanophases (Ø = 0.02-1 µm) containing Pb* are present in all grains and correspond to galena (PbS) or sesquioxide of Pb (Pb 2 O 3 ). They are associated with a chemically varied suite of amorphous silicate (± Al, Mg, Fe) phases or sulphur (e.g. FeS). They are interpreted as precipitates within monazite crystals. They are formed during replacement mechanism of monazite through fluid interactions. Two generations of Pb*-bearing nanophases exist supported by previous geochronological data. The shielding effect of garnet and rutilated quartz (host minerals), limiting fluid access, induces plentiful Pb*-bearing nanophases precipitation (fluid saturation enhanced) and limits Pb*-loss at the grain scale. This multiscale study provides new insights for interpretations of meaningless geochronological information, thanks to nanoscale investigations.