Resolution of U-Th-Pb isotope systems perturbations at the nanoscale in monazite from ultra-high temperature contexts (Antarctica, Madagascar, India)

• Main Author: Turuani, Marion
• Other Authors: Laboratoire Magmas et Volcans (LMV), Institut national des sciences de l'Univers (INSU - CNRS)-Institut de Recherche pour le Développement et la société-Centre National de la Recherche Scientifique (CNRS)-Université Clermont Auvergne (UCA)-Observatoire de Physique du Globe de Clermont-Ferrand (OPGC), Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Université Clermont Auvergne (UCA)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Université Clermont Auvergne (UCA), Université de Lyon, Anne-Magali Seydoux-Guillaume, Simon L. Harley
• Format: Doctoral or Postdoctoral Thesis
• Language: French
• Published: HAL CCSD 2022
• Subjects: Geochronology; U-Th-Pb systems; Isotope disruption; Monazite; Tomographic atom probe; Transmission electron microscope; Géochronologie; Systèmes U-Th-Pb; Perturbation isotopique; Sonde atomique tomographique; Microscope électronique en transmission; [SDU.STU]Sciences of the Universe [physics]/Earth Sciences; Napier; Antarc*; Antarctica; Antarctique*
• Online Access: https://theses.hal.science/tel-04019471; https://theses.hal.science/tel-04019471/document; https://theses.hal.science/tel-04019471/file/These-Turuani-Marion-2022.pdf

This thesis focuses on the behavior of the mineral monazite (LREEPO4) which is used in geosciences for dating planetary events. This mineral integrates U and Th in its crystal lattice, which decay to Pb by radioactive decay over time. By measuring the isotopic compositions in U, Th and Pb in monazite, it is possible to calculate the age of crystallization of the mineral and to deduce the age of the rock or the geological events that affected it. However, it is frequent to observe perturbations in the isotopic signal, leading to the calculation of meaningless dates. To explain these perturbations, it is important to improve our knowledge of this mineral by taking as case studies Precambrian monazites from the Napier Complex (Antarctica), the Andriamena Unit (Madagascar) and the Kerala Khondalite Belt (India), where the U-Th-Pb signal is perturbed, giving a range of dates between two ages. In order to understand the origin of these perturbations, it is necessary to go down to the nanometric scale by coupling two analytical techniques; atom probe tomography (APT) which allows the acquisition of isotopic information at the atomic scale and transmission electron microscopy (TEM) which allows access to nano-structural information. This nanometric characterization of monazites reveals that the observed perturbations in the isotopic signal are due to the presence of Pb nano-phases in the monazite crystals. These results allow a new interpretation of the perturbations of the U-Th-Pb isotopic systems in monazite. Cette thèse s'intéresse aux comportements du minéral monazite (LREEPO4) qui est utilisé en géosciences pour la datation des évènements planétaires. Ce minéral intègre de l'U et Th dans son réseau cristallin qui au cours du temps se désintègrent en Pb par décroissance radioactive. En mesurant les compositions isotopiques en U, Th et Pb dans la monazite, il est possible de calculer l'âge de cristallisation du minéral et d'en déduire celui de la roche ou des évènements géologiques l'ayant affecté. Toutefois, il ...