Nanoscale resetting of the Th/Pb system in an isotopically-closed monazite grain: A combined atom probe and transmission electron microscopy study
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© 2018 China University of Geosciences (Beijing) and Peking University Understanding the mechanisms of parent-daughter isotopic mobility at the nanoscale is key to rigorous interpretation of U–Th–Pb data and associated dating. Until now, all nanoscale geochronological studies on geological samples have relied on either Transmission Electron Microscope (TEM) or Atom Probe Microscopy (APM) characterizations alone, thus suffering from the respective weaknesses of each technique. Here we focus on monazite crystals from a ~1 Ga, ultrahigh temperature granulite from Rogaland (Norway). This sample has recorded concordant U–Pb dates (measured by LA-ICP-MS) that range over 100 My, with the three domains yielding distinct isotopic U–Pb ages of 1034 ± 6 Ma (D1; S-rich core), 1005 ± 7 Ma (D2), and 935 ± 7 Ma (D3), respectively. Combined APM and TEM characterization of these monazite crystals reveal phase separation that led to the isolation of two different radiogenic Pb (Pb*) reservoirs at the nanoscale. The S-rich core of these monazite crystals contains Ca–S-rich clusters, 5–10 nm in size, homogenously distributed within the monazite matrix with a mean inter-particle distance of 40–60 nm. The clusters acted as a sink for radiogenic Pb (Pb*) produced in the monazite matrix, which was reset at the nanoscale via Pb diffusion while the grain remained closed at the micro-scale. Compared to the concordant ages given by conventional micro-scale dating of the grain, the apparent nano-scale age of the monazite matrix in between clusters is about 100 Myr younger, which compares remarkably well to the duration of the metamorphic event. This study highlights the capabilities of combined APM-TEM nano-structural and nano-isotopic characterizations in dating and timing of geological events, allowing the detection of processes untraceable with conventional dating methods.
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Fougerouse, Denis; Reddy, Steven; Saxey, David; Erickson, Timmons; Kirkland, Chris; Rickard, William; Seydoux-Guillaume, A.; Clark, Christopher; Buick, I. (2018)© 2018 Elsevier B.V. The widespread use of monazite (LREEPO 4 ) in U-Pb geochronology is underpinned by the assumption that it incorporates negligible amounts of Pb during initial growth, and that radiogenic Pb remains ...
Liu, Z.; Wu, F.; Yang, Y.; Yang, J.; Wilde, Simon (2012)As an important carrier of thorium, uraniumand trace elements, monazite iswidely used in U-Th-Pb geochronology and Sm\Nd isotopic studies. The combination of U-Th-Pb age and Sm\Nd isotopic composition makes monazite one ...
Fougerouse, Denis ; Kirkland, Chris ; Saxey, David ; Seydoux-Guillaume, A.M.; Rowles, Matthew ; Rickard, William ; Reddy, Steven (2020)© 2020 The Authors. Geostandards and Geoanalytical Research © 2020 International Association of Geoanalysts Monazite U-Th-Pb geochronology is widely used for dating geological processes, but current analytical techniques ...