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Nanoscale Isotopic Dating of Monazite

Abstract : 10 Monazite U-Th-Pb geochronology is widely used for dating geological processes, but current 11 analytical techniques are limited to grains greater than 5 μm in diameter. This limitation 12 precludes the analysis of both micrometre-scale discrete monazite grains and fine textures 13 within monazite crystals that are commonly found. Here, we analyse reference materials by 14 atom probe tomography and develop a protocol for 208 Pb/ 232 Th dating of nanoscale domains 15 of monazite (0.0007 μm 3 analytical volume). The results indicates that the 208 Pb ++ / 232 ThO ++ are 16 higher than the true values. Such fractionation can be corrected using a linear regression 17 between 208 Pb ++ / 232 ThO ++ and the M/ΔM10 peak shape parameter, where M is the position of the 18 O2 + peak and ΔM10 the full-width-tenth-maximum for the same peak. This correction results 19 in 15 to 20 % analytical uncertainty on the corrected 208 Pb/ 232 Th age. Nonetheless, this approach 20 opens the possibility of obtaining 208 Pb/ 232 Th ages with sufficient precision to address 21 geological questions on an unprecedented small scale. To illustrate the approach, atom probe 22 geochronology of a small monazite grain from the contact aureole of the Fanad pluton 23 (Ireland) yielded a 208 Pb/ 232 Th atom probe age of 420 ± 60 Ma (2σ) and is consistent with the 24 known metamorphism in the region. 25 26 27 28
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Denis Fougerouse, Christopher Kirkland, David Saxey, Anne-Magali Seydoux-Guillaume, Matthew Rowles, et al.. Nanoscale Isotopic Dating of Monazite. Geostandards and Geoanalytical Research, Wiley, 2020, ⟨10.1111/ggr.12340⟩. ⟨hal-02996416⟩



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