Nanoscale Isotopic Dating of Monazite
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This is the peer reviewed version of the following article: Fougerouse, D., Kirkland, C.L., Saxey, D.W., Seydoux‐Guillaume, A.‐M., Rowles, M.R., Rickard, W.D.A. and Reddy, S.M. (2020), Nanoscale Isotopic Dating of Monazite. Geostand Geoanal Res., which has been published in final form at https://doi.org/10.1111/ggr.12340. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions.
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© 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 are limited to grains greater than 5 μm in diameter. This limitation precludes the analysis of both micrometre-scale discrete monazite grains and fine textures within monazite crystals that are commonly found in geological specimens. Here, we analyse reference materials by atom probe tomography and develop a protocol for 208Pb/232Th dating of nanoscale domains of monazite (0.0007 μm3 analytical volume). The results indicate that the 208Pb2+/232ThO2+ ratios are higher than the true values. Such fractionation can be corrected using a linear regression between 208Pb2+/232ThO2+ and the M/ΔM10 peak shape parameter, where M is the position of the (Formula presented.) peak and ΔM10 the full-width-at-tenth-maximum for the same peak. This correction results in 15–20% analytical uncertainty on the corrected 208Pb/232Th age. Nonetheless, this approach opens the possibility of obtaining 208Pb/232Th ages with sufficient precision to address geological questions on an unprecedented small scale. To illustrate the approach, atom probe geochronology of a small monazite grain from the contact aureole of the Fanad pluton (Ireland) yielded a 208Pb/232Th atom probe age of 420 ± 60 Ma (2s) and is consistent with the known metamorphism in the region.