Nanoscale Isotopic Dating of Monazite

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 208 Pb/ 232 Th dating of nanoscale domains of monazite (0.0007 μm 3 analytical volume). The results indicate that the 208 Pb 2+ / 232 ThO 2+ ratios are higher than the true values. Such fractionation can be corrected using a linear regression between 208 Pb 2+ / 232 ThO 2+ and the M /Δ M 10 peak shape parameter, where M is the position of the peak and Δ M 10 the full‐width‐at‐tenth‐maximum for the same peak. This correction results in 15–20% analytical uncertainty on the corrected 208 Pb/ 232 Th age. Nonetheless, this approach opens the possibility of obtaining 208 Pb/ 232 Th 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 208 Pb/ 232 Th atom probe age of 420 ± 60 Ma (2 s ) and is consistent with the known metamorphism in the region.