Nanoscale Isotopic Dating of Monazite

• Published in: Geostandards and geoanalytical research 2020-12, Vol.44 (4), p.637-652
• Main Authors: Fougerouse, Denis, Kirkland, Christopher L., Saxey, David W., Seydoux‐Guillaume, Anne‐Magali, Rowles, Matthew R., Rickard, William D. A., Reddy, Steven M.
• Format: Article
• Language: English
• Subjects: Analytical methods; atom probe tomography; Crystals; Earth Sciences; Fractionation; Geochemistry; Geochronometry; Geological processes; Geology; isotopes; Metamorphism; Mineralogy; Monazite; nanogeochronology; Plutons; Sciences of the Universe; Th‐Pb dating; Tomography; ultra‐high spatial resolution
• ISSN: 1639-4488; 1751-908X
• DOI: 10.1111/ggr.12340

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 O2+ 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. Key Points This method allows 208Pb/232Th dating of monazite for volumes as small as 0.0007 μm3. Molecular fractionation can be corrected using a peak shape parameter. The uncertainties are 15% to 20% for single analyses. The Fanad granite (Ireland) contact metamorphism is dated at 420 ± 60 Ma.