Yttrium mapping on fossil teeth by micro-XRF for apatite U–Pb dating

Aoki, K., Chiba, K., Kogiso, T., Murakami, K., Tsogtbaatar, K.

Selecting appropriate measurement points is crucial for apatite U–Pb dating of fossil teeth due to the impact of post-fossilization alterations. Previous studies have suggested that variations in yttrium (Y) concentrations within each sample can be qualitative indicators of such alterations (Y-screening method). To verify the validity of this method, ideal measurement areas for U–Pb dating were selected from Y mapping using a micro-focus X-ray fluorescence spectrometer (micro-XRF) on Late Cretaceous Tarbosaurus teeth with published apatite U–Pb ages, and trace element analysis and U–Pb dating of apatite was performed by laser ablation and inductively coupled plasma mass spectrometry (LA-ICP-MS). Two ages of 64.3 ± 3.4 Ma and 61.0 ± 11.0 Ma were obtained from two regions of relatively different Y concentrations in a sample that had been reported as 66.7 ± 2.5 Ma. Similarly, the sample with the reported age of approximately 30 Ma yielded ages of 51.5 ± 2.7 Ma and 57.3 ± 6.1 Ma in the areas with relatively low Y concentrations. These results show that Y concentration and an apatite U–Pb age are correlated, at least within the same sample, and support the usefulness of the Y-screening method. Although more measurements are needed to obtain conclusive proof, the Y micro-XRF mapping would be a helpful tool for non-destructive visualization of post-fossilization alteration in tooth fossil and could lead to the selection of suitable measuring points for LA-ICP-MS apatite U–Pb dating.

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