In situ apatite U-Pb, fission track and (U-Th)/He triple dating using a simple embedding approach

Abstract. Thermo- and geochronology techniques are foundational for many studies in tectonics, petrology, and surface processes. They provide essential information about cooling histories during exhumation or burial, and sediment provenance. Application of these methods has, in the past, primarily focused on applying individual techniques to separate mineral grains. More recently, analytical developments in laser-ablation geochronology have enabled the application of multiple dating techniques to an individual mineral grain (i.e., double or triple dating) to provide enhanced resolution of the mineral’s thermal history or dates and, therefore, the geologic or geomorphic interpretations derived from it. However, applying multiple geochronological methods to individual grains often requires methodological compromises that can restrict their applicability. In this study, we present a simplified and robust technique for embedding apatite grains in Teflon mounts. This approach enables the combined application of U-Pb, apatite fission track, and in-situ (U-Th)/He dating. In addition, this approach preserves the ability to quantify trace-element compositions and radionuclide zoning. The approach enables to significantly increase the number of measurements per sample, thereby opening the door to more robust results required for many applications.

To ensure high data quality from in-situ (U-Th)/He analyses, we introduce a decision matrix based on four key evaluation criteria: laser pit shape, radionuclide zoning, grain geometry, and the pit–grain relationship. We apply this integrated methodology to both the well-characterized Durango apatite standard and a sample from the crystalline basement of the Odenwald, western Germany. Our results demonstrate the robustness and reliability of this approach. Specifically, we obtain in-situ (U-Th)/He ages for Durango apatite (aliquot 1) of 31.04 ± 1.04 Ma (aliquot 1, n=35) and 31.38 ± 2.53 Ma (aliquot 2, n=22), which are in excellent agreement with accepted reference ages. Apatite U-Pb and fission track ages from the same grains, completing the triple dating approach, yield ages of 31.7 ± 1.9 Ma (U-Pb concordia age) and 35.0 ± 4.4 Ma (central AFT age).

The Odenwald sample reveals a complex, multi-phase cooling history consistent with major regional geodynamic events, including Late Cretaceous doming and Eocene exhumation associated with the formation of the Upper Rhine Graben. Importantly, our results confirm that the Teflon mounting procedure, which requires short-term heating to 300 °C, did does not cause measurable helium loss or reduce fission track density and track length in the near F-endmember Durango apatite, a mineral susceptible to annealing. This supports the viability of this approach for high-resolution, multi-system geo- and thermochronology thereby expanding the utility of these techniques to a broader range of geoscience applications.