BADDELEYITE (U-TH)/HE THERMOCHRONOLOGY – PROGRESS ON ANALYTICAL PROTOCOLS AND HELIUM DIFFUSION CHARACTERISTICS

Baddeleyite (ZrO₂) is an accessory mineral that commonly occurs in a wide range of terrestrial and extra-terrestrial rocks, including mafic and ultramafic rocks (e.g., gabbros, basalts), alkali rocks (syenite, carbonatite), mantle xenoliths (from kimberlites), impact related rocks (tectites) and sedimentary deposits as detrital mineral grains. Incorporating significant concentrations of U (200-1000 ppm) and low common Pb, baddeleyite is routinely used for U-Th-Pb and U-Th-disequilibrium geochronology, providing information about its crystallization history. However, the potential and limitation of this mineral for (U-Th)/He thermochronology, providing information on cooling history as opposed to crystallization history, has not yet been fully explored.

Here, we report on our development of analytical protocols for conventional (U-Th)/He dating and combined U-Pb and (U-Th)/He “double-dating” of baddeleyite, both resulting in reproducible, internally consistent and geologically viable dates. We also present results from incremental He outgassing experiments performed on baddeleyite from different geologic environments, suggesting a thermally activated volume diffusion and closure temperatures in excess of 300 °C. The diffusion experiments also revealed some complexities, likely related to phase alterations during the heating, which were further explored by employing a combination of micro- and nanoscale characterization techniques, including FIB-HAADF STEM, EBSD, and Atom Probe Tomography. Finally, we propose a potential mineral reference material of Neogene age that can serve to monitor the accuracy of baddeleyite (U-Th)/He dating.