Interpreting cooling dates and histories from laser ablation in-situ (U-Th-Sm)/He thermochronometry

Abstract. Recent applications of the in-situ (U-Th-Sm)/He thermochronometry technique demonstrate its potential to address some of the analytical challenges associated with the whole-grain technique. In this study, we adapted state-of-the-art apatite and zircon production-ejection-diffusion models for application to in-situ dating methods, aiming to enhance the applicability of this technique to a broad range of geologic samples and applications. Our modifications to thermal history models include accommodation of the full range of stopping distances for alpha-particles and cylindrical grain geometries. This investigation focuses on several key aspects of in-situ data interpretation: (i) exploring the relationship between in-situ dates and the position of ablation spots across individual grains, (ii) assessing differences and similarities between whole-grain and in-situ dates, (iii) determining optimal strategies and performance for reconstructing cooling histories from in-situ (U-Th-Sm)/He data, and (iv) reporting the effects of radionuclide zoning on (U-Th-Sm)/He thermochronology. Results indicate that the measured in-situ helium distribution is a function of grain size, ablation spot position and size, and cooling history. Together, these analytical and natural factors result in systematic variations in in-situ dates with distance from the grain rim. Therefore, similar to whole-grain analyses, robust interpretation requires determining grain geometry and the distance of the laser spot to the nearest prismatic face. In most cases, resulting in-situ dates are approximately 30 % older than corresponding whole-grain dates, except for samples exhibiting negligible diffusional helium loss. Reconstruction of cooling histories using in-situ (U-Th-Sm)/He data can be achieved through single measurements in several grains with varying grain size and/or effective uranium content, or within a single grain with measurements taken at different distances from the grain rim. In addition, statistical analysis of a large compilation of measured radionuclide variations in apatite and zircon grains reveals that radionuclide zoning strongly impacts whole-grain analyses, but can be directly measured with the in-situ method. Overall, our results suggest that in-situ measurements for (U-Th-Sm)/He date determination offer a means to extract meaningful cooling signals from samples with poor reproducibility from traditional whole-grain techniques.