Apatite (U-Th)/He thermochronology: understanding diffusion behavior using multidisciplinary approaches – HeDiff

Quantifying the thermal evolution of the continental crust is key to understanding processes like mountain building and sedimentary basin evolution. Thermochronometry constitutes a widely used tool for this purpose but the physical processes determining thermochronometric ages remain incompletely understood. The apatite (U-Th)/He (AHe) system has rapidly become a very popular thermochronometer; however, recent data and models demonstrate that interpretation of AHe data depends on a precise knowledge of He diffusion in apatite, which is currently not well constrained. Several studies have reported AHe ages that are much older than expected (more than 100% in some cases), casting doubt on the geological interpretation of these data. New models propose that recoil damage due to U-Th decay increases retention of He within apatite crystals. Models of damage creation and thermal annealing have been proposed to reproduce AHe age but these models are poorly constrained and do not fully explain the mechanism of He retention.

This project intends to significantly improve our understanding of the diffusion mechanisms of He in apatite by multidisciplinary approaches. All aspects of the current diffusion models will be experimentally verified and new models will be tested in order to study each diffusion step in detail. The project is based on three tasks: 1) quantifying diffusion mechanisms for He; 2) quantifying radiation-damage production and annealing; and 3) geological modeling and calibration. Mostly North American research teams are currently working on quantifying He diffusion; they mainly use empirical models based on under-vacuum experiments for the determination of He diffusivity. However, these approaches do not allow quantifying the diffusion mechanisms from a physical point of view and, additionally, the analytical procedure modifies the diffusion behavior due to the high temperatures involved. In contrast, this project is based on innovative approaches using ab initio/molecular dynamic calculations, nuclear physics experiments, controlled low-temperature laboratory diffusion experiments and geological calibrations. Only two previous studies, in 1998 and 2010, have relied on nuclear methods (implantation, elastic scattering) to determine the He-diffusion coefficient in apatite. The current project partners have been working on He diffusion since 2009 and have proposed complementary approaches for the physical acting mechanisms. The four partners are specialists of each topic, and preliminary tests have been realized for the new techniques proposed, ensuring the success of the project. This project will provide breakthrough insights into the physical processes controlling He diffusion in apatite and bring the project partner teams to an international leadership level in this research field.