Chemical influence on α-recoil damage annealing in apatite: Implications for (U-Th)/He dating

- Major influence of apatite grain chemistry on (U-Th)/He age via damage annealing
- Theoretical impact on AHe ages suggests divergence of more than 15%.
- Paris Basin (France) detrital apatite presents a large range of chemical composition.
- A new parameter of the annealing law stemming needs to be considered.
- Toward the use of AHe distribution to reconstruct the thermal history

Apatite (U-Th)/He (AHe) ages reflect the influence of several parameters such as grain size, thermal history and diffusion kinetics. Previous work has suggested that recoil damage produced by the U-Th decay chains can have a significant effect on He diffusion kinetics. Models taking into account damage production and annealing via a parameterization similar to that for apatite fission tracks (AFT) have been proposed. Complementary to these models, in this work we demonstrate that grain chemistry influences the annealing rate of diffusivity-altering damage in the same manner as for AFT, and thus is not constant across all apatites. The key parameter of the annealing law (rmr0) is examined and its theoretical impact on AHe age is tested. In certain situations, the AHe age can be significantly affected by changes in the damage-annealing rate. For samples having undergone a long stay in the AHe partial retention zone (50-80 °C), AHe ages can diverge by more than 15% over a reasonable range of annealing rates. In addition, this hypothesis is tested on a geological case: well samples of Triassic sandstones at 67.5 ± 1.5 °C at depth in the Paris Basin, France. AFT data and AHe ages have been measured on detrital apatite in addition to chemical analysis of the grains dated for AFT. Ages for both thermochronometers present similar histogram distributions, with younger ages for AHe (AFT: ~ 14-208 Ma; N = 117; AHe: ~ 4-76 Ma and one grain at 120 Ma; N = 36). The apatite grains are characterized by Dpar values from 1.2 μm to 2.9 μm and Cl content ranging from 0.05 to 0.9 wt.%. Using a thermal history reconstruction based on AFT data and geological evidence, it is shown that the non-correlation observed between AHe age and eU content can be simply explained by changing the rmr0 value. We conclude that to fully access the AHe age meaning in terms of temperature sensitivity, the parameters of the annealing law stemming from the grain chemistry need to be considered.