Impact of apatite chemical composition on (U-Th)/He thermochronometry: An atomistic point of view
Résumé
The quantification of the different parameters influencing He diffusion in apatite is an important issue for the interpretation of (U-Th)/He thermochronometric ages. Key issues include understanding the role of chemical composition and the mechanism modifying diffusivity by radiation damage, both requiring a realistic description at the atomic level. In this contribution, we restrict ourselves on the influence of the chemical composition especially on the effect of Cl-atoms on the He diffusion in the damage-free apatite crystal. For this purpose, a multi-scale theoretical diffusion study has been conducted using periodic Density Functional Theory calculations for two different apatite compositions (pure fluorine apatite and apatite with one chlorine and 3 fluorine atoms per cell called Cl0.25-apatite) representative of damage-free crystals. Different He insertion sites and diffusion pathways are first investigated. The Density Functional Theory approach coupled to the Nudged Elastic Band method is used to determine the energy barriers between the insertion sites. A statistical method, based on Transition State Theory, is used to compute the jump rate between sites and the different results are used as output for a 3D random walk simulation, which determines the diffusion trajectories and the diffusion coefficients. The calculated diffusion coefficients for pure F-apatite exhibit a slightly anisotropic behavior with an activation energy Ea = 95.5 kJ/mol and a frequency factor D0 = 1.9 × 10−3 cm2/s along the c axis; Ea = 106.1 kJ/mol and D0 = 4.1 × 10−3 cm2/s in the plane orthogonal to c. Closure temperatures for a 60 μm grain radius and 10 °C/Ma cooling rate range from 33 to 36 °C and depend on crystal geometry for a given grain size. Surprisingly, even though He diffusion is strongly blocked across the Cl atoms in Cl0.25-apatite, where Ea is significantly higher (166.7 kJ/mol), He atoms can still diffuse along the c axis through workaround pathways. Closure temperatures are dependent on the Cl content in the crystal lattice and can be ∼12 °C higher for Cl0.25-apatite than for F-apatite. These results show that various Cl contents lead to a more He retentive diffusivity in addition to their impact on damage-annealing rate. The results of this study are in good agreement with experimental results and demonstrate that a proper Density Functional Theory treatment allows to characterize He diffusion in damage-free apatite. This opens new avenues to a reliable method of quantifying rare gas diffusion in mineral structures.