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A multi-method, multi-scale theoretical study of He and Ne diffusion in zircon

Gautheron, C. ; Mbongo Djimbi, D. ; Roques, J. ; Balout, H. ; Ketcham, R.A. ; Simoni, E. ; Pik, R. ; Seydoux-Guillaume, A.M. ; Tassan-Got, L., GCA

A multi-method, multi-scale theoretical study of He and Ne diffusion in zircon

Gautheron, C. ; Mbongo Djimbi, D. ; Roques, J. ; Balout, H. ; Ketcham, R.A. ; Simoni, E. ; Pik, R. ; Seydoux-Guillaume, A.M. ; Tassan-Got, L.

Geochimica et Cosmochimica Acta, 2019, 268, 348-367

Abstract :

The quantification of He and Ne diffusion behavior in crystals rich in U and Th such as zircon is key for the interpretationof (U-Th)/4He and (U-Th)/21Ne thermochronometric ages. Multiple parameters such as chemical substitution, channelobstruction and damage can modify the diffusivity compared to a pristine structure. To investigate the impact of these param-eters, we have conducted a theoretical diffusion study combining a series of methods and approaches to address the problemacross the necessary range of scales (atomic to crystal size). First, using quantum calculation, we determine the different Heand Ne insertion sites, insertion energies and diffusion pathways at the atomic scale for an ideal pristine zircon structure (i.e.damage free). These results serve as input for a 3D random walk simulation of atomic trajectories that provides diffusion coef-ficients for damage-free zircon crystals. Second, as natural zircon crystals are not perfect, we model the impact of differenttypes of damage and diffusion pathway obstruction at the atomic level on He and Ne diffusion in 3D. The calculated Heand Ne diffusion coefficients for pure ZrSiO4exhibit strongly anisotropic behavior and very high diffusivity along thec-axis, and with 3D, closure temperatures of-197°C and-202°C respectively. The results for He are comparable to previousDFT studies but strongly different from experimental diffusion results ; results for Ne are similar in this respect. Modelling theimpact of different types of damage (vacancies, recoil, fission, voids or fluid inclusions) and obstruction on He and Ne diffu-sion reveals important implications for the (U-Th)/He and (U-Th)/Ne thermochronometers. First, obstruction alone does notsignificantly modify He and Ne diffusion except to reduce anisotropy. Second, trapping is the primary mechanism altering Heand Ne diffusion even at low dose, and we predict the maximal trapping energies for He and Ne to be 164 and 320 kJ/mol,similar to values inferred from experimental data. We also propose that the closure temperature increases non-linearly withdamage, with effective trapping energy increasing with dose until a threshold, possibly corresponding to a percolationtransition, after which retentivity decreases. Based on field data sets we also anticipate a value for this threshold of around-2–5-1017a/g, lower than previously proposed. We show Ne to be highly blocked by damage and predict similar diffusion behavior to He, but with higher retentivity. We demonstrate the importance of investigating rare gas diffusion at the atomiclevel for comparison with experimental data, in order to build a predictive diffusion law at different scales.

Voir en ligne : https://doi.org/10.1016/j.gca.2019....




publié mercredi 15 janvier 2020