Atomistic simulations of $^{40}$Ar diffusion in muscovite

Muscovite ranks among the most commonly dated minerals in $^{40}$Ar/$^{39}$Ar geochronology. Yet, its use in thermochronological reconstructions is hampered by the lack of reliable data on its $^{40}$Ar diffusional behavior. In this contribution, we investigate $^{40}$Ar lattice diffusion in muscovite at the atomic scale using Molecular Dynamics (MD) simulations combined with Nudged Elastic Band (NEB) and Transition State Theory (TST). Classical MD simulations of $^{40}$Ar recoil dynamics in $2M_1$ muscovite reveal that $^{40}$Ar initially resides predominantly in the interlayer region, close to its production site. Systematic computations of migration barriers coupling NEB with TST identify the divacancy mechanism as the more energetically favorable pathway for $^{40}$Ar diffusion in the interlayer region, with characteristic activation energy $E=66$ kcal.mol$^{-1}$ and frequency factor $D_0 = 6 \times 10^{-4}$ cm$^2$.s$^{-1}$. For typical cooling rates between 1-100 °C.Ma $^{-1}$ and grain size varying from 0.1 and 1 mm, these parameters predict closure temperatures significantly higher (~200 °C) than currently accepted maximum estimates (~500°C). Consistent with long-standing empirical evidence, our theoretical results downplay the role of purely thermally activated diffusion in promoting efficient $^{40}$Ar transport in ideal (stoichiometrically stable and undefective) muscovite. Along with experimental and field-based evidence, they call for more complex physics to explain the$^{40}$Ar retention properties of natural muscovite, most notably by considering crystal-chemical disequilibrium interactions and the reactivity of the interlayer with the external medium.