Pb diffusion in perfect and defective zircon for thermo/petro-chronological investigations: An atomistic approach

Abstract

Factors that affect the diffusion of lead (Pb) in zircon are still a debatable issue amongst geochronologists. These are anisotropy, chemical composition, metamictization, radiation damage, and defects such as vacancy and Frenkel pairs. Careful investigations of such effects require a detailed description at atomic levels. This study focuses on the details of Pb diffusion pathways (anisotropy) at the atomic scale level in perfect zircon lattices for the better understanding of thermo/petro-chronological problems in geosciences. The applications of Density Functional Theory (DFT) combined with both the Nudged Elastic Band (NEB) method and Transition State Theory (TST) together with Kinetic Monte Carlo (KMC) simulations provide a reasonable estimate of the activation energy and diffusion of lead (Pb) in a perfect zircon lattice. Results of the tested methodology give the diffusion co-efficient of anisotropic nature in perfect zircon i.e. the activation energies and frequency factors (D0) along the z-axis [001] is 687 kJ/mol and 4.16×10−3cm2/sec, along zx-axis [101]/[011] is 1331 kJ/mol and 8.57×10−4cm2/sec and along the x-axis is 5810 kJ/mol and 8.57×10−4cm2/sec respectively. The Closure temperatures for Pb in perfect zircon lattices are reasonably high for an effective grain size of 60 μm and a cooling history of 10 °C/Ma. This study also demonstrates that the closure temperature is affected not only by the geometry of a given grain, but also by defects and radiation damage. Furthermore, the calculated closure temperatures show poor calibrations with previous experiments of Pb diffusion in natural zircon, implying that radiation damage and vacancies are causing the reduction in Pb diffusion. Therefore, the gap between the computed and experimental results may probably be due to the change in the chemistry of each zircon crystal used in the experiments. This study is useful in characterizing the effect of individual entities on Pb diffusion in zircon, showing good agreement with experiments, and indicating the applicability of DFT + NEB+TST + KMC methods for characterization of Pb diffusion in perfect zircon.