Low-temperature anisotropic diffusion of helium in zircon: Implications for zircon (U–Th)/He thermochronometry

Abstract

In the last decade the zircon (U–Th)/He (ZHe) thermochronometer has been applied to a variety of geologic problems. Although bulk diffusion coefficients for He in zircon are available from laboratory step-heating experiments, little is known about the diffusion mechanism(s) and their dependence on the crystallographic structure of zircon. Here, we investigate the diffusion of He in perfectly crystalline zircon using atomistic simulation methods that provide insights into the structural pathways of He migration in zircon. Empirical force fields and quantum-mechanical calculations reveal that the energy barriers for He diffusion are strongly dependent on structure. The most favorable pathway for He diffusion is the [001] direction through the open channels parallel to the c-axis (∆E*[001] = 13:4 kJ mol-1, activation energy for tracer diffusion of a He atom along [001]). In contrast, energy barriers are higher in other directions where narrower channels for He diffusion are identified, such as [100], [101], and [110] (∆E* of 44.8, 101.7, and 421.3 kJ mol-1, respectively). Molecular dynamics simulations are in agreement with these results and provide additional insight in the diffusion mechanisms along different crystallographic directions, as well as the temperature dependence. Below the closure temperature of He in zircon [Tc ~ 180 °C, Reiners P. W., Spell T. L., Nicolescu S., and Zanetti K. A. (2004) Zircon (U–Th)/He thermochronometry: He diffusion and comparisons with Ar-40/Ar- 39 dating. Geochim. Cosmochim. Acta 68, 1857–1887], diffusion is anisotropic as He moves preferentially along the [001] direction, and calculated tracer diffusivities along the two most favorable directions differ by approximately five orders of magnitude (D[001]/D[100] ~ 105, at T = 25 °C). Above this temperature, He atoms start to hop between adjacent [001] channels, along [100] and [010] directions (perpendicular to the c-axis). The diffusion along [100] and [010] is thermally activated, such that at higher temperatures, He diffusion in zircon becomes nearly isotropic (D[001]/D[100] ~ 10, at T = 580 °C). These results suggest that the anisotropic nature of He diffusion at temperatures near the closure temperature should be considered in future diffusivity experiments. Furthermore, care should be taken when making geologic interpretations (e.g., exhumation rates, timing of cooling, etc.) from this thermochronometer until the effects of anisotropic diffusion on bulk ages and closure temperature estimates are better quantified.