Helium diffusion in zircon: Effects of anisotropy and radiation damage revealed by laser depth profiling

Laser depth profiling of laboratory-induced helium diffusion profiles in natural zircon confirms that helium diffusivity is crystallographically controlled and significantly anisotropic. Experiments on Mud Tank zircon with low degrees of alpha radiation damage (5.6 × 10¹⁶ to 1.3 × 10¹⁷ α/g) indicate that c_‖ diffusion is ∼400 to 700 times faster than a_‖ diffusion over the experimental temperature range investigated (400–600 °C). This magnitude of diffusive anisotropy implies that zircon crystals with different crystal morphologies record different helium closure temperatures. Zircon diffusion models commonly used in thermal-kinematic modeling programs do not properly account for diffusive anisotropy, and consequently, are likely to over- or underestimate helium closure temperatures in low-damage zircon. Additional experiments on pieces of a large Sri Lankan zircon crystal with strong radiation damage zoning demonstrate that both c_‖ and a_‖ diffusivity – as well as the magnitude of diffusive anisotropy – decrease with increasing radiation damage over an alpha dose range of ∼4.2 × 10¹⁷ to 8.5 × 10¹⁷ α/g. Decreases in diffusivity appear to reflect changes in the diffusion coefficient D₀ and not the activation energy for diffusion. While we did not design our experiments to explore the effect of trace element geochemistry on helium diffusion in zircon in detail, our results suggest that such an effect may be significant.