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

Abstract

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 $c_{\Vert }$ diffusion is ∼400 to 700 times faster than $a_{\Vert }$ 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 $c_{\Vert }$ and $a_{\Vert }$ 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.

Introduction

The temperature-dependent diffusivity of radiogenic helium in uranium- and thorium-bearing minerals is commonly used to investigate thermal processes that occur at or near Earth's surface (e.g., Farley, 2002, Enkelmann and Garver, 2016, Reiners et al., 2017). In addition to temperature, however, many studies have suggested that helium systematics in minerals used for (U-Th)/He thermochronology also strongly depend on the mineral's crystal structure and accumulated radiation damage (Shuster et al., 2006; Farley, 2007; Flowers et al., 2009, Guenthner et al., 2013, Baughman et al., 2017, Cherniak et al., 2009, Cherniak and Watson, 2011). This is especially true for zircon, one of the most widely used low-temperature thermochronometers (e.g. Reiners et al., 2004, Powell et al., 2016, Reiners et al., 2017).

Zircon grains display a wide range of crystal morphologies from rock-to-rock (Pupin, 1980, Corfu et al., 2003), and often from crystal-to-crystal within individual rock samples (e.g., Rivera et al., 2016, Horne et al., 2016). The zircon crystal structure is highly anisotropic (Robinson et al., 1971, Hazen and Finger, 1979, Finch and Hanchar, 2002). If helium diffusion is similarly anisotropic, we would expect zircon crystals with different morphologies to record different closure temperatures. Such an effect likely plays a role in the commonly observed (U-Th)/He date dispersion amongst crystals from individual rock samples that cannot be explained fully by other factors that affect the closure temperature for helium in zircon, such as radiation damage and grain size variations.

Most helium diffusion studies of zircon have relied on single crystal, step-wise degassing experiments that measure a crystal's bulk diffusive properties (Reiners et al., 2002, Guenthner et al., 2013). While yielding important results, such studies provide only limited and indirect information on crystallographically-dependent variations in diffusivity. Characterizing diffusive loss profiles on crystallographically oriented zircon crystals by depth profiling can provide more direct constraints on diffusive anisotropy (Cherniak et al., 2009), but to date these methods have only been used to investigate how radiation damage affects crystallographic variations in helium diffusion in severely damaged zircon (Cherniak 2019). In this study, we use an excimer laser to measure laboratory-induced ³He diffusive loss profiles in crystallographically-oriented zircon crystals with low to moderate degrees of radiation damage, the range over which diffusive anisotropy is expected to be most significant, to evaluate the competing effects of radiation damage and crystallographic anisotropy on helium diffusion.