• Zircon (U-Th)/He Dates from Radiation Damaged Crystals: A New Damage-He Diffusivity Model for the Zircon (U-Th)/He Thermochronometer

      Reiners, Peter; Guenthner, William Rexford; Reiners, Peter; Ketcham, Richard; Ganguly, Jibamitra; DeCelles, Peter (The University of Arizona., 2013)
      Zircon (U-Th)/He (zircon He) dating has become a widely used thermochronologic method in the geosciences. Practitioners have traditionally interpreted (U-Th)/He dates from zircons across a broad spectrum of chemical compositions with a single set of ⁴He diffusion kinetics derived from only a handful of crystals (Reiners et al., 2004). However, it has become increasingly clear that a "one-size-fits-all" approach to these kinetics is inadequate, leading to erroneous conclusions and incongruent data. This dissertation develops a more grain-specific approach by showing the fundamental role that intracrystalline radiation damage plays in determining the He diffusivity in a given zircon. I present three appendices that seek to quantify the radiation damage effect on He diffusion in zircon, explain how this effect manifests in zircon He dates, and show how to exploit such manifestations to better constrain sample thermal histories. Of particular importance, this dissertation represents the first comprehensive study to concentrate on the entire damage spectrum found in natural zircon and also the first to show that two different mechanisms affect He diffusion in zircon in different ways across this spectrum. In the first appendix, I and my fellow co-authors describe results from a series of step-heating experiments that show how the alpha dose of a given zircon, which we interpret to be correlated with accumulated radiation damage, influences its He diffusivity. From 1.2 × 10¹⁶ α/g to 1.4 × 10¹⁸ α/g, He diffusivity at a given temperature decreases by three orders of magnitude, but as alpha dose increases from ~2 × 10¹⁸ α/g to 8.2 × 10¹⁸ α/g, He diffusivity then increases by about nine orders of magnitude. We parameterize both the initial decrease and eventual increase in diffusivity with alpha dose with a function that describes these changes in terms of increasing abundance and size of intracrystalline radiation damage zones and resulting effects on the tortuosity of He migration pathways and dual-domain behavior. This is combined with another equation that describes damage annealing in zircon. The end result is a new model that constrains the coevolution of damage, He diffusivity, and He date in zircon as a function of its actinide content and thermal history. The second and third appendices use this new model to decipher zircon He datasets comprising many single grain dates that are correlated with effective uranium (eU, a proxy for the relative degree of radiation damage among grains from the same sample). The model is critical for proper interpretation of results from igneous settings that show date-eU correlations and were once considered spurious (appendix B). When applied to partially reset sedimentary rocks, other sources of date variability, such as damage and He inheritance, have to be considered as well (appendix C).