Simulating effects of heterogeneous 4He concentration profiles and radiation damage annealing on whole-grain zircon diffusivity analyses

Abstract

The retrieval of accurate thermal histories recorded by the (U-Th)/He system relies on empirical estimates of diffusion kinetics from natural minerals, although the difficulty in obtaining samples with homogeneous gas concentrations (required for accurate diffusivities) has limited the collection of such datasets. Whole-grain analyses are relatively time- and cost-effective, but natural 4He concentration heterogeneities caused by α-ejection, diffusional rounding, and/or parent nuclide zonation present an unquantified source of error. We employ a 1D spherical finite difference diffusion model to simulate effects of natural sources of He concentration heterogeneities, with a focus on the zircon (U-Th)/He (ZHe) system. Using He concentration profiles affected by various natural heterogeneities, models consistently predict that diffusion kinetics are strongly underestimated using such methods, especially at low gas release fractions. These underestimations are driven by He depletion near the grain boundary caused by α-ejection, and exacerbated by U and Th depletion. Conversely, models with parent nuclide (and therefore He) enrichment near the grain boundary compensate for ejection-based loss, yielding more accurate diffusion kinetics overall. A comparison of these results to real step-heating data yields few similarities, however, with He releases from real zircons either being linear in Arrhenius space, or having anomalously high diffusivities at low fractional releases. Radiation damage and its annealing are also significant factors affecting He diffusional mechanics. Using diffusivities and annealing as predicted by the current ZHe forward model, cycled step-heating of high-dose zircons (>1 × 1018 α/g) predict that diffusion decreases consistently with increased heating temperatures, similar to observations. The forward model predicts that annealing beyond this threshold dose should increase diffusivity, which is not observed, however. These results suggest that whole-grain step-heating data may provide a reasonable proxy for empirically measuring damage annealing kinetics in zircon. Major discrepancies between model results and real data reveal the need for more cycled-step heating data to understand the full complexities of He diffusion dynamics.