The effects of radiation damage and annealing on the diffusion of helium from zircon
The relationship between radiogenic ⁴He in zircon, and its associated radiation damage to the crystal lattice, have been of scientific interest since the earliest attempts at geochronology, and remain an active topic of research today in the pursuit of understanding the timing of low-temperature geological processes. Recently, the complex interrelationships between ⁴He diffusion kinetics, time, temperature, and radiation damage were quantified into a complete model for the zircon (U-Th)/He (ZHe) thermochronometric system: the Zircon Radiation Damage Accumulation and Annealing Model (ZRDAAM). Many researchers have taken issue with the predictions of this model, however, particularly at moderate to high radiation doses (~3 × 10¹⁷ – 2 × 10¹⁹ α/g). In this submission, the relationship between radiation damage and ⁴He diffusion kinetics are examined in greater detail to identify sources of discrepancy between observations and model predictions. Using zircons from three geologically distinct localities, this work represents the largest study on the diffusion kinetics of ⁴He from zircon to date, including 26 cycled step-heating analyses. A new behavior is observed in diffusion kinetics behavior, termed ‘rollover’: a thermally activated decrease in diffusivities, interpreted as low-temeprature annealing. Comparing the results of modeled and observed ZHe dating and step-heating analyses, we find significant discrepancies with model predictions in agreement with previous research. The source of these discrepancies is identified as low-temperature annealing, a process distinct from epitaxial recrystallization, resulting in significant effects on diffusional behavior, and not accounted for in the ZRDAAM. Results reveal that the damage threshold beyond which model diffusivities increase is too high, likely as a result of extensive prior geological annealing of the high-damage Sri Lankan zircons used to calibrate the ZRDAAM. Furthermore, the use of a single alpha dose appears inappropriate for capturing the full complexity of the ZHe system. Lastly, these results strongly suggest that the diffusion of ⁴He from zircon proceeds in a manner similar to a homogeneous gas, in spite of the fact that natural ⁴He concentrations are often strongly heterogeneous. This work is a significant step forward in the improvement of ZHe thermochronology, and will yield to the recovery of more accurate and robust thermal histories.