MAPPING AND QUANTIFICATION OF TOTAL ANNEALING OF ZIRCON ALPHA-DECAY RADIATION DAMAGE: REVISIONS TO ANNEALING KINETICS FOR ZIRCON-HE THERMAL HISTORIES

The zircon U/Th-He (zircon-He) system is a powerful tool for determining thermal histories of geologic regions. Advances in in the last half-decade have shown that the accumulation and annealing of alpha-decay radiation damage plays a key role in the behavior of He diffusion in zircon, and that this behavior can be parameterized to enhance the interpretation of a given sample’s time-temperature history. For damage annealing, the initial damage-diffusivity model relied upon the previously established zircon fission track annealing kinetics, but these fission track kinetics do not account for the full range of annealing mechanisms in complex natural samples, nor do they necessarily describe the annealing of the main type of damage, alpha recoil, responsible for progressive change in He diffusivity. Here, we use Raman spectroscopy to measure the degree of alpha recoil damage in a suite of natural zircon grains from a Maine pluton that have been subjected to step-heating at times of 1, 6, 12, and 24 hours and temperatures of 700, 800, 900, and 1000℃. Initial damage in this suite, as measured in terms of full-width half maximum (FWHM) on the v₃SiO₂ peak (~1008 cm^-1), ranges from >25 cm^-1 to 1 cm^-1, and our dataset consists of two-dimensional grain maps that allow us to assess the possible importance of zonation on damage annealing. We compare our annealing data to the fanning curvilinear Arrhenius model of zircon fission track annealing and find that our constraints on alpha-decay radiation damage annealing do not fall along the previous fission track model contours. This lack of conformity suggests that a new model for the annealing kinetics of alpha-decay radiation damage, one that can be integrated with the damage-diffusivity relationship to improve thermal history modeling with the zircon He thermochronometer.