EFFECTS OF INGROWN RADIATION DAMAGE AND ANNEALING ON HELIUM DIFFUSION IN ZIRCON

Noble gas migration through minerals is widely used in thermochronology to interpret timing and rates of geological events. However, the noble gas migration and loss processes are still poorly understood, and growing evidence suggests an important role for ingrown radiation damage defects. Here we show that (a) the major mechanism for He diffusion in zircon is thermally activated volume diffusion, and (b) damage annealing does not simply reverse the effects of damage accumulation.

Whereas some workers have proposed that a first-order, single-jump mechanism may dominate He loss from zircon, step heating experiments show a characteristic scaling between physical grain size and D/a²., requiring not only volume diffusion but correspondence between the crystal size and diffusion domain size. We measured diffusivities in fine, medium, and coarse grain size fractions of Boat Harbor (3.6E17 α/g), a low-damage zircon, and SLB (1.24E18 α/g), a high-damage Sri Lankan megacryst. The observed ln(D/a²) values are close to the expected theoretical values with some differences caused by non-equant grain shapes, deviations from lognormal grainsize distributions and internal fractures. The results indicate that thermally activated volume diffusion through domains approximately the size of the physical grain size is the primary He migration and loss mechanism in pristine zircon and continues to dominate throughout damage accumulation up to 1.24E18 α/g and possibly beyond.

Our experiments also address the effects of annealing of radiation damage on He diffusion. Annealing of zircon with damage of 3.6E15 α/g did not change diffusivities significantly, but annealing of zircon with 6.5E17 α/g increased diffusivity by about 5 lognormal units. In contrast, annealing of SLB zircon with 1.24E18 α/g did not increase diffusivities, although Raman analysis shows recovery of short range order in the crystal lattice. Our results may indicate that there is a threshold of radiation damage above which annealing does not restore diffusivities to that of fully crystalline zircon.