Radiation and retentivity: unraveling the thermochronometric tangle
Intervenant : Richard Ketcham
Université d'Austin, Texas
The two primary thermochronometric tools, fission-track and (U-Th)/He dating, are intimately related in that radiation damage strongly affects the diffusive environment within a crystal lattice, and in particular how a mineral retains helium. This talk will review recent and in-progress developments in understanding both the nature of radiation damage and how it anneals, and how it affects helium diffusivity. Radiation damage percolation, or formation of an interconnected network, may be linked to transitions in the diffusivity of helium in both apatite and zircon. In apatite, alpha recoil damage percolation correlates with a maximum in helium retentivity as well as age dispersion, suggesting that the connected damage network is forming an alternative but chaotic exit pathway for trapped gas. The effects of radiation damage annealing on diffusivity in highly damaged zircon can be observed in step-heating experiments. Diffusivity evolution indicates that retentivity increases with annealing, but at temperatures substantially below those suggested by a fission-track annealing, again indicating that alpha recoil damage is the most relevant for diffusivity. However, taken as a whole, the experiments suggest a hysteresis effect, and that a single dose-diffusivity curve may be inadequate to understand radiation damage effects.