He diffusion and (U-Th)/He thermochronometry of zircon: Initial results from Fish Canyon Tuff and Gold Butte

To evaluate the potential of (U-Th)/He geochronometry and thermochronometry of zircon, we measured He diffusion characteristics in zircons from a range of quickly and slowly cooled samples, (U-Th)/He ages of zircons from the quickly cooled Fish Canyon Tuff, and age-paleodepth relationships for samples from 15 to 18 km thick crustal section of the Gold Butte block, Nevada. (U-Th)/He ages of zircons from the Fish Canyon Tuff are consistent with accepted ages for this tuff, indicating that the method can provide accurate ages for quickly cooled samples. Temperature- dependent He release from zircon is not consistent with thermally activated volume diffusion from a single domain. Instead, in most samples apparent He diffusivity decreases and activation energy (E_a) increases as cycled step-heating experiments proceed. This pattern may indicate a range of diffusion domains with distinct sizes and possibly other characteristics. Alternatively, it may be the result of ongoing annealing of radiation damage during the experiment. From these data, we tentatively suggest that the minimum E_a for He diffusion in zircon is about 44 kcal/mol, and the minimum closure temperature (T_c, for a cooling rate of 10 °C/myr) is about 190 °C. Age-paleodepth relationships from the Gold Butte block suggest that the base of the zircon He partial retention zone is at pre-exhumation depths of about 9.5 - 11 km. Together with constraints from other thermochronometers and a geothermal gradient derived from them in this location, the age-depth profile suggests a He T_c of about 200 °C for zircon, in reasonable agreement with our interpretation of the laboratory measurements. A major unresolved question is how and when radiation damage effects become significant for He loss from this mineral.