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 thermo chronometry 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 lie diffusivity decreases and activation energy (E.) 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. for He diffusion in zircon is about 44 kcal/mol, and the minimum closure temperature (T-c for a cooling rate of 10 degreesC/myr) is about 190 degreesC. Age-paleodepth relationships from the Gold Butte block suggest that the base of the zircon He partial retention zone is at preexhumation 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 He T-c of about 200 degreesC 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. (C) 2002 Elsevier Science B.V. All rights reserved.