Diffusion of helium in FCT zircon

Abstract

To gain a better quantitative understanding of zircon (U-Th)/He ages and evaluate the applicability of zircon (U-Th)/ He dating, the diffusion characteristics, He diffusion kinetics, helium partial retention zone (HePRZ), closure temperature, and (U-Th)/He ages were investigated using high-precision laboratory step heating experiments based on the thermally activated diffusion process. The ln(D/a2) in Fish Canyon Tuff (FCT) zircons determined from laboratory step heating experiments was negatively correlated with reciprocal temperature, as expected for thermally activated volume diffusion. The zircon activation energies ranged from 144 to 184 kJ mol−1 with a mean of 169±12 kJ mol−1. The closure temperatures ranged from 144 to 216°C (a cooling rate of 10°C Ma−1 and an effective grain radius of 38–60 μm) with an average of 176±18°C. The calculated closure temperature increased with increasing cooling rate, yielding an average zircon He closure temperature of ~136°C at a slow cooling rate of 0.1°C Ma−1, whereas the closure value was ~199°C at a cooling rate of 100°C Ma−1. The closure temperature increased with the equivalent spherical radius assuming a constant cooling rate. The He ages from FCT zircons were negligibly affected by grain size because of the rapid cooling. He preserved in the zircon was sensitive to temperature and holding time, and the temperature range for zircon HePRZ gradually decreased with increasing holding time. The (U-Th)/He ages from 26 FCT zircons yielded an algorithmic mean of 28.3±0.3 Ma (S.E.) and a geometric mean of 28.4±0.3 Ma (S.E.), consistent with the ages of 28.4±1.9 Ma reported by other laboratories. The FCT zircons were characterized by rapid cooling, young (U-Th)/He ages with good reproducibility, and low alpha doses. Weak correlations between the He ages and effective uranium (eU) concentrations from the FCT zircons indicated radiation damage did not significantly affect He diffusivity.