Diffusion of helium in FCT zircon

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/a^2) 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.

Late Mesozoic Tectono-Thermal History in the South Margin of Great Xing’an Range,NE China:Insights from Zircon and Apatite(U-Th)/He Ages

The Late Mesozoic tectono-thermal evolution and geodynamic setting of the Great Xing’an Range(GXR),particularly in the south margin,are still ambiguous.In this study,we present original low-temperature thermochronological data of six granitoid samples collected from Maanzi pluton in the south margin of the GXR.The apatite and zircon(U-Th)/He ages vary of 70.8 ± 4.3-119.0 ±7.0 Ma and 120.0 ± 7.1-146.7 ± 8.7 Ma,respectively.Further numerical inverse modeling results reveal that the granitoid plutons experienced a rapid cooling and exhumation stage during Early Cretaceous with a rate of ~5.0-6.0 ℃/Ma and ~0.14-0.17 mm/yr,respectively,including the western minor pluton emplaced during Early Permian and the major pluton crystallized in latest Jurassic,corresponding to the extensional tectonics in NE China.The rapid exhumation processes in the south margin of the GXR during the Early Cretaceous could be related to both the rollback of Paleo-Pacific oceanic plate and the collapse of thickened crust in the Mongol-Okhotsk Orogen.