Zircon (U-Th)/He thermochronometry in the KTB drill hole, Germany, and its implications for bulk He diffusion kinetics in zircon

This study presents new down-hole zircon (U-Th)/He (ZrHe) ages, laboratory He diffusion measurements, and numerical thermal modeling of ZrHe ages from the Continental Deep Drilling Project (KTB) in Germany to investigate He diffusion kinetics in zircon in nature over geologic timescales and to test to applicability of laboratory-derived He diffusion kinetics. Single-grain laser (U-Th)/He ages, calculated using standard alpha-ejection correction procedures assuming homogenous parent-nuclide distribution, display a systematic decrease in ZrHe ages from ∼ 112 to < 1 Ma with increasing depth. Down-hole ZrHe results display consistent ages of ∼ 85 ± 15 Ma down to ∼ 4.7 km, in agreement with rapid Cretaceous cooling documented by previous thermochronometric studies from the KTB drill hole. Below ∼ 5 km, ZrHe ages systematically decrease in age and are completely reset (< 1 Ma) below ∼ 7.2 km. The temperature range (∼ 130-200 °C) in which ZrHe ages systematically decrease defines a well-behaved zircon helium partial retention zone (HePRZ). In addition, this study presents new, cycled step-heating experiments on zircon samples from the KTB drill hole. Results from these new KTB zircon diffusion experiments indicate an activation energy (Ea) of 160 kJ/mol and a frequency factory (Do) of 0.03 cm2 s−1 with an estimated closure temperature (Tc) of 181 °C, which are in excellent agreement with published He diffusion kinetics for zircon. To compare the ZrHe results and bulk diffusion kinetics, we modeled diffusion parameters using the well-established thermal history of the KTB drill hole. The computed zircon HePRZ for the KTB drill hole is consistent with observed down-hole ZrHe ages and published and KTB-specific laboratory-derived He diffusion kinetics. Our results from ZrHe analysis from the KTB drill hole suggest that He diffusion of zircon in nature may not be controlled by anisotropic diffusion behavior, but rather behaves in accordance with laboratory-derived diffusion kinetics. The observed ZrHe ages from the KTB drill hole are in excellent agreement with predicted ZrHe age data and underscore the validity and applicability of ZrHe dating as a reliable thermochronometer.