Field constraints on zircon (U-Th)/He closure from the European Alps

Thermochronology provides powerful tools for reconstructing Earth’s thermal and tectonic history. Among low-temperature thermochronometers, zircon (U–Th)/He (ZHe) dating has gained particular importance due to its sensitivity to deep-time thermal events, enabling tight temperature constraints even for thermal histories that span billions of years.

Helium diffusivity in zircon is strongly controlled by radiation-damage accumulation. This causes complex diffusion behaviour and a wide range of effective closure temperatures especially in (meta-)sedimentary rocks, where detrital zircons share their post-depositional thermal history but differ in provenance age and uranium content, leading to variable radiation-damage and annealing histories.

The widely used zircon radiation damage accumulation and annealing model (ZRDAAM; Guenthner et al., 2013) predicts complete resetting of ZHe ages for samples heated above ~200 °C during burial. Consequently, highly dispersed ZHe datasets in sedimentary rocks are commonly interpreted as reflecting mixed detrital populations and limited heating below this threshold. However, this interpretation remains largely untested against natural field laboratories.

In this study, ZHe closure and annealing is re-investigated based on two field areas with independently constrained thermal histories: (1) the Austroalpine Drau Range and adjacent Southalpine units and (2) the Helvetic Glarus Alps. Peak temperatures in these regions are well defined by vitrinite reflectance, Raman spectroscopy of carbonaceous material, Kübler-Index and fluid inclusion data as well as by metamorphic assemblages. In both areas, ZHe data systematically conflict with model predictions. The Austroalpine and Southalpine (meta-)sedimentary units targeted within this study experienced upper-diagenetic to low-grade metamorphic conditions (T>200 °C), but ZHe ages are largely not reset and show strong dispersion, contrary to ZRDAAM expectations. Combined ZHe–U–Pb double dating confirms substantial differences in pre-depositional provenance ages, but this age variation cannot explain the obvious difference to modelled age predictions.

A similar pattern is observed in the Glarus Alps, where peak metamorphic temperatures range from diagenesis to greenschist facies. ZHe ages show a systematic younging trend but retain large single-grain age dispersion and partially inherited ages even under very low-grade metamorphic conditions.

Results demonstrate significant helium retention in zircons at temperatures above 200 °C and reveal limitations of our understanding of the ZHe system. Interpreting dispersed ZHe datasets solely in terms of detrital inheritance fails to explain field constraints. Ongoing work combining U-Pb-He double dating with cathodoluminescence imaging, Raman spectroscopy, and spatially resolved U-Pb and isotopic mapping aims to identify the mechanisms responsible for this behavior and to improve the interpretation of ZHe data in sedimentary and remain-grade metamorphic rocks.

Reference:

Guenthner, W. R., Reiners, P. W., Ketcham, R. A., Nasdala, L., & Giester, G. (2013). Helium diffusion in natural zircon: radiation damage, anisotropy, and the interpretation of zircon  (U-Th)/He thermochronology. American Journal of Science, 313(3), 145-198. ://WOS:000319306100001