"Too old" zircon (U-Th)/He ages in Austro- and Southalpine units of the European Alps: an overestimate of temperature or an underestimate of helium retention?

Zircon (U-Th)/He (ZHe) dating has seen rapid growth and widespread application among low-temperature thermochronological methods. Complex diffusion kinetics, primarily due to radiation damage density, may substantially influence the diffusivity of He and cause a wide temperature range from ca. 220 to <25 °C for the transition from an open to a closed system. Complexities may augment for (meta-)sedimentary rock samples containing minerals of different initial ages with highly variable uranium content leading to differences in accumulated radiation damage and thus annealing behaviours. In such cases, individual grains may only share their postdepositional thermal path. Current diffusion models predict inheritance to play a role for those samples that remained at diagenetic temperatures below 200 °C during burial.

In this contribution, we address the question whether ZHe dates from anchizonal to very low-grade metamorphic units may be transformed into geologically meaningful age information and as such may enhance thermal history reconstructions. We applied ZHe dating on 37 samples from Austroalpine and Southalpine basement-cover series adjacent to the eastern part of the Periadriatic fault line. In an attempt to quantify maximum thermal overprint during Alpine burial we compiled evidence from paleothermal indicators (e.g. vitrinite reflectance, illite crystallinity, CM Raman spectroscopy), geological field observations, and geochronological dates. These data suggest overprint at diagenetic conditions up to low-grade metamorphism in our study area. According to current ZHe diffusion models anchizonal and higher thermal conditions should have harmonized the samples’ age response and thus should have reset the ZHe system leading to concordant Alpine ages.

However, our new thermochronological dataset is characterized by a large variability in intra- and intersample age dispersion. Most of our single grain ages ranging from 12 to 305 Ma are much older than predicted by forward modeling. Such mismatch may be explained either by an underestimate of He retention resulting from a still incomplete understanding of He diffusivity. In this scenario, metasedimentary samples with an overprint up to lower anchizonal conditions (≤270°C) are likely to preserve inherited detrital information and cooling ages will reflect both the previous and most recent thermal histories. Alternatively geothermal data compiled from the literature may have overestimated peak temperatures reached during Alpine burial.

Both alternatives will be discussed in detail as they bring up challenging methodical issues. We underline the need for combining thermal maturity studies with ZHe low-temperature thermochronology in order to extract thermal history information for such complex detrital datasets.