Aragonite (U-Th)/He geochronology: Full retentivity across geologic timescales (10⁴–10⁷ yr)

In this study we explore aragonite retentivity to helium by comparing (U-Th)/He ages to independent age constraints provided by U/Th and U-Pb dating. We utilize aragonite speleothems from Botovskaya cave in Siberia and Soreq cave in Israel where the current temperature is ∼1 °C and ∼20 °C, respectively and is almost fluctuations-free. We then explore the retentivity of the dated aragonite samples using data from petrographic thin sections and thermal models. The obtained (U-Th)/He ages are statistically identical to independent age constraints extending backwards 6 Ma. This demonstrates that aragonite with relatively common grain size (20–500 μm) is retentive to helium at surface temperatures of 0-20 °C for millions of years. Moreover, its retentivity is higher than expected using currently available helium diffusion kinetics data for aragonite. Our results indicate that aragonite (U-Th)/He geochronology is feasible for aragonite held at near surface temperatures even when the sample grain size is relatively small. Aragonite samples that experienced post-depositional heating may lose helium. (U-Th)/He ages of such samples can be used to constrain paleo surface temperatures and the timing of tectonic and thermal events.