ARAGONITE AND CALCITE (U-TH)/HE GEOCHRONOLOGY AND THERMOCHRONOLOGY

We explore and quantify helium retentivity in speleothem-derived carbonates using independent U/Th and U-Pb ages, step-heating diffusion experiments, microstructural characterization and thermal modelling.

(U-Th)/He ages of aragonite speleothem layers which were dated during this study are statistically identical to independent ages and thus indicate that aragonite speleothems as old as 6 Ma are fully retentive to helium at surface temperatures of 0-20°C. Step heating diffusion experiments suggest that crystal boundaries in aragonite speleothems often do not act as rapid paths for helium diffusion and thus the size of the diffusion domain is often dictated by larger-scale structures such as transitions between different laminae. Our experiments constrain the activation energy for He diffusion in aragonite to 31.0 ± 1.5 kcal/mol and the log of the frequency factor (D₀) to -3.3 ± 0.5 m²/s. These parameters enable to calculate helium retention at various holding temperatures as a function of grain size and the holding duration and provide further indication that aragonite of common grain size (~100 microns) is fully retentive across geological time scales at common surface temperatures (0-20^o C), and can be used for (U-Th)/He geochronology and thermochronology alike. Considering a cooling rate of 10°C/Ma and a diffusion domain radius of 100 mm, the closure temperature of aragonite is 69 ± 16°C, similar to Durango apatite.

(U-Th)/He dating of calcite speleothems often yields ages which are lower than independent ages, implying that helium loss from calcite may occasionally occur even at surface temperatures as low as 1°C. Step heating diffusion experiments reveal multi-domain diffusion behavior and suggest that helium loss occurs from diffusion domains which are much smaller than the crystal size. However, despite this caveat, the observed microstructure can be tightly linked to the retentivity. Our results demonstrate that although the use of calcite as a thermochronometer is not straightforward - it is often feasible. Furthermore, since helium diffusion from calcite can occur at common surface temperatures it may be possible to use microstructural data and independent ages in order to invert helium loss to near-surface paleo-temperatures.