Karst-hosted bauxites as a new archive for oxi-hydroxide (U-Th)/He thermochronology

Low-temperature thermochronology has traditionally relied on apatite and zircon minerals that are commonly absent from carbonate platforms, leaving large regions effectively blind to shallow crustal thermal reconstructions. Recent methodological advances now permit the application of (U-Th)/He dating to iron oxides, but so far this approach has only been tested in crystallization contexts and has rarely been used to quantify burial-exhumation trajectories.

Here we explore karst-hosted bauxite deposits as a new natural laboratory for oxide thermochronology. These lateritic bodies, developed on the Durancian structural high in southeastern France, contain abundant hematite and goethite that formed during intense Lower Cretaceous weathering and were subsequently buried beneath Upper Cretaceous to Cenozoic sedimentary sequences. Such conditions provide the requirement for oxide thermochronology: iron oxides that experienced post-crystallization heating.

We report a large dataset of (U-Th)/He ages obtained from more than one hundred individual hematite and goethite grains sampled across south of France. All ages postdate bauxite formation and independent depositional constraints, demonstrating that these minerals systematically record post-depositional thermal overprints.

Coexisting hematite and goethite systematically yield distinct age populations, with goethite consistently recording younger apparent ages. This reproducible offset demonstrates that these two iron oxides behave as independent low-temperature chronometers.

Because diffusion parameters for goethite remain poorly constrained, thermal history modelling was performed using hematite only. Thermal inversion modelling, is supported by regional stratigraphic and tectonic frameworks. It identifies two successive heating phases linked to Pyrenean compression and to Oligocene–Miocene rifting. Reheating during this latter event temporally correspond to goethite ages. This age comparisons between both phases provide empirical constraints on reset temperatures of goethite about 60-40°C.

Our results demonstrate that karst-hosted bauxites constitute a robust archive for oxide-based thermochronology, and provide the first natural framework for empirically constraining reset temperature in goethite. This approach opens new perspectives for reconstructing shallow thermal histories in carbonate-dominated regions where conventional chronometers are absent.