Fluorite (U-Th)/He geochronology: methodical development and case studies

Fluorite is a widespread mineral species that forms in highly variable geological environments ranging from pegmatitic to diagenetic conditions. The few data available suggest that this mineral is suitable for (U-Th)/He geochronology although its closure temperature is not known yet. We plan to study the diffusion of helium in fluorite and its possible dependence on crystallographic parameters on various natural fluorites in order to determine experimentally the closure temperature of the fluorite-He thermochronometer. Additionally two types of natural case studies will be investigated: (i) fluorite occurrences where the fluorite-He ages could be related to the thermal history of the region as constrained by several independent thermochronometers. This allows for deriving an empirical closure temperature. (ii) Fluorite-He, Sm/Nd and geochemical data of carbonate-hosted fluorite (mostly diagenetic or fluorite-bearing Pb-Zn ore deposits) will be used to constrain the genetic conditions and derive thermochronological data of such occurrences. The development of the method has strong potential with respect to thermochronology of apatite and zircon-free lithologies such as carbonate nappes of fold-and-thrust belts.