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

萤石是一种分布广泛的矿物种类，形成于从伟晶岩到成岩条件的高度变化的地质环境中。现有的少量数据表明，该矿物适合于（U-Th）/He年代学，尽管其闭合温度尚不清楚。我们计划研究氦在萤石中的扩散及其对各种天然萤石晶体学参数的可能依赖性，以便通过实验确定萤石-He热时计的闭合温度。此外，还将研究两种类型的自然案例研究：（一）萤石的出现，其中萤石-He年龄可能与该地区的热历史有关，受到几个独立的热年代计的限制。这允许导出经验闭合温度。(ii)碳酸盐型萤石（主要是成岩型或含萤石型铅锌矿床）的萤石-He、Sm/Nd和地球化学数据将用于约束成因条件并推导此类矿床的热年代学数据。该方法的发展在磷灰石和无锆石岩性（如褶皱冲断带的碳酸盐推覆体）的热年代学方面具有很强的潜力。

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.