Variable helium diffusion characteristics in fluorite
Variable helium diffusion characteristics in fluorite
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萤石中不同的氦扩散特性
DOI：
10.1016/j.gca.2016.05.029
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发表时间：
2016
影响因子：
5
通讯作者：
von Eynatten
中科院分区：
文献类型：
--
作者：
Stockli;Wiedenbeck;von Eynatten
准确分析氦在萤石中的扩散特征是建立新的萤石(U-Th-Sm)/He热年代仪(FHE)的关键，它可能为不适用于常规地质年代学的矿床测年提供有力的工具。对来自一系列地质环境的萤石进行的增量氦放气实验表明，这是一种热激活的体积扩散机制。扩散行为是高度可变的，参数范围在LogD_0/a_2=90.30±0.27~7.27×10~(-4)−~(-1)~E_(A)=9.96×10~(-3)×10~(-5)~182×10~(-6)×10~(-6)×10~(-6)×10~(-6)×10~(-6)×10~(-2)×10~(-6)×10~(-2)×10~(-6)×10~(-2)×10~(-6)×10~(-3)~(？)尽管天然萤石的CaF2含量在大多数情况下超过99%的重量百分比，但根据这些扩散参数计算的萤石(U-Th-Sm)/He热时计的闭合温度(TC)在46°C±0.14°C到169°C±9.9°C之间，考虑到125μm的碎片尺寸。在这里，我们确定了稀土元素和Y对钙的少量取代以及钠、氟、氧和/或萤石晶格中空位的相关电荷补偿对氦在矿物中的扩散系数有显著的影响。随着稀土离子浓度的增加，F空位减少，关键扩散路径变窄。因此，预计会有更高的关闭温度。一个经验案例研究证实了这种变异性：来自同一矿床(捷克共和国霍尔尼克鲁普卡)的两个萤石样品的温度分别为170°C和43°C，U-Th-Sm/He年龄分别为290 Ma±10.10 Ma和79°±10.10 Ma。因此，尽管随后的中生代埋藏和与之相关的区域热液加热，但具有高Tc的萤石样品自二叠纪含萤石矿形成以来可能定量地保留了氦。与之相反，低T值的萤石形成于晚白垩世，接近同一地区的磷灰石裂变径迹(AFT)和磷灰石(U-Th)/He年龄(AHe)。值得注意的是，FHE的热模拟产生了与基于AFT和AHE的成熟模拟相当的结果。
Precise analysis of the diffusion characteristics of helium in fluorite is crucial for establishing the new fluorite (U–Th–Sm)/He thermochronometer (FHe), which potentially provides a powerful tool for dating ore deposits unsuitable for the application of conventional geochronometers. Incremental helium outgassing experiments performed on fluorites derived from a spectrum of geological environments suggest a thermally activated volume diffusion mechanism. The diffusion behaviour is highly variable and the parameters range between logD0/a2= 0.30 ± 0.27–7.27 ± 0.46 s−1andEa= 96 ± 3.5–182 ± 3.8 kJ/mol. Despite the fact that the CaF2content of natural fluorites in most cases exceeds 99 weight percent, the closure temperature (Tc) of the fluorite (U–Th–Sm)/He thermochronometer as calculated from these diffusion parameters varies between 46 ± 14 °C and 169 ± 9 °C, considering a 125 μm fragment size. Here we establish that minor substitutions of calcium by rare earth elements and yttrium (REE + Y) and related charge compensation by sodium, fluorine, oxygen and/or vacancies in the fluorite crystal lattice have a significant impact on the diffusivity of helium in the mineral. With increasing REE + Y concentrations F vacancies are reduced and key diffusion pathways are narrowed. Consequently, a higher closure temperature is to be expected. An empirical case study confirms this variability: two fluorite samples from the same deposit (Horni Krupka, Czech Republic) withca.170 °C andca.43 °CTcyield highly different (U–Th–Sm)/He ages of 290 ± 10 Ma and 79 ± 10 Ma, respectively. Accordingly, the fluorite sample with the highTccould have quantitatively retained helium since the formation of the fluorite-bearing ores in the Permian, despite subsequent Mesozoic burial and associated regional hydrothermal heating. In contrast, the fluorite with the lowTcyields a Late Cretaceous age close to the apatite fission track (AFT) and apatite (U–Th)/He ages (AHe) from the same locality. Remarkably, thermal modelling of FHe yields comparable results to the well-established modelling based on AFT and AHe.