Impact of apatite chemical composition on (U-Th)/He thermochronometry: An atomistic point of view
Impact of apatite chemical composition on (U-Th)/He thermochronometry: An atomistic point of view
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DOI：
10.1016/j.gca.2015.06.017
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发表时间：
2015-10
影响因子：
5
通讯作者：
Duval Mbongo Djimbi;C. Gautheron;J. Roques;Laurent Tassan-Got;C. Gerin;É. Simoni
中科院分区：
文献类型：
--
作者：
Duval Mbongo Djimbi;C. Gautheron;J. Roques;Laurent Tassan-Got;C. Gerin;É. Simoni
影响He在磷灰石中扩散的不同参数的量化是(U-Th)/He热年代学解释的一个重要问题。关键问题包括了解化学成分的作用和通过辐射损伤改变扩散系数的机制，这两个问题都需要在原子水平上进行现实的描述。在这一贡献中，我们局限于化学成分的影响，特别是氯原子对无损伤磷灰石晶体中He扩散的影响。为此，利用周期密度泛函理论对两种不同的磷灰石成分(纯氟磷灰石和每晶胞含1个氯和3个氟原子的磷灰石，称为Cl0.25-磷灰石)进行了多尺度的理论扩散研究。首先研究了不同的He插入位置和扩散路径。将密度泛函理论方法与微动弹性带方法相结合，确定了插入位之间的势垒。基于过渡态理论的统计方法被用来计算站点之间的跳跃率，不同的结果被用作3D随机行走模拟的输出，以确定扩散轨迹和扩散系数。计算的纯F-磷灰石的扩散系数表现出轻微的各向异性，其激活能Ea=95.5kJ/−，频率因子D_0=11.9×10~(-3)−~3/S，在正交面上，E_a=约106.1×10~(-3)cm~2/S，D_0=94.1×10~(-10)cm~2/S。颗粒半径为60μm、冷却速率为10°C/mA的封闭温度范围为33°C至36°C，取决于给定颗粒尺寸的晶体几何形状。令人惊讶的是，即使He在Cl0.25-磷灰石中的氯原子之间的扩散被强烈阻止，其中Ea值明显更高(166.7 kJ/摩尔)，He原子仍然可以通过变通途径沿着轴心扩散。闭合温度取决于晶格中的氯含量，Cl0.25-磷灰石的闭合温度可以比F-磷灰石高∼12°C。这些结果表明，除了对损伤退火率的影响外，不同的氯含量还会导致更大的保持性扩散系数。本研究的结果与实验结果吻合较好，表明适当的密度泛函理论处理可以表征He在无损伤磷灰石中的扩散。这为量化稀有气体在矿物结构中扩散的可靠方法开辟了新的途径。
The quantification of the different parameters influencing He diffusion in apatite is an important issue for the interpretation of (U-Th)/He thermochronometric ages. Key issues include understanding the role of chemical composition and the mechanism modifying diffusivity by radiation damage, both requiring a realistic description at the atomic level. In this contribution, we restrict ourselves on the influence of the chemical composition especially on the effect of Cl-atoms on the He diffusion in the damage-free apatite crystal. For this purpose, a multi-scale theoretical diffusion study has been conducted using periodic Density Functional Theory calculations for two different apatite compositions (pure fluorine apatite and apatite with one chlorine and 3 fluorine atoms per cell called Cl0.25-apatite) representative of damage-free crystals. Different He insertion sites and diffusion pathways are first investigated. The Density Functional Theory approach coupled to the Nudged Elastic Band method is used to determine the energy barriers between the insertion sites. A statistical method, based on Transition State Theory, is used to compute the jump rate between sites and the different results are used as output for a 3D random walk simulation, which determines the diffusion trajectories and the diffusion coefficients. The calculated diffusion coefficients for pure F-apatite exhibit a slightly anisotropic behavior with an activation energyEa= 95.5 kJ/mol and a frequency factorD0= 1.9 × 10−3cm2/s along thecaxis;Ea= 106.1 kJ/mol andD0= 4.1 × 10−3cm2/s in the plane orthogonal toc. Closure temperatures for a 60 μm grain radius and 10 °C/Ma cooling rate range from 33 to 36 °C and depend on crystal geometry for a given grain size. Surprisingly, even though He diffusion is strongly blocked across the Cl atoms in Cl0.25-apatite, whereEais significantly higher (166.7 kJ/mol), He atoms can still diffuse along thecaxis through workaround pathways. Closure temperatures are dependent on the Cl content in the crystal lattice and can be ∼12 °C higher for Cl0.25-apatite than for F-apatite. These results show that various Cl contents lead to a more He retentive diffusivity in addition to their impact on damage-annealing rate. The results of this study are in good agreement with experimental results and demonstrate that a proper Density Functional Theory treatment allows to characterize He diffusion in damage-free apatite. This opens new avenues to a reliable method of quantifying rare gas diffusion in mineral structures.