Helium diffusion in olivine based on first principles calculations
Helium diffusion in olivine based on first principles calculations
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DOI：
10.1016/j.gca.2015.01.023
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发表时间：
2015-05
影响因子：
5
通讯作者：
Kai Wang;J. Brodholt;Xiancai Lu
中科院分区：
文献类型：
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作者：
Kai Wang;J. Brodholt;Xiancai Lu
氦作为参与地幔演化的关键微量元素，其在地幔中的输运性质对理解地球的热化学演化具有重要意义。然而，由于缺乏地幔条件下矿物的测量扩散数据，氦在地幔中的迁移率仍然不清楚。本文采用基于密度泛函理论的第一性原理计算方法，计算了橄榄石中氦的绝对扩散系数。利用爬升图像轻推弹性带法，确定了扩散路径、活化能（ea）和前因子。结果表明，氦的扩散具有中等的各向异性。氦在橄榄石中的定向扩散可以用阿伦尼乌斯形式表示如下。D [0 0]= 2.42× 10-7 exp (-128.33 kJ/mol/RT) m2 /s D [0 0 0]= 9.97× 10-7 exp (-147.62 kJ/mol/RT) m2 /s D [0 0 1]= 3.85× 10-7 exp (-128.33 kJ/mol/RT) m2 /s。为了研究压力的影响，我们还在压力达到14gpa时进行了计算。随着压强从0到14 GPa的升高，ea[10 0 0]和ea[0 0 1]增大到162.10 kJ/mol， ea[10 0 0]增大到167.89 kJ/mol。根据计算的扩散系数，我们理论上估计氦可以通过最快的1 Ma晶格扩散在上地幔最上层（~ 3 GPa, 1180 K）的2.7 m和上地幔最深处（~ 14 GPa, 1800 K）的20.7 m之间迁移。此外，作为一种可能的热测时工具，我们发现在大多数地球表面条件下，橄榄石中氦的闭合温度约为143至244°C。
As a key trace element involved in mantle evolution, the transport properties of helium in the mantle are important for understanding the thermal and chemical evolution of the Earth. However, the mobility of helium in the mantle is still unclear due to the scarcity of measured diffusion data from minerals under mantle conditions. In this study, we used first principles calculations based on density functional theory to calculate the absolute diffusion coefficients of the helium in olivine. Using the climbing images nudged elastic band method, we defined the diffusion pathways, the activation energies (E a), and the prefactors. Our results demonstrate that the diffusion of helium has moderate anisotropy. The directionally dependent diffusion of helium in olivine can be written in Arrhenius form as follows. D [1 0 0]= 2.42× 10-7 exp (-128.33 kJ/mol/RT) m 2/s D [0 1 0]= 9.97× 10-7 exp (-147.62 kJ/mol/RT) m 2/s D [0 0 1]= 3.85× 10-7 exp (-128.33 kJ/mol/RT) m 2/s Our results are in excellent agreement with previous experiments. We also performed calculations where the pressure reached up to 14 GPa in order to investigate the effect of pressure. As the pressure rose from 0 to 14 GPa, E a [1 0 0] and E a [0 0 1] increased to 162.10 kJ/mol, and E a [0 1 0] increased to 167.89 kJ/mol. Based on the calculated diffusion coefficients, we theoretically estimate that helium can migrate between 2.7 m in the upmost upper mantle (∼ 3 GPa, 1180 K) and 20.7 m in the deepest upper mantle (∼ 14 GPa, 1800 K) via the fastest lattice diffusion in 1 Ma. In addition, as a possible thermo-chronometry tool, we found that the closure temperature for helium in olivine in most earth-surface conditions ranged from about 143 to 244° C.