Fast anisotropic Mg and H diffusion in wet forsterite
Fast anisotropic Mg and H diffusion in wet forsterite
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DOI：
10.1007/s00269-022-01204-7
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发表时间：
2021-10
影响因子：
1.4
通讯作者：
J. Muir;Feiwu Zhang;A. Walker
中科院分区：
文献类型：
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作者：
J. Muir;Feiwu Zhang;A. Walker
向刚石中加入氢能显著提高Mg的扩散速率，但其原因尚不清楚。由于镁在橄榄石中的扩散会影响其导电性，因此了解这一过程非常重要。在这项研究中，我们使用密度泛函理论来预测含h Mg空位的扩散率，我们发现它们比不含h的Mg空位慢1000倍左右。含h的Mg空位比不含h的Mg空位集中了许多个数量级，而扩散是扩散率和缺陷浓度的结合。总的来说，预计水合Mg空位的存在会导致扩散速率和扩散各向异性的大幅（多个数量级）增加，并强烈倾向于向预测的[001]方向扩散。在实验数据模型中，水浓度对扩散的影响通常用一个常数的最佳拟合指数来描述。我们的结果表明，在常见的实验条件下，随着压力的降低和温度的升高，该指数将在0.5到1.6之间变化。这些结果表明，镁在整个上地幔中的扩散可能会发生很大的变化，而这种变化无法用简单的单指数模型来捕捉。与氢扩散率的比较表明，水合Mg空位的扩散也控制氢在（无铁）forsterite中的扩散，我们的结论也适用于氢扩散率和各向异性。我们还发现，阳离子扩散率可能不能解释水对橄榄石电导率影响的实验测量。
Adding hydrogen to forsterite strongly increases the diffusion rate of Mg, but the reason for this is unclear. As Mg diffusion in forsterite can influence its electrical conductivity, understanding this process is important. In this study we use density functional theory to predict the diffusivity of H-bearing Mg vacancies and we find that they are around 1000 times slower than H-free Mg vacancies. H-bearing Mg vacancies are many orders of magnitude more concentrated than H-free Mg vacancies, however, and diffusion is a combination of diffusivity and defect concentration. Overall, the presence of hydrated Mg vacancies is predicted to cause a large (multiple orders of magnitude) increase in both diffusion rate and diffusional anisotropy with a strong preference for diffusion in the [001] direction predicted. In models of experimental data, the effect of water concentration on diffusion is often described by a constant best-fitting exponent. Our results suggest that this exponent will vary between 0.5 and 1.6 across common experimental conditions with pressure decreasing and temperature increasing this exponent. These results suggest that Mg diffusion in forsterite could vary considerably throughout upper mantle conditions in ways that cannot be captured with a simple single-exponent model. Comparisons to measures of hydrogen diffusivity suggest that the diffusion of hydrated Mg vacancies also controls the diffusion of hydrogen in (iron-free) forsterite and that our conclusions above also apply to hydrogen diffusion rates and anisotropy. We also find that cation diffusivity likely cannot explain experimental measurements of the effect of water on electrical conductivity in olivine.