H‐D Interdiffusion in Single‐Crystal Olivine: Implications for Electrical Conductivity in the Upper Mantle
H‐D Interdiffusion in Single‐Crystal Olivine: Implications for Electrical Conductivity in the Upper Mantle
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DOI：
10.1029/2019jb017576
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发表时间：
2019-06
期刊：
影响因子：
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通讯作者：
Wei Sun;T. Yoshino;M. Kuroda;N. Sakamoto;H. Yurimoto
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文献类型：
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作者：
Wei Sun;T. Yoshino;M. Kuroda;N. Sakamoto;H. Yurimoto
了解地幔中的水含量和分布对于理解地球的地球化学演化和地球动力学过程至关重要，因为水可以在高压下并入名义上无水的矿物中，并显著影响地幔矿物的化学和物理性质。氢扩散控制着地球深部水的输送和电导率，但对于上地幔中最丰富的矿物橄榄石，氢扩散还没有完全了解。在这里，我们提出了在上地幔条件下（3-13 GPa和1，000 - 1，300 K）通过H和D掺杂橄榄石单晶偶的相互扩散确定的新的氢自扩散系数。本活化焓氢迁移是显着小于以前的工作确定在有限的测量温度范围内。不同水浓度的平行互扩散实验表明，橄榄石中的水含量强烈加速了氢的扩散。橄榄石上的几何平均扩散系数是温度和水含量的函数：DH=10−7.4±0.8*CH2O0.41±0.01exp−130± 17 kJ/molRTm 2/s。结合Nernst-Einstein关系式，本文的结果可以约束水对橄榄石电导率的贡献。这表明，在低温（<1,000 K）下对含水橄榄石进行的原位电导率测量产生的活化焓太低，无法外推到更高的温度。与之前单晶橄榄石电导率测量结果的比较表明，由于合成单晶的不均匀性，除Dai和Karato（2014）之外的文献数据可能高估了水对电导率的影响。由于高温下氢的扩散机制发生了变化，因此由扩散数据模拟高活化焓的地幔电导率更为可靠。考虑到重新评估的氢扩散活化焓和橄榄石中的水溶性，现有电导率模型与地球物理观测之间的比较表明，橄榄石的水合作用不能解释在海洋软流圈中观察到的极高电导率值（10−2-10−1 S/m）。
Knowledge of water content and distribution in the Earth's mantle is critical to understanding the geochemical evolution and geodynamic processes of the Earth, since water can incorporate into nominally anhydrous minerals at high pressure and dramatically affect the chemical and physical properties of mantle minerals. Hydrogen diffusion controls the transport of water and electrical conductivity in the deep Earth but is not fully understood for olivine, the most abundant mineral in the upper mantle. Here we present new hydrogen self‐diffusion coefficients determined from interdiffusion in H‐ and D‐doped olivine single‐crystal couples at the upper mantle conditions (3–13 GPa and 1,000–1,300 K). Present activation enthalpy for hydrogen migration is significant smaller than previous work determined within a limited measured temperature range. Parallel interdiffusion experiments with diversified water concentrations demonstrated that hydrogen diffusivity strongly accelerated by the water content in olivine. The geometric average diffusion coefficient on olivine is showed as a function of temperature and water content: DH=10−7.4±0.8*CH2O0.41±0.01exp−130±17kJ/molRTm2/s . Combined with the Nernst‐Einstein relation, the present results can constrain the contribution of water to the electrical conductivity on olivine. It suggests that in situ conductivity measurements on hydrous olivine at low temperatures (<1,000 K) produced too low activation enthalpy to extrapolate to the higher temperatures. Comparison with previous results by conductivity measurements on single‐crystal olivine suggests that the literature data except for Dai and Karato (2014) might overestimate water effect on conductivity because of heterogeneity of synthetic single crystals. Because of a change of dominant hydrogen diffusion mechanism at high temperature, this study suggests that the modeling of mantle conductivity with a high activation enthalpy from diffusion data is more trustworthy. Considering a reevaluated activation enthalpy on hydrogen diffusion and water solubility in olivine, comparisons between present conductivity model and geophysical observations suggest that hydration of olivine cannot account for extremely high conductive values (10−2–10−1 S/m) observed in the oceanic asthenosphere.