Electrical conductivity of enstatite as a function of water content: Implications for the electrical structure in the upper mantle
Electrical conductivity of enstatite as a function of water content: Implications for the electrical structure in the upper mantle
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DOI：
10.1016/j.epsl.2012.09.020
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发表时间：
2012-12
影响因子：
5.3
通讯作者：
Baohua Zhang;T. Yoshino;Xiaoping Wu;Takuya Matsuzaki;S. Shan;T. Katsura
中科院分区：
文献类型：
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作者：
Baohua Zhang;T. Yoshino;Xiaoping Wu;Takuya Matsuzaki;S. Shan;T. Katsura
在Kawai型多砧装置上，在3GPa压力下测定了不同含水量的无钙铝质顽火辉石的电导率。在0.1Hz至1MHz的频率范围内对氢掺杂和未掺杂的样品进行阻抗谱，以检查水对电导率的影响。在1000 - 1723K温度范围内，未掺杂氢的样品和在500 - 900K相对较低的温度范围内掺杂氢的样品被确定为两种导电机制，以最大限度地减少样品的脱水。对于未掺杂氢的样品，在较高温度范围（> 1300 K）的激活焓约为1.9 eV，表明主要的电荷转移机制是Fe2 + − Fe3+跳跃（小极化子）传导。在低于900K的温度下测得的掺氢样品的活化焓从1.11降低到0.70eV，电导率值随水含量的增加而系统地增加，表明质子导电是主要的导电机制。考虑到质子传导的跳跃传导和活化焓的水含量依赖性，所有电导率数据都拟合为公式σ = σ 0hexp（− Hh/kT）+ σ 0pCwexp [−（Hp0-α Cw1/3）/kT]，其中σ 0是指前因子，Cwis是以重量百分比表示的水含量，H是活化焓，Hp0是在非常低的水浓度下质子传导的活化焓，α是几何因子，k是玻尔兹曼常数，T是绝对温度，下标h和p分别表示跳跃和质子传导。利用目前的结果，在地球上地幔的电导率深度剖面实验室为基础的水含量的函数已经建成。我们的模型与目前可用的地球物理观测下的东太平洋隆起的比较表明，含水铝顽火辉石不能占高电导率异常的软流圈的顶部，以及含水橄榄石。
The electrical conductivity of Ca-free aluminous enstatite with various water contents has been determined at a pressure of 3GPa in a Kawai-type multi-anvil apparatus. Impedance spectroscopy was performed for both hydrogen-doped and -undoped samples in a frequency range from 0.1Hz to 1MHz to examine the effect of water on conductivity. Two conduction mechanisms were identified for hydrogen-undoped samples at temperature of 1000–1723K and for hydrogen-doped samples at relatively lower temperature range of 500–900K to minimize dehydration of samples. For the hydrogen-undoped samples, the activation enthalpy is around 1.9eV at the higher temperatures range (>1300K) suggesting that the dominant charge transfer mechanism is Fe2+−Fe3+hopping (small polaron) conduction. For the hydrogen-doped samples measured below 900K, the activation enthalpy decreases from 1.11 to 0.70eV, and the conductivity values systematically increase with increasing water content, suggesting that proton conduction is the dominant conduction mechanism. Taking hopping conduction and water content dependence of activation enthalpy for proton conduction into account, all electrical conductivity data were fitted to the formula σ=σ0hexp(−Hh/kT)+σ0pCwexp[−(Hp0−αCw1/3)/kT], where σ0is pre-exponential factor, Cwis the water content in weight percent, H is the activation enthalpy, Hp0is the activation enthalpy for proton conduction at very low water concentration, α is the geometrical factor, k is the Boltzmann constant, T is absolution temperature and subscripts h and p represent hopping and proton conductions, respectively. Using the present results, a laboratory-based conductivity-depth profile in the Earth's upper mantle has been constructed as a function of water content. Comparison of our model with the currently available geophysical observations beneath the Eastern Pacific Rise indicates that hydrous aluminous enstatite cannot account for the high conductivity anomaly at the top of the asthenosphere as well as hydrous olivine.