The effect of water on the electrical conductivity of olivine aggregates and its implications for the electrical structure of the upper mantle
The effect of water on the electrical conductivity of olivine aggregates and its implications for the electrical structure of the upper mantle
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DOI：
10.1016/j.epsl.2009.09.032
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发表时间：
2009-10
影响因子：
5.3
通讯作者：
T. Yoshino;Takuya Matsuzaki;A. Shatskiy;T. Katsura
中科院分区：
文献类型：
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作者：
T. Yoshino;Takuya Matsuzaki;A. Shatskiy;T. Katsura
在Kawai型多砧机上测量了不同含水率的San Carlos橄榄石集合体在10 Gpa压力下的电导率。对两组样品进行了电导率测量，以确定其对橄榄石中水的电导率的影响：1)氢掺杂样品和2)未掺杂氢样品。为了最大限度地减少掺氢样品中的水逸出，电导率测量在1000K以下进行。从未掺杂样品的Arrhenian行为中识别出三种导电机制，其中包括少量的水。活化热的变化表明，随着温度的升高，主要的导电机制由质子导电转变为小极化子导电。在1700K以上的温度下，活化热超过2 eV，表明电荷传输的主要机制是离子传导。电导率随含水率的增加而增大。质子传导活化能随水含量的增加而略有降低。每次测定的活化热都有相似的值(~0.9 eV)。考虑到质子传导的活化热与水浓度的关系，所有数据都符合电导率公式σ=σ0Iexp[−EI/kT]+σ0Hexp[−EH/kT]+σ0PCWexp[−(E0−αCW1/3)/kT]，，其中σ0表示指前项，cw是以重量百分比表示的水分含量，E是活化热，E0是极低水浓度下质子传导的活化焓，α是几何因子，k是玻耳兹曼常数，T是绝对温度，下标I，H和P分别表示离子、跳跃(小极化子)和质子电导。在410公里不连续处(橄榄石-瓦兹利石转变)的电导率跳跃比先前预测的要小得多。由于质子传导对块体电导率的贡献随着温度的升高而减小，软流圈顶部的高电导率异常不能用橄榄石水化作用来解释。
The electrical conductivity of San Carlos olivine aggregate of various water content was measured at a pressure of 10GPa in a Kawai-type multi-anvil apparatus. Conductivity measurements were performed on two sets of samples to determine the effect on conductivity of water in olivine: 1) a hydrogen-doped sample and 2) a hydrogen-undoped sample. To minimize water escape from the hydrogen-doped samples, the conductivity measurement was carried out below 1000K. Three conduction mechanisms were identified from the Arrhenian behavior of the undoped samples, which include a small amount of water. A change in the activation enthalpy indicated that the dominant conduction mechanism changed from proton conduction to small polaron conduction with increasing temperature. At temperatures above 1700K, the activation enthalpy exceeds 2eV suggesting that the dominant mechanism of charge transport would be ionic conduction. The conductivity increased with increasing water content. The activation enthalpy for proton conduction tends to decrease slightly with increasing water content. The activation enthalpy determined for each run had similar values (~0.9eV). Taking the water concentration dependence of activation enthalpy into account for proton conduction, all data were fitted to the electrical conductivity formula σ=σ0Iexp[−EI/kT]+σ0Hexp[−EH/kT]+σ0PCWexp[−(E0−αCW1/3)/kT], where σ0represents a pre-exponential term, CWis the water content in weight percent, E is the activation enthalpy, E0is the activation enthalpy for proton conduction at very low water concentration, α is the geometrical factor, k is the Boltzmann constant, T is absolute temperature and subscripts I, H and P denote ionic, hopping (small polaron) and proton conductions, respectively. The conductivity jump at the 410km discontinuity (olivine–wadsleyite transition) is much smaller than that previously predicted. Since the contribution of proton conduction to the bulk electrical conductivity decreases with increasing temperature the high conductivity anomaly at the top of the asthenosphere cannot be explained by olivine hydration.