Hydrogen diffusivity in wadsleyite and water distribution in the mantle transition zone
Hydrogen diffusivity in wadsleyite and water distribution in the mantle transition zone
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DOI：
10.1016/j.epsl.2005.12.035
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发表时间：
2006-03
影响因子：
5.3
通讯作者：
Ryota Hae;E. Ohtani;T. Kubo;T. Koyama;H. Utada
中科院分区：
文献类型：
--
作者：
Ryota Hae;E. Ohtani;T. Kubo;T. Koyama;H. Utada
氢扩散动力学的合成多晶wadsleyite的动力学进行了测量，以确定氢在wadsleyite，地幔过渡带的主要组成矿物，与（Mg0.89Fe0.11）2SiO 4的扩散系数。采用Kawai型多砧实验装置，以Mg（OH）2为水源，在15-16 GPa、900 ~ 1200 °C的温度范围内进行了水化实验。本文所得到的扩散速率被认为是晶粒尺寸为1.9 μm的有效扩散系数，同时考虑了晶格扩散和晶界扩散的贡献。在15 GPa下，氢在多晶硅铝石中扩散的温度依赖性被确定为DH=9.6×10− 6 exp [−123（±32）（kJ mol−1）/RT]。氢在硅镁石中的扩散速率与氢在橄榄石中的平均扩散率大致一致。近年来，一些作者提出了含水过渡带的可能性，但水的分布和输运性质仍有争议。根据电导率与水的相关性，利用能斯特-爱因斯坦关系，由实测电导率估算了地幔过渡带的含水量。结果表明，在整个地幔过渡带中，水的分布是不均匀的。
The kinetics of the hydrogen diffusivity in synthesized polycrystalline wadsleyite was measured by IR spectroscopy in order to determine the diffusion coefficient of hydrogen in wadsleyite, the major constituent mineral in the mantle transition zone, with the composition of (Mg0.89Fe0.11)2SiO4. The hydration experiments were conducted at 15–16 GPa and temperature range from 900 to 1200 °C with Mg(OH)2as the water source by Kawai-type multi-anvil apparatus. The diffusion rate obtained here is considered to be an effective diffusion coefficient with the grain size of ∼9 μm involving contributions both from the lattice diffusion and the grain boundary diffusion. The temperature dependence of diffusion of hydrogen in polycrystalline wadsleyite was determined to be DH=9.6×10−6exp [−123 (±32) (kJ mol−1)/RT] at 15 GPa. Hydrogen diffusion rate in wadsleyite is roughly consistent with the average diffusivity of hydrogen in olivine. In recent years, several authors have suggested a possibility of the hydrous transition zone, and the distribution and transport properties of water are still debated. On the basis of water-dependence of the electrical conductivity the water content in the mantle transition zone was estimated from the observed conductivity by using the Nernst–Einstein relation. The results obtained here show that the distribution of water should be quite heterogeneous throughout the mantle transition zone.