Diffusion Creep of Enstatite at High Pressures Under Hydrous Conditions
Diffusion Creep of Enstatite at High Pressures Under Hydrous Conditions
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含水条件下顽辉石在高压下的扩散蠕变
DOI：
10.1002/2017jb014400
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发表时间：
2017
期刊：
影响因子：
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通讯作者：
David L. Kohlstedt
中科院分区：
文献类型：
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作者：
Guinan Zhang;Shenghua Mei;Maoshuang Song;David L. Kohlstedt
地幔对流和大规模板块运动关键取决于岩石圈-软流圈边界的性质，从而取决于地球上地幔的粘度结构，而上地幔的粘度结构是由其组成矿物的流变特性决定的。为了限制斜方辉石（上地幔第二丰富的成分）的流动行为，使用变形 DIA 装置在高压（3.8–6.3 GPa）和高温（1323–1573 K）下对细粒（~6 µm）顽辉石样品进行三轴压缩蠕变实验。根据这项研究的结果，顽辉石的变形行为以流动定律的形式定量呈现，该定律描述了变形速率对差异应力、水逸度、温度和压力的依赖性。具体来说，蠕变速率近似线性地取决于应力，表明扩散蠕变状态下的变形。与我们的数据拟合的最小二乘回归产生了扩散蠕变的流动定律，活化能约为 200 kJ/mol，活化体积约为 14 × 10−6m3/mol。水弱化效应的大小与水逸度指数 r ≈ 0.7 的橄榄石相似。粘度对水逸度（浓度）的强烈依赖性表明，含斜方辉石地幔的粘度随一种地质环境的不同而变化，具体取决于大范围的水分布。根据橄榄石和顽火辉石之间的流变学对比，我们得出结论，除了扩散蠕变状态中的一些浅层区域外，整个上地幔的大部分区域橄榄石都弱于顽火辉石。
Mantle convection and large‐scale plate motion depend critically on the nature of the lithosphere‐asthenosphere boundary and thus on the viscosity structure of Earth's upper mantle, which is determined by the rheological properties of its constituent minerals. To constrain the flow behavior of orthopyroxene, the second most abundant constituent of the upper mantle, deformation experiments were carried out in triaxial compressive creep on fine‐grained (~6 μm) samples of enstatite at high pressures (3.8–6.3 GPa) and high temperatures (1323–1573 K) using a deformation‐DIA apparatus. Based on results from this study, the deformation behavior of enstatite is quantitatively presented in the form of a flow law that describes the dependence of deformation rate on differential stress, water fugacity, temperature, and pressure. Specifically, the creep rate depends approximately linearly on stress, indicating deformation in the diffusion creep regime. A least squares regression fit to our data yielded a flow law for diffusion creep with an activation energy of ~200 kJ/mol and an activation volume of ~14 × 10−6m3/mol. The magnitude of the water‐weakening effect is similar to that for olivine with a water fugacity exponent ofr≈ 0.7. This strong dependence of viscosity on water fugacity (concentration) indicates that the viscosity of an orthopyroxene‐bearing mantle varies from one geological setting to another, depending on the large‐scale water distribution. Based on the rheology contrast between olivine and enstatite, we conclude that olivine is weaker than enstatite throughout most of the upper mantle except in some shallow regions in the diffusion creep regime.