Measurement of Si self-diffusion coefficients of wadsleyite as a function of water content

地幔中的动力学运动解释了各种各样的地质和地球物理现象。因此，了解动态地幔流是固态地球物理学的关键。岩石变形实验表明，地幔矿物的蠕变强度因水化作用（水解弱化）而大大降低。然而，变形实验在技术上是困难的，并且对关键参数的固有缺乏控制使得实验结果的解释成为问题。硅的自扩散系数的测量有可能提供一个重要的和独立的信息来源的高温流变矿物，因为缓慢的高温蠕变的硅酸盐矿物应控制硅的自扩散。我们已经研究了水对镁橄榄石的Si自扩散的影响（Fei等人，2013年）。这项研究表明，水对地幔流变学的影响比以前认为的要小得多;虽然橄榄石流变学的含水量指数被认为是1.2，但实际上只有0.3。这个惊人的结论应该通过研究其他重要的地幔矿物来推广。硅镁石是地幔过渡带的主要组成矿物。在这项研究中，我们将测量硅的自扩散系数的无铁和含铁wadsleyite与水含量从<1到1000 ppm的温度为1400和1600 K，确定它们作为温度和水含量的函数。在这类研究中，使用单晶样品对于可靠的测量是必不可少的。我们将在前期研究的基础上合成硅铝石单晶。硅镁橄榄石的硅自扩散系数的测量将沿用我们以前对镁橄榄石的研究。对于含铁样品，预退火和扩散退火将在Mo + MoO 2缓冲的恒定氧逸度下进行。基于这个项目的结果，我们将检查和提高我们的理解的水对地幔流变学的影响。这项工作将为正确理解地幔的动力学行为提供重要的见解。

Dynamic motion in the mantle accounts for a wide variety of geological and geophysical phenomena. Understanding dynamic mantle flow is therefore essential for solid-state geophysics. Rock deformation experiments have shown a large decrease in the creep strength of mantle minerals by hydration (hydrolytic weakening). However, deformation experiments are technically difficult, and the inherent lack of control on key parameters makes interpretation of experimental results problematic. Measurements of Si self-diffusion coefficients have the potential to provide an essential and independent source of information about the high-temperature rheology of minerals, because the slow high-temperature creep of silicate minerals should be controlled by Si self-diffusion. We have studied the effect of water on the Si self-diffusion of forsterite (Fei et al., 2013). This study suggests that the effect of water on mantle rheology is much smaller than previously considered; although the water content exponent on the olivine rheology was considered to be 1.2, it is actually only 0.3. This striking conclusion should be extended by examining other important mantle minerals. Wadsleyite is a dominant constituent mineral in the mantle transition zone. In this study, we will measure Si self-diffusion coefficient of both Fe-free and Fe-bearing wadsleyite with water contents from <1 to 1000 ppm at temperatures of 1400 and 1600 K, determining them as a function of temperature and water content. In this kind of study, use of single crystal samples is essential for reliable measurement. We will synthesize wadsleyite single crystals based on the techniques developed in our previous study. The measurement of Si self-diffusion coefficients of wadsleyite will be conducted by following those in our previous study for forsterite. For the iron-bearing samples, pre-annealing and diffusion annealing will be conducted under constant oxygen fugacity buffered by Mo + MoO2. Based on the results of this project, we will examine and improve our understanding of the effects of water on mantle rheology. This work will provide important insights required to properly understand the dynamic behavior of the mantle.