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

硅锰矿硅自扩散系数随含水量变化的测量

• 批准号: 251472533
• 负责人: Professor Dr. Tomoo Katsura
• 金额: --
• 依托单位: Bayerisches Forschungsinstitut für Experimentelle Geochemie und Geophysik (BGI)
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2014
• 资助国家: 德国
• 起止时间: 2013-12-31 至 2019-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/251472533?language=en
• 关键词: Measurement; Si; self; diffusion; coefficients

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.

地幔的动力运动解释了各种各样的地质和地球物理现象。因此，了解动态地幔流动对固态地球物理学至关重要。岩石变形实验表明，地幔矿物的水化作用（水解弱化）使其蠕变强度大幅度降低。然而，变形实验技术难度大，对关键参数缺乏控制，使得实验结果的解释存在问题。硅自扩散系数的测量有可能提供关于矿物高温流变学的重要和独立的信息来源，因为硅酸盐矿物的缓慢高温蠕变应该由硅自扩散控制。我们研究了水对硅自扩散的影响（Fei et al., 2013）。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；虽然认为橄榄石流变性的含水率指数为1.2，但实际上只有0.3。这一惊人的结论应该通过研究其他重要的地幔矿物加以扩展。wadsleite是地幔过渡带的主要组成矿物。在这项研究中，我们将在1400和1600 K的温度下测量含水量从<1到1000 ppm的无铁和含铁的wadsleite的Si自扩散系数，确定它们作为温度和含水量的函数。在这种研究中，使用单晶样品是可靠测量的必要条件。我们将在前人研究的基础上合成瓦德斯莱石单晶。对wadsleyite的Si自扩散系数的测量将按照我们之前对forsterite的研究进行。对于含铁样品，在Mo + MoO2缓冲的恒定氧逸度下进行预退火和扩散退火。基于这个项目的结果，我们将检查和提高我们对水对地幔流变的影响的理解。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。