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.

地幔的动态运动解释了各种各样的地质和地球物理现象。因此，了解动态地幔流对于固态地球物理学至关重要。岩石变形实验表明，地幔矿物的蠕变强度因水合作用（水解弱化）而大幅降低。然而，变形实验在技术上存在困难，而且对关键参数固有的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为矿物高温流变学提供重要且独立的信息来源，因为硅酸盐矿物的缓慢高温蠕变应受到硅自扩散的控制。我们研究了水对镁橄榄石硅自扩散的影响(Fei et al., 2013)。这项研究表明，水对地幔流变学的影响比之前认为的要小得多。尽管橄榄石流变学上的含水指数被认为是1.2，但实际上仅为0.3。这个惊人的结论应该通过检查其他重要的地幔矿物来扩展。瓦兹利石是地幔过渡带的主要组成矿物。在本研究中，我们将在 1400 和 1600 K 的温度下测量含水量 <1 至 1000 ppm 的无铁和含铁硅锰矿的硅自扩散系数，并将其确定为温度和含水量的函数。在此类研究中，使用单晶样品对于可靠的测量至关重要。我们将根据我们之前研究开发的技术合成瓦兹利石单晶。硅锰矿硅自扩散系数的测量将按照我们之前对镁橄榄石的研究进行。对于含铁样品，预退火和扩散退火将在Mo+MoO2缓冲的恒定氧逸度下进行。根据该项目的结果，我们将检查并提高我们对水对地幔流变学影响的理解。这项工作将为正确理解地幔的动态行为提供重要的见解。