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Measurement of Si grain-boundary diffusion coefficients of forsterite as a function of water content: Estimation of the degree of creep enhancement with water within different regions of the upper mantle

镁橄榄石硅晶界扩散系数随含水量变化的测量：估计上地幔不同区域内水的蠕变增强程度

基本信息

批准号：
247132310
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/247132310?language=en
关键词：
Measurement Si grain boundary diffusion

项目摘要

Knowledge of the rheological properties of mantle minerals is essential for understanding mantle dynamics. Rock deformation experiments suggest that water incorporation weakens the creep strength of mantle minerals and greatly enhances dynamic flow in the mantle. However, this conclusion may be partly compromised by the technical difficulty of deformation experiments. Hence it is important that the large effect of water on creep claimed by deformation experiments be examined by an independent method. Measurement of Si self-diffusion coefficients is such a method because Si diffusion has the slowest rate in mantle minerals and should be therefore the rate-limiting process.Our previous measurements of the Si grain-interior diffusion of forsterite suggest that neither dislocation creep nor Nabarro-Herring creep should be greatly enhanced by water. It is instead creep mechanisms on grain boundaries that are enhanced by water: Coble creep, pressure solution creep and grain boundary sliding. Coble creep is driven by grain-boundary diffusion, and therefore enhancement of Coble creep by water could occur even under water-undersaturated conditions. The effect of water on mantle dynamics would be significant if Coble creep were greatly enhanced by water. On the other hand, interstitial water plays an essential role in pressure solution creep and grain boundary sliding. Therefore, these two mechanisms should be enhanced only under water-saturated conditions. Because regions of water-saturated conditions are thought to occur only under volcanic fronts in the mantle wedge, the effect of water on mantle dynamics would be limited if Coble creep were not enhanced by water.For these reasons, we will measure the Si grain-boundary diffusion coefficient of forsterite under water-undersaturated conditions in this project. Fine-grained pore-free forsterite aggregates will be synthesized by a vacuum sintering technique, and water up to 25,000 ppm will be doped into their grain boundaries. A thin film of 29Si-enriched forsterite will be made on a sample surface and the samples will be annealed for diffusion under the same conditions as the water-doped experiment. The diffusion profile will be measured using SIMS and the water content on grain boundaries will be measured using FTIR spectroscopy.

了解地幔矿物的流变学性质是理解地幔动力学的基础。岩石变形实验表明，水的掺入削弱了地幔矿物的蠕变强度，并大大增强了地幔中的动力流动。然而，这一结论可能部分地受到变形实验技术难度的影响。因此，重要的是，由变形实验声称的水对蠕变的大的影响，由一个独立的方法进行检查。Si自扩散系数的测量是这样一种方法，因为Si扩散在地幔矿物中具有最慢的速率，因此应该是速率限制过程。我们以前对镁橄榄石的Si颗粒内部扩散的测量表明，水不会大大增强位错蠕变或Nabarro-Herring蠕变。相反，晶界上的蠕变机制被水增强：Coble蠕变、压力溶解蠕变和晶界滑动。Coble蠕变是由晶界扩散驱动的，因此即使在水不饱和条件下，水也会增强Coble蠕变。水对地幔动力学的影响将是显着的，如果Coble蠕变大大加强水。另一方面，间隙水在压溶蠕变和晶界滑动中起着至关重要的作用。因此，这两种机制应仅在水饱和条件下得到增强。由于水饱和区一般只出现在地幔楔中的火山前缘，如果Coble蠕变不被水强化，水对地幔动力学的影响将是有限的，因此本项目将测量水欠饱和条件下镁橄榄石的Si晶界扩散系数。将通过真空烧结技术合成细粒无孔镁橄榄石聚集体，并将高达25，000 ppm的水掺杂到其晶界中。将在样品表面上制备富含29 Si的镁橄榄石的薄膜，并且将在与水掺杂实验相同的条件下对样品进行退火以进行扩散。将使用西姆斯测量扩散曲线，并使用FTIR光谱测量晶界上的水含量。