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.

对地幔矿物质的流变特性的了解对于理解地幔动态至关重要。岩石变形实验表明，掺入水会削弱地幔矿物质的蠕变强度，并大大增强了地幔中的动态流动。但是，这一结论可能因变形实验的技术难度而部分损害。因此，重要的是，必须通过独立方法检查水对因变形实验声称的蠕变的巨大影响。 SI自扩散系数的测量是一种方法，因为SI扩散的速度在地幔矿物中的速率最慢，因此应该是限制速率的过程。您先前对固定岩的SI晶粒间接扩散的测量表明，脱位脱位均不应通过水来大大提高Nabarro-Herring蠕变。相反，它是通过水增强的晶边界上的蠕变机制：卷蠕变，压力溶液蠕变和晶界滑动。 Coble蠕变是由晶界扩散驱动的，因此，即使在水不饱和条件下，水也可能会增加水蠕变。如果水被水大大增强，水对地幔动力学的影响将是显着的。另一方面，间质水在压力溶液蠕变和晶界滑动中起着至关重要的作用。因此，仅在水饱和条件下才能增强这两种机制。因为认为水饱和条件的区域仅在地幔楔形的火山阵线下发生，因此，如果没有用水增强山毛蠕变，水对地幔动力学的影响将受到限制。出于这些原因，我们将在该项目的水上饱和条件下测量Si晶粒 - 晶体结构的扩散系数。无孔的脱粒的脱粒液聚集体将通过真空烧结技术合成，并将高达25,000 ppm的水掺入其晶界。将在样品表面上制作一个富含29 si的稀铁石的薄膜，并将样品在与水掺杂实验相同的条件下退火以扩散。将使用SIMS测量扩散曲线，并使用FTIR光谱法测量晶界的水含量。