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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特性对于理解地幔动力学至关重要。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态流动。然而，这一结论可能会因变形实验的技术难度而受到部分影响。因此，通过独立的方法来检验变形实验所声称的水对蠕变的巨大影响是很重要的。硅自扩散系数的测量就是这样一种方法，因为硅扩散在地幔矿物中具有最慢的速率，因此应该是限速过程。我们之前对镁橄榄石硅晶粒内部扩散的测量表明，位错蠕变和纳巴罗-赫林蠕变都不应该被水大大增强。相反，水增强了晶界上的蠕变机制：科布尔蠕变、压溶蠕变和晶界滑动。科布尔蠕变是由晶界扩散驱动的，因此即使在水不饱和条件下，水也会增强科布尔蠕变。如果水大大增强了科布尔的蠕变，那么水对地幔动力学的影响将是显着的。另一方面，间隙水在压溶蠕变和晶界滑动中起着重要作用。因此，这两种机制只有在水饱和条件下才能得到增强。由于水饱和条件区域被认为仅出现在地幔楔的火山前缘下，如果水不增强Coble蠕变，水对地幔动力学的影响将是有限的。因此，本项目将测量水不饱和条件下镁橄榄石的Si晶界扩散系数。细粒无孔镁橄榄石聚集体将通过真空烧结技术合成，并且将高达25,000 ppm的水掺杂到其晶界中。将在样品表面制备一层富含 29Si 的镁橄榄石薄膜，并在与掺水实验相同的条件下对样品进行退火以进行扩散。将使用 SIMS 测量扩散曲线，并使用 FTIR 光谱测量晶界上的水含量。