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

了解地幔矿物的流变性对于了解地幔动力学是至关重要的。岩石变形实验表明，水的掺入削弱了地幔矿物的蠕变强度，大大增强了地幔中的动力流动。然而，变形实验的技术难度可能会在一定程度上削弱这一结论。因此，用一种独立的方法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是这样一种方法，因为硅在地幔矿物中的扩散速度最慢，因此应该是速率限制的过程。我们以前对镁橄榄岩的硅颗粒内扩散的测量表明，无论是位错蠕变还是纳巴罗-赫林蠕变，水都不应该太大地促进位错蠕变。相反，水强化了晶界上的蠕变机制：鹅卵石蠕变、压溶蠕变和晶界滑动。Coble蠕变是由晶界扩散驱动的，因此即使在水不饱和的条件下，水对Coble蠕变的促进作用也会发生。如果水对Coble蠕变有很大的促进作用，那么水对地幔动力学的影响将是显著的。另一方面，间隙水在压溶蠕变和晶界滑移中起着至关重要的作用。因此，只有在饱和水的条件下才能加强这两种机制。由于水饱和条件下的区域被认为只发生在地幔楔体的火山前缘之下，因此如果水不促进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岩的硅晶界扩散系数。通过真空烧结技术将合成细粒无孔镁橄榄石聚集体，并在其晶界掺入高达25000ppm的水。在样品表面制备一层富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扩散。扩散轮廓将使用SIMS测量，晶界上的水含量将使用FTIR光谱测量。

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.