Al,Si-interdiffusion in bridgmanite and the viscosity of the lower mantle

在所有关于地幔对流和地球热状态的模型计算中，地幔的粘性是一个关键参数。然而，作为地球上最大的储集层，下地幔的粘度只是受到了很少的限制。固体的粘度与变形蠕变速率有关。高温蠕变是由晶格缺陷运动引起的物质输运引起的。因此，可以使用Nabarro-Herring和Weertman模型从扩散数据估计粘度。在这个方案中，我给出了一些初步的数据，这些数据表明，硅在莫来石和硅酸盐钙钛矿中的扩散系数是不同的，并且硅在钙钛矿中的扩散系数强烈依赖于体系中的铝含量。在拟议的项目中，将进一步研究地球S下地幔的优势相--硅酸盐钙钛矿中的硅的扩散率：(I)利用多台顶压机系统地研究铝、硅在硅酸盐钙钛矿中的互扩散。(Ii)应使用不同的扩散偶(例如，有空位)来研究硅的扩散系数与化学成分和Al替代机制的关系。(Iii)假设塑性变形速率由组成矿物中扩散最慢的物质控制，则应根据实验结果估计下地幔的粘度。这一研究结果将对S地幔中的对流和化学混合模式提供重要的约束。层状对流与全地幔对流的相对重要性，以及深部地幔原始未枯竭储集层存活的可能性，只能通过这项研究预期产生的粘度数据来定量解决。

The viscosity of the Earth’s mantle is a key parameter in all model calculations on mantle convection and on the thermal state of the Earth. Yet, the viscosity of the lower mantle, which is the largest reservoir of our planet, is only poorly constrained. The viscosity of solids is related to the deformation creep rate. Creep deformation at high temperature is due to transport of matter by the motion of lattice defects. Therefore, viscosities can be estimated from diffusion data using the Nabarro-Herring and Weertman models. In this proposal, I show some preliminary data, which suggest that silicon diffusivities in majorite and silicate perovskite (bridgmanite) are different and that the diffusivity of silicon in perovskite is strongly dependent on the Al content in the system. In the proposed project, Si diffusivity in silicate perovskite, the dominant phase of Earth´s lower mantle, shall be further investigated: (i) Al,Si-interdiffusion in magnesium silicate perovskite shall be systematically studied using multi-anvil press. (ii) The dependence of Si diffusivity on chemical composition and Al substitution mechanism shall be studied using different diffusion couples (e.g., with vacancies). (iii) From the experimental results, the viscosity in the lower mantle shall be estimated, assuming that plastic deformation rate is controlled by the slowest diffusing species in the constituent minerals. The results of this study will provide important constraints on the mode of convection and chemical mixing in Earth´s mantle. The relative importance of layered versus whole-mantle convection and the likelihood of the survival of primordial, undepleted reservoirs in the deep mantle can only be quantitatively addressed with the viscosity data that are expected to result from this study.