Evaluation of the aspect ratio of ferropericlase under lower-mantle conditions

下地幔的流变性是了解地球动力学和演化的关键。下地幔由桥镁石(Brg)组成，并含有少量的铁方镁石(Fpc)。哪一相控制下地幔流变性的问题至关重要，因为FPC的粘性比Brg低2~3个数量级。如果FPC颗粒被隔离，Brg将控制整体下地幔粘度。如果FPC颗粒相互连接，FPC将控制整体下地幔粘度，尽管其分数很小。如果FPC颗粒被拉长，则FPC将发生互联。地质时间尺度上的大的总应变应该产生细长的颗粒。然而，这一论点并不认为随着时间的推移，颗粒会变圆，从而降低表面能。预计在下地幔极低的应变率条件下，由于表面能最小化而导致的圆整应能克服伸长作用。在这个项目中，我们将评估在给定应变率下的FPC在下地幔物理条件下的长径比，为通过数值模拟预测有效的下地幔粘性提供必要的数据。样品将通过新开发的带有阶梯状金刚石活塞的变形组合在传统的多砧压机上进行变形，压力为23 Gpa，温度为2000K，以拉长FPC颗粒。变形样品中的FPC长宽比将用扫描电子显微镜测量。根据FPC的长径比和整体剪切应变，可以得到延伸率作为应变率的函数。然后，将具有拉长FPC颗粒的样品在23、38和53 Gpa的压力以及1800、2100和2400K的温度下在准静液条件下使用超高压多砧压机进行持续3和30小时的退火。然后，使用扫描电子显微镜再次测量退火样的FPC长宽比。为了评价位错和扩散蠕变对圆化的贡献，将用透射电子显微镜测定位错密度。圆化率将根据退火前后的长宽比、退火时间、位错密度、压力和温度来制定。最后，我们将基于这些信息来评估下地幔条件下的FPC纵横比。

The rheology of the lower mantle is key to understanding the dynamics and evolution of the Earth. The lower mantle consists of bridgmanite (Brg), with smaller amounts of ferropericlase (fPc). The question of which phase controls lower-mantle rheology is essential, because fPc is 2~3 orders of magnitude less viscous than Brg. If fPc grains are isolated, Brg will control the bulk lower-mantle viscosity. If fPc grains are interconnected, fPc will control the bulk lower-mantle viscosity despite its small fraction. The interconnectivity of fPc will occur if fPc grains are elongated. Large total strains on geologic time scales should produce elongated grains. This argument however does not consider that grains will become round over time to decrease surface energy. It is expected that rounding due to surface energy minimization should overcome elongation under the extremely low strain-rate conditions in the lower mantle. In this project, we will evaluate the aspect ratio of fPc at a given strain rate under the physical conditions of the lower mantle to provide necessary data to predict the effective lower-mantle viscosity by numerical modelling. Samples will be deformed by newly developed deformation assemblages with stepped diamond pistons at a pressure of 23 GPa and a temperature of 2000 K in a conventional multi-anvil press to elongate fPc grains. The fPc aspect ratios in the deformed samples will be measured using a scanning electron microscope. From the fPc aspect ratio and bulk shear strain, the elongation rate will be obtained as a function of the strain rate. Samples with elongated fPc grains will then be annealed under quasi-hydrostatic conditions at pressures of 23, 38 and 53 GPa and temperatures of 1800, 2100 and 2400 K for durations of 3 and 30 hours using an ultrahigh-pressure multi-anvil press. The fPc aspect ratios of the annealed samples will then be measured again using a scanning electron microscope again. To evaluate the contribution of dislocation and diffusion creep to rounding, the dislocation density will be determined by transmission electron microscopy. The rounding rate will be formulated according to the aspect ratios before and after annealing, annealing duration, dislocation density, pressure and temperature. We will finally evaluate the fPc aspect ratio under lower-mantle conditions based on this information.