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

下地幔条件下方镁石长径比的评估

• 批准号: 449701262
• 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: 2020
• 资助国家: 德国
• 起止时间: 2019-12-31 至 2023-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/449701262?language=en
• 关键词: Evaluation; aspect; ratio; ferropericlase; under

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.

下地幔的流变学是了解地球动力学和演化的关键。下地幔由硼镁石（Brg）和少量的铁方镁石（fPc）组成。下地幔流变学受哪个相控制的问题是至关重要的，因为fPc的粘性比Brg小2~3个数量级。如果fPc颗粒是孤立的，Brg将控制整体下地幔粘度。如果fPc颗粒是相互连接的，fPc将控制整体下地幔粘度，尽管它的小部分。如果fPc晶粒被拉长，则fPc将发生互连。地质年代尺度上的大的总应变应该产生细长的颗粒。然而，这一论点没有考虑到颗粒会随着时间的推移而变圆以降低表面能。预计由于表面能最小化的圆化应克服下地幔极低应变率条件下的伸长。在这个项目中，我们将在下地幔的物理条件下，在给定的应变速率下，评估fPc的纵横比，提供必要的数据，通过数值模拟预测下地幔的有效粘度。样品将通过新开发的具有阶梯式金刚石活塞的变形组装件在23 GPa的压力和2000 K的温度下在传统的多砧压机中变形以拉长fPc晶粒。将使用扫描电子显微镜测量变形样品中的fPc纵横比。从fPc纵横比和体剪切应变，将获得作为应变速率的函数的伸长率。然后将具有细长fPc晶粒的样品在准流体静力学条件下在23、38和53 GPa的压力和1800、2100和2400 K的温度下使用超高压多砧压机退火持续3和30小时。然后再次使用扫描电子显微镜测量退火样品的fPc纵横比。为了评估位错和扩散蠕变对圆化的贡献，将通过透射电子显微镜测定位错密度。根据退火前后的纵横比、退火持续时间、位错密度、压力和温度来制定圆化率。最后，我们将根据这些信息评估下地幔条件下的fPc长宽比。