Calibration of a new model for mantle viscosity: the role of grain boundaries from bicrystal experiments

地球内部的固体岩石可以流动，类似于冰川中的冰，只要有足够的时间和温度。这种流动或粘性变形在短时间和长时间尺度上对各种过程有很大的影响。在很长的时间尺度上，岩石的粘性变形控制着地球构造板块的运动。在短时间内，岩石的粘性变形控制着上覆的、引发地震的断层上应力积累的速率。然而，我们对这些岩石如何变形的理解存在很大的差距，这导致在模拟地球上这些大规模过程时存在很大的不确定性。不确定性的最大来源之一是理解晶界，即晶体之间的区域，在极端条件下如何变形。这种缺乏了解对预测地球的过程有重大影响。例如，如果晶粒边界相对于晶体内部来说比较脆弱，那么在大的地震断层上，应力的积累速度可能会增加十倍。为了解决这个缺点，我们将在极端条件下进行实验，让两个晶体相互滑动。在某些情况下，我们会向边界加水，以测试水是否会增加晶体滑动的速度。来自许多实验的数据将被用来创建一个方程，描述晶体在各种条件下滑动的速度。为了研究单个晶界如何影响由许多晶体组成的岩石的性质，这些方程将被纳入预测晶体集合体行为的数值模拟中。这些模拟将用于了解晶界在各种重要的大规模地质过程中的重要性。

The solid rocks within Earth's interior can flow, analogous to ice in a glacier, given sufficient time and temperature. This flow, or viscous deformation, has a strong influence on a variety of processes over short and long time scales. Over long time scales, the viscous deformation of rocks controls the motion of Earth's tectonic plates. Over short timescales, the viscous deformation of rocks controls the rate at which stresses buildup on overlying, earthquake-generating faults. However, there are major gaps in our understanding of how these rocks deform, which results in significant uncertainties in modeling these large-scale processes on Earth.One of the largest sources of uncertainty is in understanding how grain boundaries, that is the regions between crystals, deform at extreme conditions. This lack of understanding has major implications for predicting processes in Earth. For instance, if grain boundaries are weak relative to the interiors of crystals, then the rates at which stresses build up on large, earthquake-generating faults may increase tenfold. To address this shortcoming, we will carry out experiments at extreme conditions in which we slide two crystals past each other. In some cases, we will add water to the boundary to test if water increases how fast the crystals slide. The data from many experiments will be used to create an equation that describes how fast the crystals slide under a wide range of conditions. To investigate how individual grain boundaries influence the properties of a rock made up of many crystals, these equations will be incorporated into numerical simulations that predict the behavior of an aggregate of crystals. These simulations will be used to understand the importance of grain boundaries in a variety of important large-scale geologic processes.