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强的影响。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。在短时间尺度上，岩石的粘性变形控制着上覆地震断层的应力积聚速度。然而，我们对这些岩石如何变形的理解存在重大差距，这导致了在地球上模拟这些大规模过程的重大不确定性。不确定性的最大来源之一是如何理解晶界，即晶体之间的区域，在极端条件下是如何变形的。这种认识的缺乏对预测地球的过程有重大影响。例如，如果晶界相对于晶体内部较弱，那么在产生地震的大型断层上积聚应力的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，我们将在边界上加水来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用于创建一个方程，该方程描述了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。这些模拟将用于理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.