Is Grain-Boundary Sliding the Dominant Deformation Mechanism in Earth's Mantle?

地幔的流动允许热量通过对流从地球内部向外传递，这种对流的风格也在决定板块构造如何塑造地球最外层的方面发挥了主要作用。了解地幔的粘性对于理解几个这类大规模地球动力学问题至关重要，包括地球的热演化和岩石圈板块与其下的软流层的相互作用。然而，将在人类时间尺度上进行的实验室实验的结果外推到在地质时间尺度上运行的地球动力学过程是非常困难的。因此，必须建立准确的标度关系，描述地幔粘度对温度、应力和粒度等关键因素的依赖关系。地幔岩石的变形有几种微观机制。晶界滑动，即单个颗粒相互运动以适应变形，直到最近才作为地幔岩石中的一种粘度控制机制受到关注。该项目强调了一种新的、变革性的实验室实验方法，旨在为上地幔中的主要矿物橄榄石的晶界滑动建立精确的比例关系。这种关系可以应用于许多其他研究，包括地幔流动的数值模拟、地表形变的大地测量和暴露的地幔岩石的野外研究，这些研究都依赖于对给定条件下的地幔粘度的准确了解。该项目的动机是最近的实验结果和野外观测表明，晶界滑动可能是地幔剪切带中的主要机制。然而，这些预测依赖于实验得出的关系式，以前的实验往往忽略了颗粒大小的影响，因为它可能是重要的。在变形实验过程中，即使是颗粒尺寸的微小变化(例如，由于颗粒长大)也会导致在确定粘度对应力和温度的依赖关系时出现重大误差。这些误差是如此之大，以至于晶界滑动可以控制整个上地幔的粘度，而不仅仅是低温剪切带。粘性对应力、颗粒大小和温度的依赖关系目前还不够清楚，不足以推断和应用从实验室实验获得的结果到地球地幔中发生的地球动力学过程。我们的新方法依赖于在颗粒尺寸稳定的样品中测试温度依赖关系和应力依赖关系。为了精确控制晶粒度，我们在变形实验中引入了两个重要的变化。首先，我们制作的样品的晶粒度明显大于稳定的晶粒度。其次，我们将这些试件扭转变形为大剪切应变。由于稳定的晶粒度是外加应力的函数，变形到高应变的样品将重新结晶，产生稳定的晶粒度。使用这种方法，我们可以通过控制外加应力来简单地控制晶粒度。我们的初步结果表明，在这些实验中，基于流动行为、颗粒尺寸演变和颗粒取向，获得了稳定的颗粒尺寸。压力和温度相关性测试将在积累了显著应变后进行。这项研究有可能准确地描述晶界滑动对橄榄石变形的重要性，大大提高我们预测上地幔粘度的能力。

Flow of the Earth's mantle allows heat to be transferred by convection outward from the planet's interior, and the style of that convection also has a primary role in determining how plate tectonics shape the outermost portions of the Earth. Knowledge of the viscosity of the Earth's mantle is essential to understanding several of these large-scale geodynamic problems, including the Earth's thermal evolution and the interaction of lithospheric plates with the underlying asthenosphere. There exists, however, great difficulty in extrapolating the results of laboratory experiments conducted on a human timescale to geodynamic processes that operate on geological timescales. Thus accurate scaling relationships must be developed that describe the dependence of mantle viscosity on key factors such as temperature, stress, and grain size. Several microscopic mechanisms contribute to deformation of mantle rocks. Grain-boundary sliding, where individual grains move past each other to accommodate deformation, has only recently been brought attention to as a viscosity-controlling mechanism in mantle rocks. This project emphasizes a new, transformative approach to laboratory experiments designed to develop a precise scaling relationship for grain-boundary sliding in olivine, the dominant mineral in the upper mantle. This relationship can then be applied to many other studies, including numerical simulations of mantle flow, geodetic observations of earth surface deformation, and field studies of exposed mantle rocks, which all rely on accurate knowledge of mantle viscosity under a given set of conditions.This project is motivated by recent experimental results and field observations that indicate that grain-boundary sliding may be the dominant mechanism in shear zones in the Earth's mantle. However, these predictions rely on experimentally derived relationships, and previous experiments have often neglected the effect of grain size when it was likely important. Even small changes in grain size (e.g., due to grain growth) during deformation experiments can lead to significant errors in the determination of the dependence of viscosity on stress and temperature. These errors are so great that grain-boundary sliding could control the viscosity of the entire upper mantle rather than merely low-temperature shear zones. The dependence of viscosity on stress, grain size, and temperature is not currently known well enough to allow extrapolation and application of results obtained from laboratory experiments to geodynamic processes occurring in the Earth's mantle. Our new approach relies on testing temperature dependence and stress dependence in samples where the grain size is stable. To precisely control grain size, we have introduced two important changes to our deformation experiments. First, we fabricate samples with grain sizes significantly larger than the stable grain size. Second, we deform these samples in torsion to large shear strains. Because the stable grain size is a function of the applied stress, samples deformed to high strains will recrystallize to produce a stable grain size. Using this method, we can control grain size simply by controlling the applied stress. Our initial results demonstrate that a stable grain size is attained in these experiments based on flow behavior, grain-size evolution, and grain orientations. Tests of stress and temperature dependencies will be carried out after significant strains have been accumulated. The study proposed has the potential to accurately characterize the importance of grain-boundary sliding to olivine deformation, substantially furthering our ability to predict upper mantle viscosity.