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

地幔的流动使得热量可以通过对流从地球内部向外传递，而这种对流的方式在决定板块构造如何塑造地球最外层的过程中也起着主要作用。了解地幔的黏性对于理解一些大规模的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底层软流圈的相互作用。然而，要把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起作用的地球动力学过程，存在很大的困难。因此，必须建立精确的标度关系，以描述地幔粘度对温度、应力和晶粒尺寸等关键因素的依赖。地幔岩石的变形有几种微观机制。晶界滑动，即单个颗粒相互移动以适应变形，直到最近才作为地幔岩石的一种粘度控制机制引起人们的注意。该项目强调了一种新的、变革性的实验室实验方法，旨在为橄榄石（上地幔的主要矿物）的晶界滑动建立精确的尺度关系。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、地表变形的大地测量观测和暴露的地幔岩石的实地研究，这些都依赖于在给定条件下对地幔粘度的准确了解。最近的实验结果和实地观测表明，晶界滑动可能是地幔剪切带的主要机制，这一项目的动机是。然而，这些预测依赖于实验推导的关系，而以前的实验往往忽略了晶粒尺寸的影响，而这可能是重要的。在变形实验中，即使晶粒尺寸的微小变化（例如，由于晶粒长大）也会导致粘度对应力和温度的依赖性的测定出现重大误差。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，而不仅仅是控制低温剪切带。黏度对应力、晶粒尺寸和温度的依赖性目前还不够清楚，无法将实验室实验结果外推和应用于地幔中发生的地球动力学过程。我们的新方法依赖于在晶粒尺寸稳定的样品中测试温度依赖性和应力依赖性。为了精确控制晶粒尺寸，我们在变形实验中引入了两个重要的变化。首先，我们制备了晶粒尺寸明显大于稳定晶粒尺寸的样品。其次，我们将这些样品在扭转中变形为大剪切应变。由于稳定的晶粒尺寸是外加应力的函数，在高应变下变形的样品会再结晶以产生稳定的晶粒尺寸。利用这种方法，我们可以简单地通过控制外加应力来控制晶粒尺寸。我们的初步结果表明，在这些实验中，基于流动行为、晶粒尺寸演变和晶粒取向，获得了稳定的晶粒尺寸。在积累了显著的应变后，将进行应力和温度依赖性测试。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，从而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.