A new method for mapping stresses in mantle rocks: Dislocation density from electron-backscatter diffraction

地球深部固体岩石的粘性流动从根本上控制着各种大规模过程，从构造板块边界的形成，到与冰盖消退有关的海平面上升的时间尺度，再到引发地震的断层最深处的动力学。了解极端条件下岩石流动的最常见方法是在实验室中对相对较短时间尺度的小样本进行实验。然而，地球上的相关过程发生在许多公里和数百万年的时间里，其规模远远大于实验室所能达到的规模。对于科学家来说，在预测真实地球过程的行为时，要知道如何最好地从小尺度推断到大尺度，他们需要对极端条件下控制岩石流动的微观物理机制有充分的了解。有几个复杂的模型描述了岩石的粘性行为。然而，它们的细节仍然存在相当大的不确定性，并且相应地缺乏适当的分析技术来测试这些细节。因此，我们建议采用一种新的技术来量化岩石粘性流动的复杂性。这项技术最近被开发出来用于分析金属中的小尺度结构，它允许在组成晶体中的缺陷——被称为位错——在电子显微镜下以高分辨率快速精确地绘制出来。我们将对上地幔中的优势矿物橄榄石单晶进行实验室变形实验，建立位错密度与变形条件之间的关系。确定这种关系将使我们能够在非常精细的尺度上绘制在各种环境下变形的合成和天然富橄榄岩的变形条件。然后，我们将把这个映射的结果与目前橄榄石变形模型的预测结果进行比较，利用结果排除那些没有捕捉到基本物理过程的模型，并改进和更新那些能够捕捉到的模型。因此，该项目将产生一种适用于各种矿物的地球科学新方法，并将解决上地幔岩石变形方面的几个突出问题。

The viscous flow of solid rock in Earth's deep interior fundamentally controls a variety of large-scale processes ranging from the formation of tectonic plate boundaries to the timescale of sea-level rise associated with receding ice sheets to the dynamics of the deepest portions of earthquake-generating faults. The most common approach to learning about the flow of rocks at extreme conditions is to conduct experiments in a laboratory on small samples at relatively short timescales. The relevant processes in the Earth, however, occur over many kilometers and millions of years, scales much larger than those accessible in the laboratory. For scientists to know how best to extrapolate from small scales to large scales when predicting the behavior of real Earth processes, they need a robust understanding of the microscopic physical mechanisms controlling the flow of rocks at extreme conditions.Several sophisticated models exist describing the viscous behavior of rocks. However, considerable uncertainty persists regarding their details, and there is a corresponding lack of suitable analytical techniques to test those details. Therefore, we propose to adapt a new technique for quantifying the intricacies of the viscous flow of rocks. This technique, recently developed for analyzing small-scale structures in metals, allows defects in the constituent crystals-known as dislocations-to be mapped quickly and precisely at high resolution in an electron microscope. We will conduct laboratory deformation experiments on single crystals of olivine, the dominant mineral in the upper mantle, to establish a relationship between the density of dislocations and the deformation conditions. Determining this relationship will enable us to map deformation conditions at very fine scales in synthetic and natural olivine-rich rocks deformed in variety of settings. We will then compare the results of this mapping to predictions from current models for olivine deformation, using the results to rule out models that do not capture the essential physical processes and to refine and update the ones that do. The project will thus yield a new method in the Earth sciences applicable to a wide range of minerals, and it will address several outstanding problems in the deformation of upper-mantle rocks.