How anisotropic is the viscosity of Earth's mantle?

来自现场、实验室和数值研究的证据表明，岩石强度随外力的方向而变化。这种粘度的各向异性(岩石的流动能力)影响着大规模的地球动力学过程，是板块构造活动的组成部分。地幔结构中先前存在的非均质性引起的各向异性粘度与古代和现代板块边界的形成都有很强的相关性。各向异性粘度也显著影响剪切带的形成位置和动力学，如圣安德烈亚斯断层。尽管各向异性粘性对大尺度地球动力学过程的影响很大，但在变形岩石中缺乏量化其大小的数据。因此，通过我们的实验，我们将探索各向异性粘性对这种板块边界减弱的影响程度，使它们能够局部化和狭窄，这是板块构造的基本特征。因此，这项建议中概述的研究直接解决了“地幔岩石粘度的各向异性如何，以及这种各向异性在变形过程中如何演变？”这一问题。我们强调，我们的实验取得了全新的突破，并将提供关于粘性各向异性的第一个实验结果。这项研究不仅将影响我们对地球内部动态演化的理解，还将为高温应用中使用的结构陶瓷材料的机械性能提供重要的约束。粘度各向异性的最重要来源是组成岩石的颗粒的强烈排列和熔体在应力驱动下偏析成富熔体的层。我们的研究利用一种创新的实验方法来确定粘度的各向异性，这是由于强烈的晶体择优取向或明显的富熔带分层造成的。实验将在橄榄石固体集合体和部分熔融橄榄石+玄武岩集合体上进行。薄壁圆柱体样品首先在扭转时变形为高应变，以在微观结构中产生显著的各向异性，并确定平行于最容易晶体滑动面的主导取向或由富熔体带所定义的分层的剪切粘度。然后将该样品变形为一个小的附加拉伸应变，以分别确定垂直于易滑动面或富熔体带的粘度。粘度的各向异性程度用这两种粘度的比率来表示。我们的初步结果表明，粘性各向异性比基于传统实验方法的模型预测的更强，并且在不同的时间尺度上演化。这项研究的一个长期目标是深入了解下地壳和上地幔变形的部分熔融区的流变性、输运和地震性质的各向异性。第二个目标是为理论分析提供约束，这些理论分析对于理解地幔动力学涉及的基本物理机制至关重要。第三个目标是开发从实验室条件到地幔时间和空间尺度的实验结果的外推标度定律。

Evidence from field, laboratory, and numerical studies indicates that rock strength varies with the direction of the applied force. This anisotropy in viscosity (the ability of a rock to flow) influences large-scale geodynamic process and is integral to the operation of plate tectonics. Anisotropic viscosity arising from pre-existing heterogeneities in mantle structure strongly correlates with both ancient and present-day plate boundary formation. Anisotropic viscosity also markedly affects the location and dynamics of formation of shear zones such as the San Andreas fault. Despite the major influence of anisotropic viscosity on large-scale geodynamic processes, there is a paucity of data quantifying its magnitude in deformed rocks. With our experiments, therefore, we will explore the extent to which anisotropic viscosity influences the weakening of such plate boundaries, enabling them to be localized and narrow, the essential feature of plate tectonics. Consequently, the research outlined in this proposal directly addresses the question "How anisotropic is the viscosity of mantle rocks, and how does this anisotropy evolve during deformation?" We emphasize that our experiments break totally new ground and will provide the first experimental results on viscous anisotropy. This research will impact not only our understanding of the dynamic evolution of Earth's interior, it will also provide important constraints on the mechanical properties of structural ceramic materials used in high-temperature applications.The most important sources of anisotropy in viscosity are strong alignment of the grains that make up a rock and stress-driven segregation of melt into melt-rich layers. Our research utilizes an innovative experimental approach to determine the anisotropy in viscosity resulting from either a strong crystallographic preferred orientation or a pronounced layering of melt-rich bands. Experiments will be carried out on solid aggregates of olivine and partially molten aggregates of olivine plus basalt. A thin-walled cylindrical sample is first deformed to high strain in torsion to produce significant anisotropy in the microstructure and determine the shear viscosity parallel to either the dominant orientation of the easiest crystallographic glide plane or the layering defined by the melt-rich bands. This sample is then deformed to a small additional strain in tension to determine the viscosity normal to the easy glide plane or melt-rich bands, respectively. The degree of anisotropy in viscosity is expressed as the ratio of these two viscosities. Our initial results indicate that viscous anisotropy is both stronger and evolves on a different timescale than predicted by models based on results from traditional experimental approaches. One long-term goal of this research is to provide insight into the anisotropy in rheological, transport, and seismic properties of deformed partially molten regions of the lower crust and upper mantle. A second goal is to furnish constraints for theoretical analyses that are critical for understanding the underlying physical mechanisms involved in mantle dynamics. A third goal is to develop scaling laws for extrapolation of experimental results from laboratory conditions to mantle temporal and spatial scales.