How anisotropic is the viscosity of Earth's mantle?

地幔粘度的各向异性有多大？

• 批准号: 1214876
• 负责人: David Kohlstedt
• 金额: $ 32万
• 依托单位: University of Minnesota-Twin Cities
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2012
• 资助国家: 美国
• 起止时间: 2012-07-01 至 2017-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1214876&HistoricalAwards=false
• 关键词: How; anisotropic; viscosity; Earth; mantle

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.

来自现场、实验室和数值研究的证据表明，岩石强度随施加力的方向而变化。 这种粘度的各向异性（岩石流动的能力）影响大规模的地球动力学过程，并且是板块构造运行不可或缺的一部分。 由地幔结构中预先存在的异质性引起的各向异性粘度与古代和现代板块边界的形成密切相关。 各向异性粘度还显着影响圣安德烈亚斯断层等剪切带形成的位置和动态。 尽管各向异性粘度对大规模地球动力学过程具有重大影响，但量化其在变形岩石中的大小的数据却很少。 因此，通过我们的实验，我们将探索各向异性粘度在多大程度上影响此类板块边界的弱化，从而使它们局部化和狭窄，这是板块构造的基本特征。 因此，该提案中概述的研究直接解决了“地幔岩石的粘度各向异性如何，以及这种各向异性在变形过程中如何演变？”的问题。 我们强调，我们的实验开辟了全新的领域，并将提供有关粘性各向异性的第一个实验结果。 这项研究不仅会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构成岩石的颗粒的强烈排列以及应力驱动的熔体偏析成富熔层。 我们的研究利用创新的实验方法来确定由于强晶体择优取向或熔体丰富带的明显分层而产生的粘度各向异性。 实验将在橄榄石固体骨料和橄榄石加玄武岩部分熔融骨料上进行。 首先将薄壁圆柱形样品变形至高扭转应变，以在微观结构中产生显着的各向异性，并确定与最简单的晶体滑移面的主导方向或由富熔带定义的分层平行的剪切粘度。 然后使该样品变形至小的附加张力应变，以确定分别垂直于易滑移平面或富熔带的粘度。 粘度的各向异性程度表示为这两种粘度的比率。 我们的初步结果表明，与基于传统实验方法结果的模型预测的相比，粘性各向异性更强，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部分熔融区域的流变、输运和地震特性的各向异性。 第二个目标是为理论分析提供约束，这对于理解地幔动力学所涉及的潜在物理机制至关重要。 第三个目标是制定尺度定律，用于将实验结果从实验室条件外推到地幔时间和空间尺度。