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，岩石强度随所施加力的方向而变化。 这种粘性的各向异性（岩石流动的能力）影响着大规模的地球动力学过程，并且是板块构造运动不可或缺的一部分。 各向异性粘度所产生的地幔结构中预先存在的不均匀性强烈相关的古代和现代板块边界的形成。 各向异性粘度也显着影响剪切带，如圣安德烈亚斯断层的位置和动力学的形成。 尽管各向异性粘度对大尺度地球动力学过程有重要影响，但在变形岩石中，其大小的量化数据却很少。 因此，通过我们的实验，我们将探索各向异性粘性在多大程度上影响这种板块边界的弱化，使它们能够局部化和狭窄，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，还将对高温应用中使用的结构陶瓷材料的机械性能提供重要的限制。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进入熔体富集层的应力驱动偏析。 我们的研究利用了一种创新的实验方法来确定粘度的各向异性，无论是从一个强大的结晶择优取向或显着分层的熔体丰富的带。 实验将在橄榄石的固体聚集体和橄榄石加玄武岩的部分熔融聚集体上进行。 首先将薄壁圆柱形样品在扭转中变形至高应变，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的结晶滑移面的主导取向或由熔体富集带限定的分层的剪切粘度。 然后使该样品在张力下变形至小的附加应变，以分别确定垂直于易滑移面或富熔体带的粘度。 粘度的各向异性程度表示为这两种粘度的比率。 我们的初步研究结果表明，粘性各向异性都是强大的，并在不同的时间尺度上比预测模型的基础上从传统的实验方法的结果。 这项研究的一个长期目标是提供深入了解流变学，运输和地震属性的变形部分熔融地区的下地壳和上地幔的各向异性。 第二个目标是为理论分析提供约束条件，这对于理解地幔动力学中所涉及的基本物理机制至关重要。 第三个目标是制定标度律，以便将实验室条件下的实验结果外推到地幔时空尺度。