Effects of Deformation-Induced Microstructure, Texture and the Spatial Distribution of Phases on the Steady-State Rheology and Attenuation Response(s) of Mantle Materials

摘要：变形诱导的微观结构、结构和相的空间分布对地幔材料稳态流变学和衰减响应的影响布朗大学地质科学系首席研究员reid F. Cooper对地球上地幔结构的地球物理学认识是基于地震波的传播速度和吸收（衰减）。地震资料的解释取决于对适当矿物组合作为各种热力学和微观结构因素的函数的动态力学响应的理解。在适当的地震和远震频率下表征动力响应的实际数据库非常少；因此，大多数地震学解释，例如对地球上地幔结构的解释，都是基于有关衰减响应与塑性（蠕变）响应的相关性的假设，而塑性（蠕变）响应的数据要多得多。然而，最近对各种地球和工程材料衰减的实验研究清楚地表明，衰减-蠕变相关假设充其量是可疑的，特别是因为变形中的空间和时间尺度效应没有得到充分考虑。本项目是一个实验和理论研究变形诱导的微观结构，纹理和矿物的空间缩放对(i)稳态流变学和（ii）衰减响应的影响。研究了典型上地幔物质——由白云石（多晶橄榄石）和辉石（橄榄石和正辉石的多晶混合物）以及天然橄榄石单晶组成的合成聚集体。实验工作强调(a)橄榄石-正辉石相分离的空间尺度特征作为应力（或应变速率）和累积应变的函数；(b)通过基于衍射的极点图分析对材料内部的变形微观结构进行统计表征；(c)应力-松弛和“应力-倾斜”响应随累积应变的函数特征；(d)直接测量变形试件的衰减响应。理论工作强调将Hart[1970]的塑性状态方程模型应用于晶体和聚集体的衰减响应。这项研究解决了地震学家直接感兴趣的物理性质问题，特别是在试图隔离影响方面，例如，“水”独立于温度，对衰减响应的影响。但是，衰减也会受到各种尺度（亚颗粒结构、晶格优先取向等）的岩石组构的影响，而在适当的时间-温度条件下，这些岩石组构还没有被矿物组合表征。应力松弛技术在衰减物理研究中的应用（和验证测试）可以让我们了解如何进行时空外推，以及有可能分离各种热力学因素对衰减的影响。该研究不仅追求矿物中衰减的微观机制的还原主义物理学，而且还超越了——在无情的偏应力作用下，岩石的塑性的非平衡，耗散热力学。因此，科学本身通过使用大塑性应变来解决材料“自组装”的问题；与具有独特物理（以及经济）特性的分层材料相关的理论可以通过这项研究来制定。

Research Program Abstract: EAR-0609869Effects of Deformation-Induced Microstructure, Texture and the Spatial Distribution of Phases on the Steady-State Rheology and Attenuation Response(s) of Mantle MaterialsReid F. Cooper, Principal InvestigatorDepartment of Geological SciencesBrown UniversityThe geophysical understanding of the structure of Earth's upper mantle is predicated on the propagation velocity and absorption (attenuation) of seismic waves. Interpretation of seismological data depends on understanding the dynamic mechanical response of appropriate mineral assemblages as functions of a variety of thermodynamic and microstructural factors. The actual database for characterization of dynamic response at appropriate seismic and teleseismic frequencies is quite minimal; consequently, most seismological interpretations, e.g., of structure in Earth's upper mantle, are predicated on assumptions concerning the correlation of the attenuation response with the plastic (creep) response, where far more data are available. Recent experimental studies of attenuation in a variety of Earth and engineering materials indicate clearly, however, that the attenuation-creep correlation assumption is dubious at best, specifically because of inadequate consideration of spatial and temporal scaling effects in deformation. This project is an experimental and theoretical study of the effects of deformation-induced microstructure, texture and the spatial scaling of minerals on (i) the steady-state rheology and (ii) the attenuation response(s). Model upper-mantle materials--synthesized aggregates of dunite (polycrystalline olivine) and of harzburgite (polycrystalline mixture of olivine and orthopyroxene) as well as natural olivine single crystals--are studied. The experimental work emphasizes (a) characterization of spatial scaling of olivine-orthopyroxene phase separation as a function of the stress (or strain rate) and accumulated strain; (b) statistical characterization of deformation microstructure within the material via diffraction-based pole-figure analysis; (c) characterization of the stress-relaxation and "stress-dip" responses as a function of accumulated strain; and (d) direct measurement of attenuation response of deformed specimens. The theoretical work emphasizes application of Hart's [1970] plasticity equation-of-state model to the attenuation response of crystals and aggregates. This research addresses physical properties issues of direct interest to seismologists, particularly in the attempt to isolate effects, e.g., of "water," independent from that of temperature, on the attenuation response. But attenuation will be affected, too, by rock fabrics at a variety of scales (subgrain structure, lattice-preferred orientation and beyond), which have not yet been characterized on mineral assemblages at appropriate time-temperature conditions at all. Application (and proof-testing) of a stress-relaxation technique to the study of attenuation physics may allow knowledge of how to perform spatiotemporal extrapolation, as well as have the potential to isolate the effects of competing various thermodynamic factors on attenuation. The research pursues not only the reductionist physics of the microscopic mechanism(s) of attenuation in minerals, but moves beyond--to the nonequilibrium, dissipative thermodynamics of plasticity in rocks subjected to a relentless deviatoric stress. Thus, the science itself addresses issues of material "self-assembly" specifically through the use of large plastic strain; theories relating to hierarchical materials with unique physical (and, thus, economical) properties can be anticipated to be formulated through this research.