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

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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.