Abstract
Combined observations from natural and experimental deformation microstructures are often used to constrain the rheological properties of the upper mantle. However, relating natural and experimental deformation processes typically requires orders of magnitude extrapolation in strain rate due to vastly different time scales between nature and the lab. We examined a sheared peridotite xenolith that was deformed under strain rates comparable to laboratory shearing time scales. Microstructure analysis using an optical microscope and electron backscatter diffraction (EBSD) was done to characterize the bulk crystallographic preferred orientation (CPO), intragrain misorientations, subgrain boundaries, and spatial distribution of grains. We found that the microstructure varied between monophase (olivine) and multiphase (i.e., olivine, pyroxene, and garnet) bands. Olivine grains in the monophase bands had stronger CPO, larger grain size, and higher internal misorientations compared with olivine grains in the multiphase bands. The bulk olivine CPO suggests a dominant (010)[100] and secondary activated (001)[100] that are consistent with the experimentally observed transition of the A to E-types. The bulk CPO and intragrain misorientations of olivine and orthopyroxene suggest that a coarser-grained initial fabric was deformed by dislocation creep coeval with the reduction of grain size due to dynamic recrystallization. Comparing the deformation mechanisms inferred from the microstructure with experimental flow laws indicates that the reduction of grain size in orthopyroxene promotes activation of diffusion creep and suggests a high activation volume for wet orthopyroxene dislocation creep.
Highlights
Introduction published maps and institutional affilAnalysis of rock microstructure and fabric by solid-state deformation serves as a tool for reconstructing the mechanical and thermo-chemical conditions of deformation (e.g., [1])
Pole figures were calculated either as area-weighted or as one point per grain contoured by the density of the orientation distribution function (ODF) with a half-width of 10◦
Using the three orthogonal Electron backscatter diffraction (EBSD) maps, we show the microstructure of the sample in 3-D
Summary
Analysis of rock microstructure and fabric (e.g., grain size and texture) by solid-state deformation serves as a tool for reconstructing the mechanical and thermo-chemical conditions of deformation (e.g., [1]). Many variables can affect the grain size and texture of peridotitic samples, including: differential stress, mechanism of deformation, water content, pressure, temperature, melt fraction, degree of annealing, pre-existing texture, and the fraction and distribution of auxiliary minerals such as pyroxenes and garnet [2,3,4,5,6,7,8,9,10,11]. Most constraints for the microstructural evolution come from laboratory experiments where the stress, pressure, temperature, chemical environment, and initial fabric are wellconstrained [12,13,14,15,16,17,18,19].
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