How does viscosity contrast influence phase mixing and strain localization?

Author Posting. © American Geophysical Union, 2020. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 125 (2020): e2020JB020323, doi: 10.1029/2020JB020323.Ultramylonites—intensely deformed rocks with fine grain sizes and well‐mixed mineral phases—are thought to be a key component of Earth‐like plate tectonics, because coupled phase mixing and grain boundary pinning enable rocks to deform by grain‐size‐sensitive, self‐softening creep mechanisms over long geologic timescales. In isoviscous two‐phase composites, “geometric” phase mixing occurs via the sequential formation, attenuation (stretching), and disaggregation of compositional layering. However, the effects of viscosity contrast on the mechanisms and timescales for geometric mixing are poorly understood. Here, we describe a series of high‐strain torsion experiments on nonisoviscous calcite‐fluorite composites (viscosity contrast, ηca/ηfl ≈ 200) at 500°C, 0.75 GPa confining pressure, and 10−6–10−4 s−1 shear strain rate. At low to intermediate shear strains (γ ≤ 10), polycrystalline domains of the individual phases become sheared and form compositional layering. As layering develops, strain localizes into the weaker phase, fluorite. Strain partitioning impedes mixing by reducing the rate at which the stronger (calcite) layers deform, attenuate, and disaggregate. Even at very large shear strains (γ ≥ 50), grain‐scale mixing is limited, and thick compositional layers are preserved. Our experiments (1) demonstrate that viscosity contrasts impede mechanical phase mixing and (2) highlight the relative inefficiency of mechanical mixing. Nevertheless, by employing laboratory flow laws, we show that “ideal” conditions for mechanical phase mixing may be found in the wet middle to lower continental crust and in the dry mantle lithosphere, where quartz‐feldspar and olivine‐pyroxene viscosity contrasts are minimized, respectively.This work was funded through a National Science Foundation grant (EAR‐1352306) awarded to P. S., with additional support for A. J. C. provided by the McDonnell Center for the Space Sciences (Washington University in St. Louis), the J. Lamar Worzel Assistant Scientist Fund (WHOI), and the Penzance Endowed Fund in Support of Assistant Scientists (WHOI). Partial support for electron microscopy was provided by the Institute of Materials Science and Engineering (Washington University in St. Louis).2021-02-0

Experimental deformation of forsterite, wadsleyite and ringwoodite: Implications for seismic anisotropy of the Earth's mantle

The rheological properties of the major minerals of the Earth's mantle are still not well constrained. However, these properties are crucial for the understanding of a wide range of processes in the Earth's interior such as mantle convection. The purpose of this work is to address the issue of the rheology of the lowermost upper mantle and of the transition zone through the mechanical properties at high pressure of olivine (with forsterite composition Mg2SiO4) and of its high-pressure polymorphs wadsleyite and ringwoodite. Indeed, the properties of the Earth's mantle can be inferred as a first approximation from the mechanical properties of those polymorphs which volumetrically dominate the mineralogy of the region of concern. Deformation experiments have been performed on hot-pressed forsterite samples and on pre-synthesized wadsleyite and ringwoodite samples under pressure conditions of the Earth's mantle and at 1300-1400°C. The possible influence of the phase transformation from forsterite to wadsleyite on rheology has been also investigated. Deformation has been achieved by shear using the Kawai-type multianvil apparatus. Complementary experiments on forsterite have been performed in the newly developed Deformation-DIA. Some of them have been carried out on a synchrotron beam line to perform in-situ stress and strain measurements. In order to gain a maximum of information on the deformation mechanisms and on the Crystallographic Preferred Orientation (CPO), a special attention has been devoted to the microstructural characterisation of the samples. Electron BackScattering Diffraction (ESBD) and Transmission Electron Microscope (TEM) have been mainly used. An important pressure-induced change in deformation mechanism is shown in forsterite. The deformation of forsterite at high pressure and temperature is dominated by the [001](hk0) slip system rather than the [100](010) glide which is extensively observed at low pressure and high temperature.. Concerning the high-pressure polymorphs, their plastic behaviour has been studied with a strong emphasis on the formation of CPO. ViscoPlastic Self Consistent (VPSC) modelling is used to link the CPO with known elementary deformation mechanisms of these phases. The main features of the CPO of wadsleyite are characterized by the alignment of the [100] axes parallel to the shear direction and the alignment of the [001] axes toward the normal to the shear plane. Too many uncertainties remain on the ringwoodite CPO for them being used to interpret seismic anisotropy. Finally, we suggest that strain-induced CPO might be responsible for the seismic anisotropy observed in the lowermost upper mantle and in the upper part of the transition zone. The low seismic anisotropy of the lowermost upper mantle can be explained from the slip system change in forsterite and the CPO of wadsleyite point toward a dominant tangential flow in the upper part of the transition zone.L'étude de la plasticité des minéraux du manteau terrestre sous pression joue un rôle majeur dans la compréhension et la modélisation des grands processus actifs à l'intérieur de la Terre tels que la convection mantellique. Cependant, les propriétés des minéraux du manteau sont toujours, à ce jour, mal connues. L'objectif de ce travail est d'étudier la rhéologie de la partie inférieure du manteau supérieur et de la zone de transition, à travers l'étude des propriétés mécaniques de la forsterite (Mg2SiO4) et de ses deux polymorphes de haute pression (wadsleyite et ringwoodite). En effet, ces phases sont les constituants principaux des zones étudiées et on peut considérer, en première approximation, qu'elles contrôlent les propriétés du manteau. Des échantillons de forsterite frittés et de wadsleyite et de ringwoodite synthétisés sous pression ont été déformés dans les conditions de pression du manteau et à 1300-1400°C. L'influence de la transformation de phase forsterite-wadsleyite sur la rhéologie a également été étudiée. Les expériences de déformation en cisaillement ont été menées dans la presse multi-enclumes de type « Kawai ». Quelques expériences complémentaires sur la forsterite ont été menées dans la nouvelle presse Deformation-DIA. Certaines ont été réalisées sur synchrotron afin de mesurer contraintes et déformations in situ. Les microstructures des échantillons obtenus ont été caractérisées par Microscopie Electronique en Transmission et leurs textures ont été déterminées à l'aide de la technique de diffraction des électrons rétrodiffusés. En ce qui concerne la forsterite, nous avons mis en évidence un important changement de système de glissement induit par la pression. A haute pression et température, la déformation de la forsterite est dominée par le glissement [001](hk0) alors que le glissement [100] a largement été observé à basse pression et haute température dans les travaux antérieurs. La plasticité de la wadsleyite et de la ringwoodite a été étudiée principalement aux travers des textures. La méthode de simulation ViscoPlastic Self Consistent a été utilisée pour faire le lien entre les textures et les mécanismes de déformation supposés pour ces deux phases. Les grandes caractéristiques des textures de la wadsleyite sont l'alignement des axes [100] avec la direction de cisaillement alors que les axes [001] sont normaux au plan de cisaillement. Pour la ringwoodite, aucune texture fiable ne peut être proposée. Enfin, les textures produites par la déformation plastique des trois polymorphes peuvent être proposées comme étant à l'origine de l'anisotropie sismique du manteau supérieur et de la zone de transition. Le changement de système de glissement dominant de la forsterite permet d'expliquer la faible anisotropie sismique observée dans la partie inférieure du manteau supérieur et la texture de la wadsleyite indique un écoulement horizontal dominant dans la partie supérieure de la zone de transition

Déformation expérimentale de la fosterite, de la wadsleyite at de la ringwoodite (conséquences pour l'anisotropie sismique du manteau terrestre)

L'objectif de ce travail est d'étudier la rhéologie du manteau terrestre à travers les propriétés mécaniques de la forsterite, de la wadsleyite et de la ringwoodite (Mg2SiO4). Les échantillons ont été déformés dans les presses multi enclumes Kawai et D-DIA. Les microstructures et les textures des échantillons ont été caractérisées par Microscopie Electronique en Transmission et par diffraction des électrons rétrodiffusés. A haute pression, [001] {hkO}.est dominant dans la forsterite. Les textures de la wadsleyite sont caractérisées par [100] parallèle à la direction de cisaillement et [001] normal au plan de cisaillement. Pour la ringwoodite, aucune texture fiable ne peut être proposée. Enfin, le changement de système de glissement dominant de la forsterite permet d'expliquer la faible anisotropie sismique observée dans la partie inférieure du manteau supérieur et la texture de la wadsleyite indique un écoulement horizontal dominant dans la partie supérieure de la zone de transition.LILLE1-BU (590092102) / SudocSudocFranceF

How does viscosity contrast influence phase mixing and strain localization?

Ultramylonites—intensely deformed rocks with fine grain sizes and well‐mixed mineral phases—are thought to be a key component of Earth‐like plate tectonics, because coupled phase mixing and grain boundary pinning enable rocks to deform by grain‐size‐sensitive, self‐softening creep mechanisms over long geologic timescales. In isoviscous two‐phase composites, “geometric” phase mixing occurs via the sequential formation, attenuation (stretching), and disaggregation of compositional layering. However, the effects of viscosity contrast on the mechanisms and timescales for geometric mixing are poorly understood. Here, we describe a series of high‐strain torsion experiments on nonisoviscous calcite‐fluorite composites (viscosity contrast, ηca/ηfl ≈ 200) at 500°C, 0.75 GPa confining pressure, and 10−6–10−4 s−1 shear strain rate. At low to intermediate shear strains (γ ≤ 10), polycrystalline domains of the individual phases become sheared and form compositional layering. As layering develops, strain localizes into the weaker phase, fluorite. Strain partitioning impedes mixing by reducing the rate at which the stronger (calcite) layers deform, attenuate, and disaggregate. Even at very large shear strains (γ ≥ 50), grain‐scale mixing is limited, and thick compositional layers are preserved. Our experiments (1) demonstrate that viscosity contrasts impede mechanical phase mixing and (2) highlight the relative inefficiency of mechanical mixing. Nevertheless, by employing laboratory flow laws, we show that “ideal” conditions for mechanical phase mixing may be found in the wet middle to lower continental crust and in the dry mantle lithosphere, where quartz‐feldspar and olivine‐pyroxene viscosity contrasts are minimized, respectively.National Science Foundation (NSF): Award EAR-135230