title: Disclinations provide the missing mechanism for deforming olivine-rich rocks in the mantle.
abstract: Mantle flow involves large strains of polymineral aggregates. The strongly anisotropic plastic response of each individual grain in the aggregate results from the interactions between neighbouring grains and the continuity of material displacement across the grain boundaries. Orthorhombic olivine, which is the dominant mineral phase of the Earth's upper mantle, does not exhibit enough slip systems to accommodate a general deformation state by intracrystalline slip without inducing damage. Here we show that a more general description of the deformation process that includes the motion of rotational defects referred to as disclinations can solve the olivine deformation paradox. We use high-resolution electron backscattering diffraction (EBSD) maps of deformed olivine aggregates to resolve the disclinations. The disclinations are found to decorate grain boundaries in olivine samples deformed experimentally and in nature. We present a disclination-based model of a high-angle tilt boundary in olivine, which demonstrates that an applied shear induces grain-boundary migration through disclination motion. This new approach clarifies grain-boundary-mediated plasticity in polycrystalline aggregates. By providing the missing mechanism for describing plastic flow in olivine, this work will permit multiscale modelling of the rheology of the upper mantle, from the atomic scale to the scale of the flow.
authors.affiliation: Unité Matériaux et Transformations, UMR 8207 CNRS and Université Lille 1, 59650 Villeneuve d'Ascq, France.
authors.forename: Patrick
authors.initials: P
authors.lastname: Cordier
authors.affiliation: Geosciences Montpellier, UMR 5342 CNRS and Université de Montpellier 2, 34095 Montpellier, France.
authors.forename: Sylvie
authors.initials: S
authors.lastname: Demouchy
authors.affiliation: Laboratoire d'Etude des Microstructures et de Mécanique des Matériaux, UMR 7239 CNRS and Université de Lorraine, Ile du Saulcy, 57045 Metz Cedex, France.
authors.forename: Benoît
authors.initials: B
authors.lastname: Beausir
authors.affiliation: Laboratoire d'Etude des Microstructures et de Mécanique des Matériaux, UMR 7239 CNRS and Université de Lorraine, Ile du Saulcy, 57045 Metz Cedex, France.
authors.forename: Vincent
authors.initials: V
authors.lastname: Taupin
authors.affiliation: Geosciences Montpellier, UMR 5342 CNRS and Université de Montpellier 2, 34095 Montpellier, France.
authors.forename: Fabrice
authors.initials: F
authors.lastname: Barou
authors.affiliation: Laboratoire d'Etude des Microstructures et de Mécanique des Matériaux, UMR 7239 CNRS and Université de Lorraine, Ile du Saulcy, 57045 Metz Cedex, France.
authors.forename: Claude
authors.initials: C
authors.lastname: Fressengeas