Journal Article

Disclinations provide the missing mechanism for deforming olivine-rich rocks in the mantle

Beausir, Benoît, Fressengeas, Claude, Cordier, Patrick, Taupin, Vincent, Demouchy, Sylvie, Barou, Fabrice

2014

Overview

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. Using electron backscattering diffraction maps of deformed olivine to resolve the disclinations at grain boundaries, combined with a disclination-based model of a high-angle tilt boundary in olivine, reveals the missing mechanism for describing plastic flow in polycrystalline olivine: an applied shear induces grain-boundary migration through disclination motion. How olivine deforms Olivine is the most abundant upper-mantle mineral — and the weakest under a wide range of thermo-mechanical conditions — so its deformation under stress is an important constraint on mantle behaviour. Patrick Cordier et al . present a model for the deformation of olivine aggregates that includes the motion of rotational defects — referred to as 'disclinations' — a factor neglected in previous studies of mantle rheology. The authors conclude that such disclination motion could provide the missing mechanism for describing plastic flow in olivine, which should enable multiscale modelling of the rheology of the upper mantle, from the atomic scale to large-scale mantle flow.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 147/135 / 704/2151/210 / 704/2151/330 / 704/2151/562 / Aggregates / Boundaries / Boundary conditions / Deformation / Earth Sciences / Grain boundaries / Humanities and Social Sciences / multidisciplinary / Partial differential equations / Petrography / Point defects / Rheology / Science / Sciences of the Universe / Upper mantle