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.

Residual stress preserved in quartz from the San Andreas Fault Observatory at Depth

“…The crystal is divided into slightly misoriented domains (Fig. 1B), with some regions strain free (green) and others with residual elastic strain that was not released, due to local dislocations, disclinations, vacancies, and other defects (e.g., Kröner and Anthony, 1975;Cordier et al, 2014). A typical region is in the center of the scan (red for e xx and blue for e yy ).…”

Stress evolution and associated microstructure during transient creep of olivine at 1000–1200 °C

“…Disclinations (rotational topological defects) were known to exist in liquid crystals (Friedel, 1922;Frank, 1958) but were thought to be too energetically costly in crystalline solids. Nevertheless, they were recently described along the grain boundaries of copper and olivine (Beausir & Fressengeas, 2013;Cordier et al 2014). …”

Continuous mush disaggregation during the long-lasting Laki fissure eruption, Iceland

“…Numerous misclassified pixels (e.g., low-anorthite pixels in macrocryst cores) nonetheless remained, indicating that higher count spectra (>5000 counts) would be required to discriminate between compositionally different plagioclase populations robustly. Alternatively, QEMSCAN phase maps could be converted into grain maps by integrating crystal orientation information from electron backscattered diffraction (EBSD) analyses (e.g., Prior et al 1999;Cordier et al 2014).…”