The mechanical properties of olivine-rich rocks are key to determining the mechanical coupling between Earth’s lithosphere and asthenosphere. In crystalline materials, the motion of crystal defects is fundamental to plastic flow1,2,3,4. However, because the main constituent of olivine-rich rocks does not have enough slip systems, additional deformation mechanisms are needed to satisfy strain conditions. Experimental studies have suggested a non-Newtonian, grain-size-sensitive mechanism in olivine involving grain-boundary sliding5,6. However, very few microstructural investigations have been conducted on grain-boundary sliding, and there is no consensus on whether a single or multiple physical mechanisms are at play. Most importantly, there are no theoretical frameworks for incorporating the mechanics of grain boundaries in polycrystalline plasticity models. Here we identify a mechanism for deformation at grain boundaries in olivine-rich rocks. We show that, in forsterite, amorphization takes place at grain boundaries under stress and that the onset of ductility of olivine-rich rocks is due to the activation of grain-boundary mobility in these amorphous layers. This mechanism could trigger plastic processes in the deep Earth, where high-stress conditions are encountered (for example, at the brittle–plastic transition). Our proposed mechanism is especially relevant at the lithosphere–asthenosphere boundary, where olivine reaches the glass transition temperature, triggering a decrease in its viscosity and thus promoting grain-boundary sliding.
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We acknowledge funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme under grant agreement number 787198 – TimeMan. The TEM facility in Lille is supported by the Conseil Régional du Nord-Pas de Calais and the European Regional Development Fund (ERDF). H.I. is mandated by the Belgian National Fund for Scientific Research (FSR-FNRS). This study was partially supported by the Agence Nationale de la Recherche through the ANR INDIGO grant (ANR-14-CE33-0011) for the low-pressure experiments, by the German Alexander von Humboldt Foundation and the Free State of Bavaria for the high-pressure experiments, and by the JSPS KAKENHI grant (number JP18K03799) to S.K. and cooperative research program of the Earthquake Research Institute, Tokyo.
The authors declare no competing interests.
Peer review information Nature thanks Pamela Burnley and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Extended data figures and tables
The spatial resolution is 6 nm. The three crystalline grains (red, blue and green) are indexed against forsterite (Pbnm space group, two diffraction patterns are provided as insets). The figure is the combination of the reliability map (darker areas being less well indexed) and the inverse pole figure along the vertical direction (using the colour code shown in the inset). In between the grains is an amorphous phase (orange, no relation to the colour code for the forsterite orientation), with a diffraction presented in the inset.
Extended Data Fig. 2 Chemical analysis (STEM-EDX) of the intergranular amorphous phase in specimen NF1050-1.
a, e, i, STEM bright-field images of the three areas investigated. b, f, j, Combined STEM and chemical Si maps, showing the presence of Si in the amorphous phase. c, g, k, Combined Si and Mg maps showing continuity of composition (Mg, Si) between the forsterite grains and the amorphous phase. Mg enrichment of the pyroxene grain is evident. d, h, i, Combined Si and Mg maps after quantification, performed using an existing method76 on the basis of the stoichiometric oxides, with standard specimens used to obtain the k factors77 of Mg and Si. In d, where the amorphous layer is larger, the thickness correction can be done accurately, showing that the amorphous phase has exactly the composition of forsterite. In h and i, the amorphous layers are smaller and thinner, with non-homogeneous thicknesses rendering quantification less reliable, so the local deviations observed in the thinnest regions should be taken with care.
Despite extensive serration of the vertical grain boundary, the forsterite grain in the middle (in Bragg conditions) shows no dislocation activity (no defects). The grain on the left shows indications of strain due to defects only where indicated by the arrow.
Experiment M639 was deformed in the multi-anvil press at 1,200 °C and 5 GPa. Fast Fourier transforms of the crystals are shown in the insets.
a–d, HRTEM of four grain boundaries in the starting material used in ref. 20 before deformation experiments in the Paterson press.
HRTEM of a grain boundary shows an amorphous layer; insets show the fast Fourier transforms from different regions.
a, Evidence for cleavage-like intergranular fracturing (boundaries indicted by arrows). b, Some grain boundaries display evidence for a internal flow-like structure. c, Inclined view of such a boundary, showing the cellular structures inside the boundary. At this temperature, the displacements are very small.
a, Arrows indicate where grain-boundary opening occurs in response to tensile stress components. These displacements must be accompanied by some shear along the neighbouring boundaries. Without markers, these shear displacements cannot be quantified. b, Assemblage of two micrographs. This region, which was probably under horizontal tensile loading, shows large displacements along vertical boundaries. Owing to differential ion-thinning rates between crystalline and amorphous materials, the region with the black star shows no remaining amorphous material (as in a). In the boundary on the left, located in a thicker region, some amorphous olivine (‘am’) is preserved. The extensional displacement at this boundary (about 190 nm) is used to evaluate the local strain (Methods). The boundaries indicated by the white asterisks show strong morphological evidence of ductile flow. The one on the right is still filled with amorphous olivine. c, The grain at the centre was probably subjected to complex triaxial loading, which has been accommodated by amorphization (some is remaining, ‘am’) and flow involving rotational, tensile and shear components (arrows). On the bottom right is a grain boundary still filled with amorphous material under tensile loading. The neighbouring grain boundary (white diamond) must also have experienced some shear. This is also probably the case for the boundary (white triangle) that is close to an opening boundary.
Extended Data Fig. 9 Evidence (Fresnel micrograph, Δf ≈ −20 μm) of grain-boundary sliding in specimen M640 (5 GPa, 1,000 °C).
The symbols represent markers, which help to visualize the shear (represented by the white arrow). Some shear bands evidenced by the Fresnel contrast are shown by black arrows. Owing to the shape of the grain, the pure shear sliding displacement in the upper part of the grain boundary transforms into opening in the central part.
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Samae, V., Cordier, P., Demouchy, S. et al. Stress-induced amorphization triggers deformation in the lithospheric mantle. Nature 591, 82–86 (2021). https://doi.org/10.1038/s41586-021-03238-3