Electron Microscopy for Materials Science, University of Antwerp, Antwerp, Belgium.
Univ. Lille, CNRS, INRAE, Centrale Lille, UMR 8207, UMET, Unité Matériaux et Transformations, Lille, France.
Nature. 2021 Mar;591(7848):82-86. doi: 10.1038/s41586-021-03238-3. Epub 2021 Mar 3.
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 flow. 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 sliding. 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.
富含橄榄石岩石的力学性质是确定地球岩石圈和软流圈之间力学耦合的关键。在晶体材料中，晶体缺陷的运动是塑性流动的基础。然而，由于富含橄榄石岩石的主要成分没有足够的滑移系统，因此需要额外的变形机制来满足应变条件。实验研究表明，橄榄石中存在一种非牛顿的、晶粒尺寸敏感的机制，涉及晶界滑动。然而，对晶界滑动的微观结构研究很少，对于单一或多种物理机制是否起作用也没有共识。最重要的是，在多晶塑性模型中纳入晶界力学的理论框架还没有建立起来。在这里，我们确定了富含橄榄石岩石中晶界变形的一种机制。我们表明，在镁橄榄石中，在应力作用下晶界会发生非晶化，而富含橄榄石岩石的延展性的出现是由于这些非晶层中晶界迁移率的激活。这种机制可能会引发深部地球的塑性过程，在深部地球中会遇到高应力条件（例如，在脆性-塑性转变处）。我们提出的机制在岩石圈-软流圈边界尤其相关，在那里橄榄石达到玻璃化转变温度，导致其粘度降低，从而促进晶界滑动。
