Plastic strain-induced olivine-ringwoodite phase transformation at room temperature: main rules and the mechanism of the deep-focus earthquake

Deep-focus earthquakes that occur at 350-660 km are theorized to be caused by strain-induced olivine-spinel phase transformation (PT). We introduce and apply dynamic rotational diamond anvil cell with rough diamond anvils to deform San Carlos olivine. While olivine was never transformed to spinel at any pressure at room temperature, we obtained olivine-ringwoodite PT under severe plastic shear at 15-28 GPa within seconds. This is conceptual proof of the difference between pressure- and plastic strain-induced PTs and that plastic straining can accelerate this PT from million years to timescales relevant for the earthquake. The PT pressure linearly reduces with increasing plastic strain, corresponding increasing dislocation density and decreasing crystallite size. The main rules of the coupled severe plastic flow, PT, and microstructure evolution are found.