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

TL;DR

This study demonstrates that severe plastic shear can induce olivine to transform into ringwoodite at room temperature and high pressure, providing insights into the mechanisms behind deep-focus earthquakes.

Contribution

It introduces the concept that plastic strain can trigger olivine-spinel phase transformation rapidly at room temperature, unlike pressure alone.

Findings

Plastic shear induces olivine-ringwoodite transformation at 15-28 GPa.

Transformation pressure decreases with increasing plastic strain.

Main rules of plastic flow, phase transformation, and microstructure evolution are identified.

Abstract

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.