Severe Strain-Induced Olivine-Ringwoodite Transformation at Room Temperature: Key to Enigmas of Deep-Focus Earthquake

Abstract

Deep-focus earthquakes at 350–660 km are presumably caused by olivine-spinel phase transformation (PT). This cannot, however, explain the observed high seismic strain rate, which requires PT to complete within seconds, while metastable olivine does not transform for over a million years. Recent theory quantitatively describes how severe plastic deformations (SPD) can solve this dilemma but lacking experimental proof. Here, we introduce dynamic rotational diamond anvil cell with rough diamond anvils to impose SPD on San Carlos olivine. While olivine never transformed to spinel at room temperature, we obtained reversible olivine-ringwoodite PT under SPD at 15–28 GPa within tens of seconds. The PT pressure reduces with increasing dislocation density, microstrain, plastic strain, and decreasing crystallite size. Results demonstrate a new strain-induced PT mechanism compared to a pressure/temperature-induced one. Combined with SPD during olivine subduction, this mechanism can accelerate olivine-ringwoodite PT from millions of years to timescales relevant to earthquakes.