Mechanism of the γ–β phase transformation of Mg₂SiO₄ at high temperature and pressure

Abstract

THE transformation of Mg_1.8Fe_0.2SiO₄ olivine first to the β-phase (modified spinel) and then to the γ-spinel phase occurs with increasing depth in the Earth's mantle as a result of the increasing pressure^1,2. The mechanisms of these two transformations have an important influence on mantle rheology and may also be related to the origin of deep-focus earthquakes^3–8. We report here the results of experiments on the phase transformation of Mg₂SiO₄ olivine at 15 GPa pressure in a multi-anvil cell. At this pressure and a temperature of 900 °C, early formed metastable ^γ-spinel transforms partially to the β-phase. The observed microstructures, which are similar to those in shocked meteorites^9–11, show that the γ-to-β transformation can occur either by diffusion-controlled growth or by a martensitic (shearing) mechanism, depending on how far the pressure-temperature conditions deviate from their values at phase equilibrium. Our results suggest that the diffusion-controlled mechanism is most likely to operate at the β/γ phase boundary in the mantle, but a martensitic β-to-γ transformation might occur in subduction zones and could reduce the shear strength of the subducting slab