Grain-scale simulation of olivine phase transition under stress: implications for mantle discontinuities

Abstract

Olivine and its polymorphs are the dominant minerals in the upper mantle and transition zone. The olivine phase transitions, determined primarily by pressure and temperature, control mantle discontinuities and influence mantle dynamics. Pressure is a first-order control on olivine phase transition and relates primarily to depth; therefore, it is commonly used to interpret the depths of mantle discontinuities. However, mantle dynamic models predicted stress levels of 100-300 MPa or as high as 1 GPa. Previous work has provided a complete picture of how such stresses would affect the positions where mineral reactions occur (and hence large-scale mantle structure). In this work, we plan to focus on the feedback between pressure and stress on the olivine phase transition at grain scale, and then the results can be extrapolated and upscaled to mantle scale deformation.   We use the Open Phase Studio software based on the phase field model to simulate olivine phase transitions. The phase field model uses order parameters to distinguish different phases and describe their evolution. The parameter value of 1 indicates the bulk of the phase, and a value of 0 indicates the absence of this phase and is a smooth function of position. The smooth transition of a phase parameter indicates a diffuse interface between phases. The total free energies, interface properties, and microstructure control the phase field evolution. Open Phase Studio considers the Helmholtz free energies of each phase and uses their elastic energies to account for the pressure and stress effects on phase evolution. This software currently focuses on models of alloys, but appropriate values for silicates can be input. As a foundation, we first consider an Al-Li alloy to understand the behaviour of models. Then, we input olivine thermodynamic data via temperature-composition (T-x) phase diagrams for olivine composition and their elastic moduli to test the phase transition under different stress boundary conditions. We present our preliminary results here.