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

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 100-300 MPa stress levels or as high as several GPa. Such stresses would affect the positions where mineral reactions occur and, hence, large-scale mantle structure. In this work, we 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, including temperature-related, elastic and interfacial free energies, interface properties, and initial microstructure, govern the evolution of the phase field. We applied this model to the Forsterite (Mg₂SiO₄)-Wadsleyite (Mg₂SiO₄) phase transition under different stress boundary conditions. We considered both isotropic and anisotropic boundary stress conditions. Under isotropic stress conditions, we plotted the Forsterite-Wadsleyite phase transition boundary based on our simulation results. The results indicate that local pressure variations, characterized by lower pressure within the Wadsleyite grain, hinder the occurrence of the phase transition. The depth offset would be ~30 km depressed due to this problem, which would be seismically detectable. Under anisotropic stress conditions, the Wadsleyite phase grows faster towards the maximum compression direction, leading to an elongated grain shape; however, the deviatoric stress does not shift the phase transition boundary significantly. At the same pressure, the deviatoric stress slightly slows down the Wadsleyite growth in volume.