Modeling of olivine-spinel phase transformation of germanate olivine (Mg2GeO4) by using the phase-field method

Olivine is the main constituent of the upper mantle, and its phase transformation affects the rheology of the subduction zone. It is crucial to reveal the kinetics of the olivine (& alpha;)-spinel (& gamma;) phase transformation under differential stress. To investigate the effect of microstructural properties on phase transformations, such as grain boundary energy and plastic strain, we conducted a phase-field simulation using germanate olivine, an analog of silicate olivine. We conducted simulations under various confining pressures of 1-5 GPa, temperatures of 1000 and 1200 K, with and without plastic strain, and various grain boundary energies. The volume fraction of spinel decreases as the overpressure (confining pressure) increases because the effect of the elastic strain energy owing to the large overpressure on spinel growth is larger than those of the interface mobility and chemical free energy. The spinel growth has two types: "spinel needle" parallel to the maximum compression direction and "spinel platelet" perpendicular to the maximum compression direction. The spinel needles constantly grow owing to the interface mobility controlled by the grain boundary diffusivity, whereas the spinel platelets grow while the plastic strain caused by the phase transformation and deformation decreases the elastic strain energy. The spinel growth produces further plastic strain; thus, the spinel growth progresses by this positive feedback between the transformation and deformation.