Mechanism and kinetics of the post-spinel transformation in Mg₂SiO₄

Mechanisms and kinetics of the post-spinel transformation in Mg₂SiO₄ were examined at 22.7-28.2 GPa and 860-1200°C by in situ X-ray diffraction experiments using synchrotron radiation combined with microstructural observations of the recovered samples. The post-spinel phases nucleated on spinel grain boundaries and grew with a lamellar texture. Under large overpressure conditions, reaction rims were formed along spinel grain boundaries at the initial stage of the transformation, whereas under small overpressure conditions, the transformation proceeded without formation of reaction rims. Mg₂SiO₄ spinel metastably dissociated into MgSiO₃ ilmenite and periclase, and stishovite and periclase as intermediate steps in the transformation into the stable assemblage of MgSiO₃ perovskite and periclase. Topotactic relationships were found in the transformation from spinel into ilmenite and periclase. Kinetic parameters in the Avrami rate equation, time taken to 10% completion, and the growth rate were estimated by analysis of the kinetic data obtained by in situ X-ray observations. The empirical activation energy for 10% transformation decreases with increasing pressure because the activation energy for nucleation becomes smaller at larger overpressure conditions. Extrapolations of the 10% transformation to ∼700°C, which is the lowest temperature expected for the cold slabs at ∼700 km depth, suggest that overpressure of more than ∼1 GPa is needed for the transformation. Because the growth rate is estimated to be large even at low-temperatures of ∼700°C and overpressures of 1 GPa, the depth of the post-spinel transformation in the cold slabs is possibly controlled by nucleation kinetics.