Parameters of phase transitions in the mantle and its influence on mantle convection

Reconstruction of the mantle flows within the mantle is essential for understanding of the Earth evolution. A temperature and pressure increase in the mantle causes phase transitions and related density changes in its material. The transition boundary in the pressure–temperature phase diagram is determined by the curve of phase equilibrium. If the slope is nonzero, a phase transition in hot ascending and cold descending mantle flows occurs at different depths and, therefore, either enhances (gamma>0) or slows down convection (gamma< 0). Endothermic phase transition at a depth of 660 km in the olivine partially slows down mantle flows. The mantle material has a multicomponent composition. Therefore, phase transitions in the mantle are distributed over an interval of pressures and depths. In this interval, the concentration of one phase smoothly decreases and the concentration of the other increases. The widths of phase transition zones in the Earth’s mantle vary from 3 km for the endothermic transition in olivine at a depth of 660 km to 500 km for the exothermic transition in perovskite, and the high-to-low spin change in the atomic state of iron takes place at a depth of about 1500 km. We present results of calculations for 2D and spherical models, demonstrating the convection effect of phase transitions as a function of the transition zone width. Transitions of both types with different slopes of the phase curve and different intensities of mantle convection are examined. The mixing of material under conditions of partially layered convection is examined with the help of markers. We analyze 2D and 3S mantle flow models with strong viscosity variations and phase transition to investigate this joint effect. For 2-D models we employ the generalized Moresi method. The 3S models are calculated with the CITCOM code.