An investigation into thermodynamic consistency of data for the olivine, wadsleyite and ringwoodite form of (Mg,Fe)2SiO4

We have performed a thermodynamic analysis to determine the internal consistency of state-of-the-art experimental data for olivine, wadsleyite, and ringwoodite solid solutions. The analysis has been carried out between 200 and 2200 K and 0 and 20 GPa. We conclude that the volume-pressure-temperature data of wadsleyite and ringwoodite show very large inconsistencies, which hamper the accurate prediction of bulk sound velocities in the transition zone to within tomographic accuracy. Owing to the compositional effect in two-phase regions, thermodynamic properties, crucial for geodynamic modeling, differ up to an order of magnitude relative to values in single-phase regions at conditions prevailing in the earth’s transition zone. Thermal expansivity in the olivine-wadsleyite two-phase region is ∼23 times larger than that in the olivine region. For the wadsleyite-ringwoodite two-phase region, thermal expansivity is ∼6 times larger than that in the wadsleyite region. It may be anticipated that this differential thermal expansivity significantly affects future geodynamic model calculations.