Thermophysical properties and phase diagrams in the system MgO–SiO ₂ –FeO at upper mantle and transition zone conditions derived from a multiple-Einstein method

We applied a lattice vibrational technique, based on representing the vibrational density of states with multiple-Einstein frequencies, to determine consistency of data on thermophysical properties and phase diagrams in the system MgO–FeO–SiO ₂ . We present analyses of these data in the temperature range between 0 and 2000 K and pressure range between 0 and 20 GPa. The result is a database containing phases relevant to the Earth upper mantle and transition zone. We show that consistency of different datasets associated with the dissociation of the ringwoodite form of Fe ₂ SiO ₄ depends on the crucible material that has been used to perform partitioning experiments between ringwoodite and ferropericlase, and that this results in different phase diagrams for FeSiO ₃ and the post-spinel part of Mg ₂ SiO ₄ –Fe ₂ SiO ₄ . We show that the existence of a phase field coesite + ringwoodite in the phase diagram of FeSiO ₃ is possible and that it might be used to fine-tune pressure scales. We demonstrate that the phase boundary between coesite and quartz is very sensitive to the low-temperature heat capacity of coesite and that heat capacity data of β-quartz are too large to be reconciled with the phase boundary between β-quartz and coesite. We compare our results with seismic data associated with the 410 km seismic discontinuity.