Abstract
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–SiO2. 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 Fe2SiO4 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 FeSiO3 and the post-spinel part of Mg2SiO4–Fe2SiO4. We show that the existence of a phase field coesite + ringwoodite in the phase diagram of FeSiO3 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.
Highlights
Our work aims at developing a thermodynamic database for planetary materials, enhancing the interpretation of geophysical observations
In the present paper we focus on the thermodynamic data of endmembers in the system FeO–SiO2, with the ultimate goal to arrive at formulations for solid solution phases in the ternary system MgO–FeO–SiO2, resulting in better approximations of thermodynamic properties of mantle materials
Additional relations to the system MgO–SiO2 are available through experimental data on the exchange of iron and magnesium atoms between solid solution phases. This exchange is mainly determined by the Gibbs energy of the magnesium and iron endmembers of the solid solution phases in P–T space, and to a smaller extent by their mixing properties
Summary
Our work aims at developing a thermodynamic database for planetary materials, enhancing the interpretation of geophysical observations. Enthalpy and entropy values at 1100 K and 4.7 GPa are kept identical to the original database having 60 Einstein frequencies in the VDoS of each substance constrained our model for coesite by heat capacity data of Atake et al (2000).
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