Placing constraints on phase equilibria and thermophysical properties in the system MgO-SiO2 by a thermodynamically consistent vibrational method |
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| Authors: | Michel HG Jacobs Bernard HWS de Jong |
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| Institution: | a Department of Theoretical Geophysics, Faculty of Geosciences, Utrecht University, Budapestlaan 4, NL-3584, CD Utrecht, The Netherlands
b Petrology Group, Faculty of Geosciences, Utrecht University, Budapestlaan 4, NL-3584 CD Utrecht, The Netherlands |
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| Abstract: | We use a lattice vibrational technique to derive thermophysical and thermochemical properties and phase equilibria in the system MgO-SiO2 at pressures and temperatures relevant to Earth’s mantle. The technique is based on an extension of Kieffer’s model to incorporate details of the phonon spectrum, and it includes treatment of intrinsic anharmonicity. We use a least squares inversion technique applied to available experimental data, and show that it results in an accurate representation of thermodynamic properties and sound wave velocities of high-pressure phases in the system MgSiO3. The vibrational method has been validated against laboratory experimental data in the temperature range between 0 and 2500 K and at pressures between 1 bar and 30 GPa. The technique results in a phase diagram consistent with the majority of thermophysical and thermochemical data. It is shown that intrinsic anharmonicity affects significantly slopes and positions of the phase boundaries. Our analysis indicates inconsistencies in a number of data sets of thermophysical properties for stishovite, majorite and ortho-enstatite necessitating new measurements. For akimotoite elasticity data at high-pressure and high-temperature conditions and 1 bar heat capacity measurements are needed. For stishovite elasticity measurements are necessary to reconcile elasticity data with V-P-T measurements. Additionally V-P-T measurements at pressures higher than 10 GPa are needed, which should be reconciled with V-P-T data at lower pressures. Raman and infrared spectroscopic data are necessary for both clino-enstatite and majorite. Additionally structural data are needed to resolve the discrepancy between values for the degree of disorder in majorite. Volume expansion data for ortho-enstatite are needed and effects causing differences in measured volume expansion should be elucidated. |
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