High-temperature and high-pressure Raman spectra of Fo89Fa11 and Fo58Fa42 olivines: Iron effect on thermodynamic properties

Abstract

The intrinsic anharmonicity plays an important role in the thermodynamic properties of minerals at the high-temperature conditions of the mantle. To investigate the effect of iron on the thermodynamic properties of olivine, the most abundant mineral in the upper mantle, we measured in situ high-temperature and high-pressure Raman spectra of natural Fo 89 Fa 11 and synthetic Fo 58 Fa 42 samples. Fo 58 Fa 42 dissociates to enstatite + quartz + Fe 2 O 3 (+ Fe) at 893 K. All the Raman-active modes systematically shift to lower frequencies at elevated temperature, whereas to higher frequencies with increasing pressure. The A g mode at approximately 960 cm-1 is more sensitive to the variations of temperature and pressure than the other internal modes. The crystal-field splitting of the vibrational energy states gets slightly weakened at high temperature, but strengthened at elevated pressure. We calculated the isobaric ( iP) and isothermal ( iT) mode Grüneisen parameters for these olivine samples. The intrinsic anharmonic parameters (a i) are in negative for both the lattice and internal vibrations, and our calculations indicate that the intrinsic anharmonicity makes positive contributions to the thermodynamic properties of olivine at high temperatures, such as the internal energy (U), heat capacities (C V and C P) and entropy (S). Iron incorporation further increases the magnitudes of these anharmonic contributions. In addition, the Fe effect on the intrinsic anharmonicity may also apply to other thermodynamic properties in olivine, such as equations of state and equilibrium isotopic fractionations, which are relevant and important to constrain many physical and chemical properties in the upper mantle. This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America. The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.