Abstract
Hydrogen substitution has significant effect on the physical properties of olivine, the most abundant mineral in the upper mantle. We collected high-temperature polarized Fourier Transform infrared (FTIR) spectra on hydrous forsterite (Mg-pure olivine) crystals, which were synthesized at 12 GPa, 1473â1673 K. The modes at 3612, 3578, 3566, 3551 cmâ1 show comparable negative temperature dependence, and the magnitude of (âvi/âT)P decreases dramatically with frequency increasing. Whereas, the peak at 3477 cmâ1, which is attributed to protonation along the O1...O2 edge on the Si tetrahedron, has a positive temperature dependence. The absorbance intensities of all these OH bands remained almost the same when quenched to room temperature. On the other hand, we also evaluate the hydration effect on the thermodynamic properties (heat capacities). For the anhydrous forsterite sample, the intrinsic anharmonicity could significantly increase the heat capacity by 5~6% when extrapolated to 2000 K. Hydration further increase such difference to ~9%, in both the cases of M-substitution or Si-substitution. Hence, hydration in olivine has significant impact on the anharmonic contribution to the thermodynamic properties, as well as Equations of State and equilibrium isotope fractionation ÎČ-factor at high-P,T conditions in the deep mantle.
Highlights
Water is incorporated into many nominally anhydrous minerals (NAMs) in the form of hydrogen (H) point defects inside the crystal structures, which is fundamental to our understanding of water cycling in the deep Earth
There is no significant difference between the anhydrous and hydrous samples in the harmonic approximation
In the case of Si-substitution, the polarizations of the OH bands at 3612 and 3551 cmâ1 are in agreement with the orientations of O1âVSi and O2âH pointing away from Si, while the ones at 3566 and 3477 cmâ1 could be properly attributed to the O3...O3 and O1...O2 edges in SiO4 tetrahedron, considering their polarizations
Summary
Water is incorporated into many nominally anhydrous minerals (NAMs) in the form of hydrogen (H) point defects inside the crystal structures, which is fundamental to our understanding of water cycling in the deep Earth. The presence of water inside the crystal structure can dramatically affect the physical and chemical properties of olivine, such as thermal and electrical conductivity [8,9], deformantion [10], diffusivity [11,12,13,14,15]. Vacancies [28,29,31,36,37,38,39,40,41,42,43] Another hydration mechanism with F-substitution is proposed in both Fe-free and Fe-bearing olivine [44].
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