10:00 AM - 10:15 AM
[SIT26-05] In-situ IR high pressure experiment on hydrous forsterite
Keywords:FT-IR, DFT calculations, nominally anhydrous minerals, in-situ IR observation, upper mantle
FT-IR spectra are sensitive to the strength of OH covalent bond. A clue to reveal the positions can be obtained by using the FT-IR method. However, the experimental FT-IR spectra were obtained from quench polycrystalline recovered samples. Since hydrogen atom is very light, therefore, quenched recovered sample may not hold positions of original hydrogen atoms. In fact, Faust and Williams (1996) reported behavior of hydroxyl bonding of phase B at high pressure using in-situ IR spectra. Phase B, which is known as hydrous phase in the subduction zone, have the two bands of 3345 and 3404 cm-1 in IR spectra associated with the two hydroxyl sites. They reported the pressure dependence of the peaks. In their experiment, opposite peak shifts of the two bands of 3345 and 3404 cm-1 were observed and it is demonstrated that hydrogen position in phase B changes with pressure.
In this study, we performed in-situ IR observations at high pressures for forsterite (Fo) single crystals using DAC. At first I developed in-situ IR observation method at high pressure for low water content of nominally anhydrous minerals in the upper mantle. Using this technique, polarized IR spectra parallel to crystal axis of synthetic hydrous Fo were obtained under high pressure for the first time. First principle calculation (DFT calculation) for hydrogen position in Fo was carried out at various pressures. Hydrogen position in Fo was estimated based on the comparison of the vibrational frequencies measured by the FT-IR methods and simulation by the first-principles methods. The band shift observed in experiments under high pressure could be explained by the hydrogen position change in Si Site with pressure calculated by DFT.