[SIT22-P10] Thermal equations of state of MgSiO4H2 phase H up to 63 GPa determined by in situ X-ray diffraction measurement in a multianvil apparatus
キーワード：Thermal equations of state、mantle、phase H、multianvil apparatus、water
Hydrous minerals deliver a certain amount of water into the Earth’s interior via subduction of oceanic plates. Following to the theoretical prediction on the thermodynamic stability of MgSiO4H2 phase H at above 35 GPa in 2013 (Tsuchiya 2013), some experimental studies have succeeded to identify the formation of this phase under high pressure and temperature conditions (Nishi et al. 2014; Ohtani et al. 2014; Walter et al. 2015). Phase H is now considered to be the key hydrous phase responsible for the water cycle in the whole mantle as the highest-pressure form of DHMS. However, the qualities of in-situ X-ray diffraction (XRD) profiles of phase H obtained in earlier studies were sometimes not enough to satisfy the precise determination of its cell parameters because phase H easily dehydrates at temperatures where spectroradiometry can be applied in laser-heated diamond anvil cell (DAC) experiments. Therefore, XRD peaks of phase H have been used basically only for its phase identification within very limited temperature ranges. Nishi et al. (2014) succeeded to obtain the cell parameters of phase H from multiple independent XRD peaks by using multianvil apparatus, which can accurately control the temperature of the sample. However, the data in limited pressure ranges of the previous study (32-42 GPa) were not applicable to construct the equation of state (EOS) of MgSiO4H2 phase H.
Here, we have determined the thermal equations of state of phase H by using in situ X-ray diffraction measurements in conjunction with a multi-anvil apparatus through its stability field from 34 to 63 GPa up to 1300 K. We succeeded to expand the experimental conditions up to 63 GPa by state-of-the-art high-pressure technology using sinter diamond anvils and Al2O3 pressure medium, which achieved higher pressures by ~20 % compared to those using the conventional MgO pressure medium in the similar design of the cell assembly used in Nishi et al. 2014. The data analysis based on the Mie-Grüneisen-Debye model using the third-order Burch-Murnaghan equations at the reference pressure of 35 GPa yielded Vref, =49.61±0.01 (Å3), Kref=344.6±4.1 (GPa), Kref’=3.05±0.32, θref=974±146 (K), γref =1.8±0.1, and q =1.79±0.55. The compressibility of phase H observed in this study is in good agreement with that derived by theoretical calculation in the pressure regions where hydrogen bond symmetrisation is predicted to occur.
Here, we have determined the thermal equations of state of phase H by using in situ X-ray diffraction measurements in conjunction with a multi-anvil apparatus through its stability field from 34 to 63 GPa up to 1300 K. We succeeded to expand the experimental conditions up to 63 GPa by state-of-the-art high-pressure technology using sinter diamond anvils and Al2O3 pressure medium, which achieved higher pressures by ~20 % compared to those using the conventional MgO pressure medium in the similar design of the cell assembly used in Nishi et al. 2014. The data analysis based on the Mie-Grüneisen-Debye model using the third-order Burch-Murnaghan equations at the reference pressure of 35 GPa yielded Vref, =49.61±0.01 (Å3), Kref=344.6±4.1 (GPa), Kref’=3.05±0.32, θref=974±146 (K), γref =1.8±0.1, and q =1.79±0.55. The compressibility of phase H observed in this study is in good agreement with that derived by theoretical calculation in the pressure regions where hydrogen bond symmetrisation is predicted to occur.