The equation of state and stability of topaz at high-pressure/high-temperature conditions have been investigated by in situ synchrotron X-ray diffraction (XRD) and Raman spectroscopy in this study. No phase transition occurs on topaz over the experimental pressure–temperature (P-T) range. The pressure–volume data were fitted by the third-order Birch–Murnaghan equation of state (EoS) with the zero-pressure unit–cell volume V₀ = 343.86 (9) ų, the zero-pressure bulk modulus K₀ = 172 (3) GPa, and its pressure derivative K’₀ = 1.3 (4), while the obtained K₀ = 155 (2) GPa when fixed K’₀ = 4. In the pressure range of 0–24.4 GPa, the vibration modes of in-plane bending OH-groups for topaz show non-linear changes with the increase in pressure, while the other vibration modes show linear changes. Moreover, the temperature–volume data were fitted by Fei’s thermal equation with the thermal expansion coefficient α₃₀₀ = 1.9 (1) × 10⁻⁵ K⁻¹ at 300 K. Finally, the P-T stability of topaz was studied by a synchrotron-based single-crystal XRD at simultaneously high P-T conditions up to ~10.9 GPa and 700 K, which shows that topaz may maintain a metastable state at depths above 370 km in the upper mantle along the coldest subducting slab geotherm. Thus, topaz may be a potential volatile-carrier in the cold subduction zone. It can carry hydrogen and fluorine elements into the deep upper mantle and further affect the geochemical behavior of the upper mantle.

The phase transitions of natural San Carlos Fe‐, Al‐ and Ca‐bearing orthopyroxene and two synthetic orthopyroxenes containing Al (up to 12 wt. % Al₂O₃) and Fe (up to 15 wt. % FeO) were investigated at high pressures and temperatures up to ∼30 GPa and 700 K using synchrotron‐based single‐crystal X‐ray diffraction. These orthopyroxenes undergo α‐opx → β‐opx → γ‐opx phase transitions with increasing pressure and temperature. We quantitatively described how the composition influences the pressures of the phase transitions. The results indicated that the variations in Mg, Fe, and Al contents do not significantly influence the pressure of the α‐opx → β‐opx transition (P_α‐β). For the β‐opx → γ‐opx phase transition, increasing Fe content decreases the P_β‐γ while increasing Al content increases the P_β‐γ. The incorporation of Ca increases the P_α‐β and decreases the P_β‐γ. The thermal equations of states of α‐opx and β‐opx for these three orthopyroxenes were determined using the unit‐cell volume collected at high pressures and temperatures, which have been used to establish a density model to interpret the cold stagnant slabs. Our experimental results have indicated that β‐opx and γ‐opx are possible metastable phases of natural orthopyroxenes within cold stagnant slabs in the mantle transition zone.

The thermoelasticity and stability of diaspore (α-AlOOH, Al_1.002Fe_0.003OOH) were investigated in this study by in situ synchronous X-ray diffraction (XRD) and Raman spectroscopy methods at high pressure and high temperature conditions. The results indicate that diaspore is stable within the pressure and temperature (P-T) region examined in this study. With increasing pressure, the Raman peaks move toward the high wave number direction, the intensity of the Raman peaks increases, and the vibration mode of diaspore changes linearly. Pressure-volume data from in situ high-pressure XRD experiments were fitted by the third-order Birch-Murnaghan equation of state (EoS) with the zero-pressure unit-cell volume V₀ = 118.15 (4) ų, the zero-pressure bulk modulus K_V0 = 153 (2) GPa, and its pressure derivative K'_V0 = 2.4 (3). When K'_V0 was fixed at 4, the obtained K_V0 = 143 (1) GPa. The axial compressional behavior of diaspore was also fitted with a linearized third-order Birch-Murnaghan EoS, showing slight compression anisotropy with K_a0 = 137 (5) GPa, K_b0 = 169 (7) GPa and K_c0 = 178 (6) GPa. In addition, the temperature-volume data from in situ high-temperature XRD experiments were fitted by Fei’s thermal equation with the thermal expansion coefficients α_V = 2.7 (2) × 10^–5 K⁻¹, α_a = 1.13 (9) × 10^–5 K⁻¹, α_b = 0.77 (5) × 10^–5 K⁻¹, and α_c = 0.85 (9) × 10^–5 K⁻¹ for diaspore, which shows that diaspore exhibits slightly anisotropic thermal expansion. Furthermore, in situ synchrotron-based single-crystal XRD under simultaneously high P-T conditions indicates that the P-T stability of diaspore is up to ∼10.9 GPa and 700 K. Combined with previous results, we infer that diaspore can be subducted to ∼390 km under cold subduction conditions based on existing experimental data and is a good candidate for transporting water to the deep Earth.

Compressional behavior of a natural hypersthene has been investigated at pressures up to 10.1 GPa at room temperature using in situ synchrotron X-ray diffraction in a diamond anvil cell. No phase transition was observed within the experimental pressure range. The pressure–volume data have been fitted with a third-order Birch–Murnaghan equation of state, resulting in K₀ = 113.6(6.2) GPa and $K_0^{\mathrm{\prime }}=7.7\left(2.0\right)$ for the bulk modulus and its pressure derivative with a fixed unit-cell volume V₀ = 854.0(6) ų. The unit-cell parameters, a, b, and c, decrease nonlinearly with elevated pressure and show an anisotropic axial compressibility with a ratio β_a: β_b: β_c: =1:2.05:1.3. The comparison with previous results of orthopyroxene reveals that Al substitution causes a decrease in unit-cell volume and an increase in the bulk modulus, whereas the incorporation of Fe has opposite effects. The relation between unit-cell volume and En/Ts contents has been obtained, V₀ (ų) = 873.7(1.1) − 40.8(1.44) × En-90.6(14.7) × Ts, which permits the prediction of unit-cell volume for Fe- and Al-bearing orthopyroxene system.

The melt content is significant characteristic for the low velocity layer, so it is very necessary to set up the quantitative relationship between the low velocity anomaly and the melt fraction. We describe the computational methods for melt volume fractions and discussed the parameter selections for the theoretical computations. •We discuss the seismic wave velocity characteristics and the equilibrium geometry model in the partial melting system.•Equations for computing the elastic properties atop the LVL are presented.•Parameter selection of the equilibrium geometry model is shown.