P-wave velocities (V_P) and electrical conductivities (σ) in serpentinite, collected from Ailaoshan orogenic belt, Yunnan province, China, were measured at 2.0–3.5 GPa and high temperature with the pulse transmission–reflection combined method and impedance spectroscopy by the means of a multi-anvil pressure apparatus, the YJ-3000 press. V_P decreased and σ increased markedly at temperature higher than about 560°C. It is argued that the onset of the dehydration of serpentine is the main cause for V_P decreasing and σ increasing. Combining the present experimental data with the results of previous experiments, we were led to speculate that the formation of the low-velocity layer between the subducted slab and mantle wedge is closely related to the dehydration of serpentine.

Thermodynamic properties of MgAl₂O₄ spinel are significant in understanding the phase relations of the Earth’s crust and upper mantle, but available values from experimental measurements are limited at ambient pressure conditions to date. Based on an iterative numerical approach and experimental data from the literature, we determined the self-consistent unit-cell volume, elastic moduli, and, particularly, thermodynamic properties, including thermal expansion, heat capacity, entropy, and the Grüneisen parameter of MgAl₂O₄ spinel over a wide temperature and pressure range. The obtained thermal expansion, heat capacity, entropy, and Grüneisen parameter of MgAl₂O₄ spinel show nonlinearly and are negatively correlated with pressure. Most importantly, we found that the pressure effects on thermal expansion and entropy increase with temperature, whereas the pressure effect on the heat capacity and the Grüneisen parameter decreases to a minimum at ~400 K and ~700 K, respectively, then increases or remains almost constant above this temperature, respectively.

The self-consistent molar volumes, elastic properties and thermodynamic properties of β-(Mg_0.87,Fe_0.13)₂SiO₄wadsleyite have been determined over a wide temperature and pressure range based on an iterative numerical approach and experimental data from the literature. The obtained molar volumes, adiabatic bulk modulus, and shear modulus generally agree with the available experimental and theoretical results. The thermodynamic properties of α-(Mg_0.9,Fe_0.1)₂SiO₄were also reinvestigated. Comparisons on thermodynamic properties show that the thermal expansions, heat capacities, and entropies of both β-(Mg_0.87,Fe_0.13)₂SiO₄and α-(Mg_0.9,Fe_0.1)₂SiO₄exhibit negative and nonlinear behavior with increasing pressure. Besides, the pressure plays a more important role in thermodynamic properties of the α-(Mg_0.9,Fe_0.1)₂SiO₄than that of the β-(Mg_0.87,Fe_0.13)₂SiO₄. Using the Clausius-Clapeyron equation combined with molar volumes and entropies of both compositions, the phase transition boundary of Fe-bearing olivine and wadsleyite is estimated. The calculated transition pressure shows a nonlinear relationship with increasing temperature, and the value is generally lower than the previous studies on Fe-free olivine and wadsleyite at the same temperature. On the basis of our phase relation, the temperature at the 410 km discontinuity is estimated as 1795 ± 70 K under anhydrous conditions. The results not only confirm the feasibility of the Clausius-Clapeyron equation in obtaining the solid-solid phase transition, but also suggest that the existence of Fe might decrease the transition pressure of olivine-wadsleyite, which would consequently cause a higher temperature at the 410 km discontinuity.