Elasticity and pressure-induced structural changes in vitreous MgSiO₃-enstatite to lower mantle pressures

Knowledge of the pressure-density relations and elastic properties of silicate melts at high pressure and temperature conditions is fundamental for a better understanding of the dynamics and properties of the Earth's interior. Here we report the sound velocities and elastic properties of vitreous MgSiO₃-enstatite, a simple analog for melts in the Earth's mantle, determined up to 33.4±1.2GPa in a diamond anvil cell by Brillouin scattering spectroscopy. The adiabatic bulk and shear moduli of MgSiO₃ glass at ambient conditions are K_S0=78.4(6) GPa and μ₀=41.1(3) GPa, corresponding to compressional and shear wave velocities of V_P=6.969(30) km/s and V_S=3.868(20) km/s. On compression both longitudinal V_P and transverse V_S modes exhibit anomalous change in slope at 8GPa and between 15.0(1) and 17.9(2) GPa, that are associated with polyamorphic phase transitions in the glass. The acoustic data were used to determine the density and compressibility of MgSiO₃ glass at high pressure in both compression and decompression experiments. In contrast to velocities and elastic moduli, the density increases smoothly without singularities upon compression over the investigated pressure range. Densities of recovered glasses at room pressure are 1.5 to 2.4% higher than uncompressed glasses, indicating pressure-induced irreversible densification. The anomalous changes in velocities and compressibility with pressure are consistent with structural modifications that have been identified as the main densification mechanisms of the glass by in situ spectroscopic studies and molecular dynamic calculations. The results suggest that the residual densification of recovered glasses is associated with irreversible structural changes in the intermediate-range order (i.e., ring statistics) rather than with changes in the coordination number of Si upon pressurization. If the changes in compressibility of MgSiO₃ glasses identified in the present Brillouin study also occurs in the analog melts, they may have important implications for the present-day dynamics of the mantle and early evolution of the Earth during a magma ocean phase.