Effect of Al2O3 on Sound Velocity of MgSiO3 Glass at High Pressure

Silicate glass has been used as an analog for silicate melts to understand the nature of dense magmas in the Earth's mantle. To understand the effect of Al2O3 on the sound velocity and structure of MgSiO3 glass, in this study, combined with Brillouin scattering and diamond anvil cells (DACs), the acoustic velocity of MgSiO3.5 mol%Al2O3 (MA1) and MgSiO3.24.5 mol%Al2O3 (MA2) glass were measured up to 20 and 42 GPa, respectively. Our studies show that the incorporation of Al2O3 could increase the sound velocity of MgSiO3 glass. Using the obtained velocities, the bulk and shear moduli (K-S, G), density (rho) and Poisson's ratio (nu) are calculated at high pressures, and the results indicate that Al2O3 could induce the stiffness of MgSiO3 glass. However, the effect of Al2O3 content on the stiffness of MgSiO3 glass is non-linear, and MA1 and MA2 exhibit similar K-S and G at high pressures. With the increase of pressure, the transverse acoustic mode (V-S) of MA1 and MA2 shows abnormal changes at 17.8 GPa and 31.8 GPa, which are related to the transition of coordination number (CN) for Si-O in Al-bearing MgSiO3 glass. Compared with previous studies on sound velocity of MgSiO3 glass, the incorporation of Al2O3 delays the transition pressure of Si-O coordination to a higher pressure. Our study has profound implications for understanding the density and sound velocity of Al-bearing MgSiO3 melt in the Earth's interior.