Sound Velocities of Basaltic Glass at Earth's Deep-Mantle Pressures

Abstract

The elasticity of compressed silicate melts is essential to decipher seismic properties and dynamic evolution of deep mantle magmas. Here, we report the compressional (V_P) and shear (V_S) wave velocities of a basaltic glass (Na_0.036Ca_0.220Mg_0.493Fe_0.115Al_0.307Ti_0.012K_0.002Si_0.834O₃) up to 63 GPa at 300 K using Brillouin and impulsive stimulated light scattering spectroscopies coupled with diamond anvil cells. The V_P and V_S data from 2.4 to 62.8 GPa exhibit convex upward trend, but the pressure-dependent velocity slopes become flatter at around 38 ± 3 GPa. The V_P and V_S values increase from 6.86 ± 0.03 to 11.62 ± 0.20 km/s and from 3.70 ± 0.03 to 5.28 ± 0.07 km/s, respectively. These velocity behaviors are likely associated with the 4- to 6-fold silicon coordination number increase. Comparison with literature data on MgSiO₃ glass, silicate, and pyrolitic glasses at high pressure suggests that the addition of Al₂O₃ can enhance the V_S and V_P of the basaltic glass, whereas FeO and CaO substitutions decrease its velocities. Using our results as an analog for basaltic melts, which can incorporate significant amounts of Al₂O₃ and FeO, the countercationic effects of Al₂O₃ and FeO on velocities and density of the basaltic melt could make it less visible seismically at the mid-lower mantle depths of ∼1,500 km.