Structural Evolution of Basaltic Melts in the Deep Earth: Insights From High-Pressure Sound Velocity of Glass

Abstract

The densification mechanisms of silicate melts under high pressure are of key interest in understanding the evolution of the early Earth and its present-day internal structure. Here, we report Brillouin spectroscopy-derived transverse acoustic wave velocities $\left(V_S\right)$ from a basaltic glass at high pressures up to 163 GPa and ambient temperature to provide insight into pressure-induced changes in its elasticity and, by extension, its density. We find that the pressure dependence of $V_S$ below 110–140 GPa follows a trend nearly tantamount to those of pyrolite and Fe- and (Fe,Al)-bearing MgSiO₃ glasses, indicating that the large compositional differences among these glasses do not exert variable acoustic wave velocity trends. However, at higher pressures we observe a small departure from the $V_S$ profiles of the Al-poor compositions toward higher acoustic wave velocities to eventually become stiffer. This pressure-induced steepening in $V_S$ is comparable to that of (Mg, Fe, Al)(Si, Al)O₃ glass, and suggests a possible structural change toward a denser state caused by more rapidly changing Al–O coordination in network-forming Al. Coupled with the high Fe content in basalt, this may render basaltic melt denser than surrounding minerals in the deep lower mantle, and may provide an additional mechanism for the existence of ultralow-velocity zones.