Acoustic Wave Velocities of Ferrous-Bearing MgSiO₃ Glass up to 158 GPa With Implications for Dense Silicate Melts at the Base of the Earth's Mantle

We conducted in situ high-pressure acoustic-wave velocity measurements of Fe²⁺-bearing MgSiO₃ glass up to 158 GPa by Brillouin scattering spectroscopy to clarify the effect of iron on the elasticity and structural evolution of silicate melts in the lower mantle. The change in trend of the V_S profile, likely induced by the structural transition of Si-O coordination number from 6 to 6+ proposed in previous studies of silicate glasses, was confirmed to be located at ∼106 GPa. Given the iron contents of partial melts derived from a pyrolitic or chondritic mantle, the transition pressure would be at around 84–97 GPa, which is well within the lowermost-mantle pressure regime. Our data show the substitution of 12 mol% Fe in MgSiO₃ glass decreases the V_S by ∼5.5%. This implies that iron affects the buoyancy relations between melts/crystals and the melts at the lowermost mantle will have the higher coordination number than 6.