Ultrasonic velocity and attenuation measurements on basalt melts to 1500°C: Role of composition and structure in the viscoelastic properties

Viscoelastic properties of basalt melts of a wide range of composition are determined from ultrasonic compressional wave velocity Vp and quality factor Qp measurements in the frequency range of 3.4–22 MHz and the temperature range of 1150°–1500°C. The basalt samples are grouped into two types: low magnesia (group A) containing 5–9 wt % MgO and high magnesia (group B) containing 18–23 wt % MgO. The Vp values of the melts determined at 1400°C are in the range 2.53–2.64 km/s for group A basalts and 2.79–2.86 km/s for group B basalts. The relaxed (low frequency) velocity Vr is a principally linear function of the melt density ρ. Least squares fits of the data give velocity‐density relations Vr (km/s) = 0.691 + 0.712ρ (g/cm3) for group A basalts and Vr = 0.328 + 0.907ρ for group B basalts at 1400°C. The mean molecular weights M are 65.0 ± 0.3 and 58.2 ± 0.5 g/mol for groups A and B basalts, respectively, suggesting that M is also a principal factor determining the melt velocity. The relaxed modulus Mr of the basalt melts range from 16.5 to 22.8 GPa and is also a linear function of the melt density. The values of longitudinal ηL and volume ηv viscosities are calculated from the measured Vp and Qp and the available shear viscosity ηs data. The viscosities and related activation energies decrease almost linearly as NBO/T (nonbridging oxygens per tetrahedrally coordinated cation) increases.