Keywords:ab-initio molecular dynamics, basalt, viscosity reversal
Transport properties such as diffusivity and viscosity of melts determined the evolution of the early Earth’s magma oceans. They also govern many present-day processes of the Earth, from mantle convection to plate tectonics, magma migration and volcanic activity. Unfortunately, carrying out experimental measurements of melt diffusivity and viscosity at most mantle conditions are rendered impossible due to technical challenges. In this theoretical work we report the structure, density, diffusivity, electrical conductivity, and viscosity of a model basaltic (Ca11Mg7Al8Si22O74) melt. The calculations have been performed using first-principles molecular dynamics simulations at temperatures from 2200 K (0 to 80 GPa) to 3000 K (40-70 GPa) and 4000 K (120 GPa). There is good agreement between our results on the melt structure and viscosity at low pressure with that of experiments and previous theoretical predictions. A significant finding is that, although the density and coordination numbers around Si and Al increase with pressure, however, the Si-O and Al-O bonds become more weak and ionic. The temporal atomic interactions at high pressure are fluxional and fragile. Thus, making the atoms more mobile and reversing the trend in transport properties at pressures close to 50 GPa. The reversed melt viscosity under lower mantle conditions allows new constraints on the timescales of the early Earth’s magma oceans. It also provides the first tantalizing explanation for the horizontal deflections of superplumes at ~1000 km below Earth’s surface.