Molecular dynamics calculation of the bulk viscosity of liquid iron–nickel alloy and the mechanisms for the bulk attenuation of seismic waves in the Earth’s outer core

Abstract

Bulk viscosity is closely related to bulk dissipation of seismic waves. To understand the potential bulk dissipation in the Earth’s outer core, molecular dynamics simulations were conducted in the present study to calculate the bulk viscosity of liquid iron–nickel (Fe–Ni) alloy at the temperatures and pressures of the Earth’s outer core. Results from these simulations showed that bulk viscosities of liquid Fe–Ni alloy are in the order of 10 −3 Pa s. These values are in the same order of magnitude as those of pure liquid iron [Phys. Chem. Minerals 27 (2000) 164], indicating that nickel has a negligible effect on the bulk viscosity of liquid iron. Consequently, the large bulk viscosity of the outer core inferred from seismological studies should be accounted for through concentration fluctuations or other mechanisms with light elements such as sulfur or oxygen involved in [Nature 285 (1980) 204; Science 277 (1997) 219], or using the particle suspension model [J. geophys. Res. 88 (1983) 2445]. In addition, using the activation energy and volume calculated from these bulk viscosity data, it was found that the seismically induced phase change model for a partial melt system [J. Geophys. Res. 73 (1968) 7675] might not be capable to account for the bulk attenuation of the outer core.