Microstructure and transport properties of pyrolite melt at mantle conditions

The microstructure and transport properties of silicate melts are key physical parameters for elucidation the geodynamic processes of earth evolution from the early magma ocean. Here, we systematically investigate the equation of state, microscopic structure and transport properties of pyrolite melt over 2000-3500 K and 0-60 GPa by molecular dynamics (MD) simulations. The calculated results reveal obviously pressure induced increases of the average coordination numbers of Si/Al-O, accompanied by the progressive replacement of SiO4 tetrahedra by higher-coordination polyhedra (SiO5 and SiO6). The fraction of non-bridging oxygen decreases, whereas the abundances of bridging oxygen and tricluster oxygen increase, indicating the pyrolite melt networks are gradually evolved to the denser and more highly polymerized configurations. The density-pressure relationships derived from the third-order Birch-Murnaghan equation are consistent with existing theoretical results. The viscosities of pyrolite melts at 2000-3500 K are ~10-3-10-1 Pa·s and increase monotonically with pressure along all isotherms. These findings provide important insights into the formation and evolution of the Earth and establish a framework for interpreting the geodynamic processes of other terrestrial planets.