Low Viscosity of Peridotite Liquid: Implications for Magma Ocean Dynamics.

Transport properties of silicate melts control magma ocean dynamics on the early terrestrial planets and rocky exoplanets. Here we calculate the viscosity (transport of momentum) of peridotite liquid at potential magma ocean conditions (0–159 GPa, 2,200–6,000 K) using ab initio molecular dynamics simulations. We find that, unlike MgSiO3 or basaltic melts, the viscosity of the highly depolymerized peridotite liquid (a) increases monotonically with pressure without an anomalous drop and (b) is lower than those of other melts over the entire mantle pressure range. Low viscosity would promote fractional crystallization in a less polymerized magma ocean and thus contribute to mantle heterogeneity from its earliest stage. Given the compositional dependence of magma ocean properties, emphasis on multicomponent bulk silicate Earth‐like composition, instead of simple end‐members, are rendered necessary, in order to better understand high‐energy planetary accretion processes and their aftermaths. Plain Language Summary: In the aftermath of impacts, widely envisaged scenarios taking place on early terrestrial planets, the silicate mantle (i.e., rocks) experiences extreme temperatures that lead to extensive melting (i.e., magma ocean) on those planets. According to our supercomputer simulations, the viscosity of a magma depends on its composition, which can be as fluid as water for a silicate Earth‐like composition. Low viscosity entails vigorous convection, promoting fractional crystallization in the magma ocean. In light of this early differentiation process, mantle heterogeneity observed in the present‐day Earth may be remnants of the solidified magma ocean that survived over geologic time. Key Points: The viscosity of peridotite liquid is calculated at probable magma ocean conditions up to 159 GPa and 6,000 KIsothermal viscosities of peridotite liquid increase monotonically with pressure and are constantly lower than those of MgSiO3 liquidLow viscosity points to turbulent mixing in a magma ocean and favors fractional crystallization during its solidification [ABSTRACT FROM AUTHOR]