Dynamics of a terrestrial magma ocean under planetary rotation: A study in spherical geometry

During a later stage of the Earth's accretion, impacts of mars-sized bodies on the Earth caused one or more deep terrestrial magma oceans of global extent. During that time, the Earth rotated very fast with a rotation period between two and five hours. Accordingly, the effect of planetary rotation is not only of key importance for the chemical structure and the development of chemical heterogeneities, but also sets the stage for the initiation of plate tectonics on Earth. This study investigates the effect of planetary rotation on the dynamics of silicate crystals in spherical geometry during an early stage of magma ocean solidification. Our results in a rotating spherical shell reveal a substantial influence of planetary rotation on the early magma ocean crystallization, leading to crucial differences in solidification with respect to crystal accumulation depth and latitude. Without and with slow planetary rotation, crystals are kept in suspension at all latitudes. Differently, at moderate and high rotation rate clear dependencies of the crystal dynamics on latitude do arise. At the poles, all crystals settle at the bottom, while in the equatorial region they stay entrained in the bottom half. However, at mid-latitudes they are kept in suspension and are distributed over the entire depth of the magma ocean. At even higher rotation rate all crystals accumulate at the bottom of the spherical shell, suggesting that crystallization would start at the bottom. At lower latitudes crystals are repeatedly re-entrained by convective motions, hindering the development of a stable layering at low latitudes, whereas at higher latitudes a potentially stable gravitational layering does develop. Our numerical experiments reveal that planetary rotation is likely to induce an inhomogeneous solidification of a terrestrial magma ocean with respect to depth and latitude, depending on the rotational strength. This has important consequences on the solidification time and further favors major chemical segregation resulting in large-scale chemical heterogeneities developing during magma ocean crystallization. The origin and distribution of mantle heterogeneities like the LLSVPs could be an imprint of planetary rotation on the magma ocean crystallization during Earth's early evolution. (C) 2019 Elsevier B.V. All rights reserved.