Onset of solid‐state mantle convection and mixing during magma ocean solidification

Abstract

The energy sources involved in the early stages of the formation of terrestrial bodies can induce partial or even complete melting of the mantle, leading to the emergence of magma oceans. The fractional crystallization of a magma ocean can cause the formation of a compositional layering that can play a fundamental role for the subsequent long‐term dynamics of the interior and for the evolution of geochemical reservoirs. In order to assess to what extent primordial compositional heterogeneities generated by magma ocean solidification can be preserved, we investigate the solidification of a whole‐mantle Martian magma ocean, and in particular the conditions that allow solid‐state convection to start mixing the mantle before solidification is completed. To this end, we performed 2‐D numerical simulations in a cylindrical geometry. We treat the liquid magma ocean in a parameterized way while we self‐consistently solve the conservation equations of thermochemical convection in the growing solid cumulates accounting for pressure‐, temperature‐, and, where it applies, melt‐dependent viscosity. By testing the effects of different cooling rates and convective vigor, we show that for a lifetime of the liquid magma ocean of 1 Myr or longer, the onset of solid‐state convection prior to complete mantle crystallization is likely and that a significant part of the compositional heterogeneities generated by fractionation can be erased by efficient mantle mixing. We discuss the consequences of our findings in relation to the formation and evolution of compositional reservoirs on Mars and on the other terrestrial bodies of the solar system.

Lunar Cumulate Mantle Overturn: A Model Constrained by Ilmenite Rheology

“…Some fraction of KREEP, including the heat‐producing elements (HPEs) K, U, and Th, would have been entrained during overturn and carried into the lunar interior along with the IBCs. Mantle overturn processes also occurred in different terrestrial planets, which have been a focus of research (e.g.,Boukare et al, ; Maurice et al, ; Scheinberg et al, ; Tosi et al, ).…”

“…Some fraction of KREEP, including the heat-producing elements (HPEs) K, U, and Th, would have been entrained during overturn and carried into the lunar interior along with the IBCs. Mantle overturn processes also occurred in different terrestrial planets, which have been a focus of research (e.g., Boukare et al, 2018;Maurice et al, 2017;Scheinberg et al, 2014;Tosi et al, 2013). ©2019 The wavelength of lunar cumulate mantle overturn may provide important clues to understanding the origin and asymmetric concentration of mare basalts on the near side of the Moon.…”

The Thermal State and Interior Structure of Mars

“…In addition, the difference in crustal thickness between the northern and southern Parameterized as well as 2-D and 3-D convection models (Breuer & Moore, 2015) of the thermal evolution of Mars have been used to explain, for example, the formation of the crustal thickness dichotomy, the formation of a super plume underneath Tharsis (e.g., Golabek et al, 2011;Keller & Tackley, 2009;Roberts & Zhong, 2006), and the magmatic and crust formation history (e.g., Breuer & Spohn, 2006;Fraeman & Korenaga, 2010;Hauck & Phillips, 2002;Morschhauser et al, 2011;Ruedas et al, 2013). Other mantle convection models studied the cooling and solidification of a putative liquid magma ocean (e.g., Elkins-Tanton et al, 2005;Maurice et al, 2017;Tosi, Plesa, et al, 2013) and the effects of large-scale impacts on the interior dynamics (e.g., Roberts & Arkani-Hamed, 2017;Ruedas & Breuer, 2017). In this study we compare the results of the largest set of numerical simulations to date of the thermal evolution of Mars in 3-D spherical geometry with available observations in order to identify key parameters that control the interior evolution.…”

Linking the evolution of terrestrial interiors and an early outgassed atmosphere to astrophysical observations

“…When the surface reaches the RT, the interior dynamics are switching to a viscous flow regime according to the local Reynolds number and melt-solid separation can become an important energy transfer mechanism. The transition from inviscid to viscous convection is validated by recent studies that demonstrate that solid-state convection begins during magma ocean crystallisation (Ballmer et al 2017;Maurice et al 2017). We thus capture the first-order nature of the transition that is important for bulk heat transport, but the details of how a lid forms and evolves at the surface is complex, notably depending on melt migration in the near-surface environment.…”