Convective Mixing in magma chambers
Final Report Abstract
We have focused on: (1) the conditions for the occurrence of different types of terrestrial magma oceans and their longevity, (2) the efficiency of convective mixing and degassing in magma oceans, chambers and pools, (3) the influence of a preserved heterogeneity resulting from magma ocean solidification on the thermochemical evolution of terrestrial planets. To study the solidification of liquid and mushy terrestrial magma oceans we modeled the cooling and the solidification of terrestrial magma oceans using either a 1D approach based on mixing length theory and parameterized Ra-Nu relationships, or a dynamic approach in a cylindrical geometry. We showed that the last stages of magma ocean evolution correspond to a mushy mantle occurring in two separate layers, whose lifetime and depth depend on the rheology. In addition, we showed that for a lifetime of the liquid magma ocean of one 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 investigated the implications of the deep mantle heterogeneity inherited from the solidification of a terrestrial magma ocean using parameterized convection and dynamic models in 2D Cartesian geometry. We showed that the presence of an enriched basal layer has a dramatic influence on the thermo-chemical evolution of a Mars-like planet, strongly delaying deep cooling, and significantly affecting nearly all present-day characteristics of the planet. In particular, the enrichment of the layer in iron and heatproducing elements generates large volumes of hot molten regions near the core-mantle boundary, which could be erroneously interpreted as core material due to their intrinsic low viscosity and seismic velocities in these molten regions. To improve the efficiency and the accuracy of the modeling of heterogeneous magma ocean dynamics, we developed a new numerical method for efficient and accurate modelling of convective mixing. We extended the Particle-In-Cell method by allowing the particle sensitivity kernel to account for the history of deformation in the vicinity of particles. This method, called DPIC, allows much more natural and uniform spatial sampling by particles with a reasonable additional calculation cost (~ 50% compared to the PIC approach) using the same number of particles. At comparable precision, this new approach reduces the computational cost by an order of magnitude. To study mixing and degassing efficiency in convecting magma oceans, we conducted a series of 2D numerical experiments of highly convecting, incompressible, magma oceans. We explored parameter space corresponding to Rayleigh numbers varying between Ra=108 −1013, and a constant Prandtl number Pr=1. We developed a scaling that allows predicting the amount of degassing at any given time and Rayleigh number value. We predicted that rather than being completely outgassed as previously thought, less than 50% of a global terrestrial magma ocean would be outgassed during the earliest stages of magma ocean evolution. This finding shatters the commonly accepted view of vigorously convecting magma ocean outgassing being quasi-instantaneous, and has important ramifications to the understanding of the origin of planetary atmospheres and habitability.
Publications
- On the Cooling of a Deep Terrestrial Magma Ocean, Earth Planet. Sci. Lett., 448, 140-149, 2016
J. Monteux, D. Andrault, H. Samuel
(See online at https://doi.org/10.1016/j.epsl.2016.05.010) - Onset of solid-state convection and mixing during magma ocean solidification, J.Geophys. Res. Planets, 2017
M. Maurice, N. Tosi, H. Samuel, A.-C. Plesa, C. Huettig, D. Breuer
(See online at https://doi.org/10.1002/2016JE005250) - A Deformable Particle-In-Cell Method for Advective Transport in Geodynamic Modelling, Geophys. Journal International, 214, 1744-1773, 2018
H. Samuel
(See online at https://doi.org/10.1093/gji/ggy231) - A mushy Earth’s mantle for more than 500 Myr after the magma ocean solidification, Geophys. J. International, 221, 1165-1181, 2020
J. Monteux, D. Andrault, M. Guitreau, H. Samuel, S. Demouchy
(See online at https://doi.org/10.1093/gji/ggaa064)
