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Convection in the primitive mantle in ineraction with global magma oceans

Abstract : A common scenario considered during the formation of Earth-like bodies is that of magma oceans. Indeed, the accretion energy as well as the heat produced by the radioactive decay of short-period elements is more than enough to melt entirely the primitive mantle, thereby forming a global magma ocean. The pressure-dependence of the solidification temperature as well as the steep isentropic temperature profile at the base of the mantle could lead to a crystallization of that global magma ocean from the middle. The primitive solid mantle could therefore be bounded by two global magma oceans: one above and one below.This PhD thesis focuses on two aspects of such a system. First, the solid part of the mantle and the magma oceans being of similar composition, convecting matter in the solid is not necessarily stopped by the solid/liquid interface but could instead go through it by melting/freezing provided that the phase change timescale is short enough compared to the viscous timescale needed to build a solid topography in the liquid oceans. A linear stability analysis and direct numerical simulations show the phase change at the boundary greatly affects convection in the solid part of the mantle. The critical Rayleigh number decreases, convective patterns have a larger wavelength, and the heat flux carried through the solid increases of up to several orders of magnitude compared to cases with classical boundary conditions.The second aspect explored in this thesis is the long-term evolution of the primitive mantle. Coupling convection in the solid with simple evolution models for the magma oceans allowed us to build a global evolution model of the primitive mantle monitoring the thermo-compositional evolution of the solid mantle and magma oceans. A linear stability analysis shows convection sets in the solid before the surface magma ocean crystallizes entirely. A preliminary direct numerical simulation shows the fractional crystallization of the basal magma ocean may lead to the formation of large thermo-chemical piles at the base of the solid mantle. These piles are similar to the large low-shear velocity provinces (LLSVP) observed today.The presence of global magma oceans could therefore have important consequences on the long-term evolution of the Earth: first, fractional crystallization of the magma oceans and convection in the solid part affect the resulting thermal and compositional structures; and second, the global heat budget could be tremendously affected by the high heat flux carried out by the solid part owing to the phase change boundary conditions.
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Adrien Morison. Convection in the primitive mantle in ineraction with global magma oceans. Earth Sciences. Université de Lyon, 2019. English. ⟨NNT : 2019LYSEN061⟩. ⟨tel-02482874v1⟩

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