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Setting Earth's Initial Conditions – metal/silicates mixing, equilibration, and segregation in a magma ocean – SEIC

Submission summary

The primary objective of this research is to quantify how core-forming metals mixed with and separated from mantle-forming silicates and oxides in the early Earth. Like other terrestrial planets, the Earth’s basic radial structure of a dense iron-rich metallic core surrounded by a less dense silicate and oxide mantle formed during accretion, when the energy released by large impacts produced widespread melting. Core-mantle differentiation was fundamental in establishing the initial conditions for the later thermal and chemical evolution of the Earth. Although several important geochemical fingerprints of core formation have recently been identified, their full significance is somewhat obscured by our incomplete understanding of the physical mechanisms at work. In particular, the interpretation of the Hafnium-Tungsten chronometer in terms of a core formation timescale depends critically on the amount of metal- silicate chemical re-equilibration. This depends on the details of the segregation process: efficient chemical re-equilibration requires small scale mixing between the metal and silicate phases. It is now recognized that most of the mass of the Earth was accreted from already differentiated planetary bodies, and the question is therefore to know wether or not the metallic cores of these bodies (of diameter 10 to 1000 km) were able to fragment down to a scale (~1 cm) at which they can re-equilibrate chemically and thermally with the silicate melt. Metal-silicate segregation and mixing processes in magma oceans were characterized by huge Reynolds numbers, and also very large values of the Bond and Weber numbers, which compare the importance of buoyancy forces and inertia to surface tension between the silicates and metals. This implies that these processes were extremely turbulent, and that surface tension effects were important only at the smallest scales of the flow. Direct numerical simulations have been limited so far to laminar conditions and are therefore of limited relevance for the question of fragmentation in highly turbulent conditions.
We propose here a combined experimental and numerical study of metal-silicate turbulent mixing and fragmentation. The experiments will involve a pair of immiscible fluids, a low viscosity silicon oil and an aqueous salt (NaI, KSCN, NaCl) solution, a choice of materials which will allow us to reach fully turbulent conditions, with high values of the Reynolds, Weber, and Bond numbers. These pairs of fluids have density and viscosity ratios similar to that of the iron-silicate pair. The dynamical conditions will still be far from the geophysical configuration in terms of non-dimensional numbers, but it is significant that we will be able to reach fully turbulent conditions, in contrast with previous studies. The experimental study will be supplemented by 3D direct numerical simulations, using the code JADIM developed at the IMFT, which can handle multi-phase flow and includes an accurate formulation of the surface tension. The results of the experimental and numerical study will be used to derive and test scaling laws for the fragmentation timescale, the final drop distribution, and the re-equilibration timescale. We intend to extrapolate these laws to the regime of a magma ocean, thereby placing constraints on the initial state of the core-mantle system, including its degree of thermal and chemical equilibration. In parallel, geochemical models of core formation will be developed further, the ultimate goal being to include in geochemical models constraints from our physical understanding of mixing and equilibration during core formation.

Project coordination

Renaud DEGUEN (Institut de Mécanique des Fluides de Toulouse) – rdeguen@gmail.com

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

IMFT Institut de Mécanique des Fluides de Toulouse

Help of the ANR 245,956 euros
Beginning and duration of the scientific project: December 2012 - 36 Months

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