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Solid Earth An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/se-2020-49
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/se-2020-49
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  17 Apr 2020

17 Apr 2020

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This preprint is currently under review for the journal SE.

Timescales of chemical equilibrium between the convecting solid mantle and over-/underlying magma oceans

Daniela Paz Bolrão1, Maxim Dionys Ballmer2,1, Adrien Morison3, Antoine Billy Rozel1, Patrick Sanan1, Stéphane Labrosse3, and Paul James Tackley1 Daniela Paz Bolrão et al.
  • 1Institute of Geophysics, ETH Zurich, 8092 Zurich, Switzerland
  • 2Department of Earth Sciences, University College London, London, WC1E 6BT, UK
  • 3Université de Lyon, ENSL, UCBL, CNRS, LGL-TPE, 46 allée d'Italie, 69364 Lyon, France

Abstract. After accretion and formation, terrestrial planets go through at least one magma ocean episode. As the magma ocean crystallises, it creates the first layer of solid rocky mantle. Two different scenarios of magma ocean crystallisation involve that the solid mantle either (1) first appears at the core-mantle boundary and grows upwards, or (2) appears at mid-mantle depth and grows in both directions. Regardless of the magma ocean freezing scenario, the composition of the solid mantle and liquid reservoirs continuously change due to fractional crystallisation. This chemical fractionation has important implications for the long-term thermo-chemical evolution of the mantle, as well as its present-day dynamics and composition. In this work we use numerical models to study convection in a solid mantle bounded at either or both boundaries by magma ocean(s), and in particular, the related consequences for large-scale chemical fractionation. We use a parameterisation of fractional crystallisation of the magma ocean(s) and (re-)melting of solid material at the interface between these reservoirs. When these crystallisation/re-melting processes are taken into account, convection in the solid mantle occurs readily and is dominated by large wavelengths. Related material transfer across the mantle magma-ocean boundaries promotes chemical equilibrium, and prevents extreme enrichment of the last-stage magma ocean (as would otherwise occur due to pure fractional crystallisation). The timescale of equilibration depends on the convective vigour of mantle convection and on the efficiency of material transfer between the solid mantle and magma ocean(s). For Earth, this timescale is comparable to that of magma ocean crystallisation suggested in previous studies (Lebrun et al., 2013), which may explain why the Earth's mantle is rather homogeneous in composition, as supported by geophysical constraints.

Daniela Paz Bolrão et al.

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Daniela Paz Bolrão et al.

Daniela Paz Bolrão et al.

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Short summary
We use numerical models to investigate the thermo-chemical evolution of a solid mantle during magma-ocean stage. When applied to the Earth, our study shows that the solid mantle and a magma ocean tend to chemical equilibration before crystallisation of this magma ocean. Hence, our findings suggest that a very strong chemical stratification of the solid mantle is unlikely to occur (as predicted by previous studies), which may explain why the Earth’s mantle is rather homogeneous in composition.
We use numerical models to investigate the thermo-chemical evolution of a solid mantle during...
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