Fractional Crystallization of a Martian Magma Ocean and Formation of a Thermochemical Boundary Layer at the Base of the Mantle

The light plagioclase-enriched crust as well as the KREEP layer at the surface of the Moon are believed to be remnants of the bottom-up crystallization of a global Lunar Magma Ocean. &#160;In such a setup, the primitive Lunar solid mantle is coated by a liquid magma ocean of similar composition. We propose here to study the dynamic and evolution of the primitive Lunar solid mantle, accounting for the presence of the Lunar Magma Ocean.We solve numerically the equations of solid-state convection in the solid part of the mantle. &#160;This model is coupled to 1D models of crystallization of the magma oceans to self-consistently compute the thickening of the solid part as heat is evacuated from the mantle. &#160;We take into account fractional crystallization at the freezing front.Moreover, the boundaries between the solid and the magma oceans are phase-change interfaces. &#160;Convecting matter in the solid arriving near the boundary or getting away from it forms a topography which can be erased by melting or freezing. &#160;Hence, provided the melting and freezing occurs rapidly compared to the time needed to build the topographies by viscous forces, dynamical exchange of matter can occur between the solid mantle and the magma oceans. &#160;We take this effect into account in our model with a boundary condition applied to the solid.We find that the boundary condition allowing matter to cross the interfaces between the solid and the magma oceans greatly affects the convection patterns in the solid as well as its heat flux. &#160;Larger-scale convection patterns are selected compared to the classical case with non-penetrative boundary conditions; and the heat transfert in the solid is more efficient with these boundary conditions. &#160;This affects the long term thermal evolution of the mantle as well as the shape of chemical heterogeneities that can be built by fractional crystallization of magma oceans.

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The initial condition for the long-term evolution of terrestrial planets as determined by Reactive Freezing of the Basal Magma Ocean

10.5194/egusphere-egu2020-11800 ◽

2020 ◽

Author(s):

Maxim Ballmer ◽

Rob Spaargaren ◽

Ananya Mallik ◽

Daniela Bolrão ◽

Adrien Morison ◽

...

Keyword(s):

Fractional Crystallization ◽

Mantle Convection ◽

Terrestrial Planets ◽

Initial Condition ◽

Magma Ocean ◽

Molten Layer ◽

Compositional Evolution ◽

Mantle Boundary ◽

Reactive Crystallization

Terrestrial planets evolve through various stages of large-scale melting, or magma oceans, due to the energy release during accretion and differentiation. Any magma ocean is thought to become progressively enriched in FeO and incompatible elements upon freezing due to fractional crystallization. The resulting upwards enrichment of the related cumulate (=crystal) packages drives gravitational overturn(s) of the incipient mantle, and ultimately stabilizes a FeO-enriched molten layer at the core-mantle-boundary (CMB)1. Such a molten layer, previously termed basal magma ocean (BMO)2, is thought to also fractionally crystallize, but downwards instead of upwards, and over much longer timescales than the surficial magma ocean. This BMO fractional crystallization due to slow planetary cooling analogously implies the stabilization of a thick FeO-enriched layer at the CMB. Such a layer would essentially remain stable forever, as being too dense to be entrained by convection of the overlying mantle. However, at least for Earth, geophysical observations rule out the preservation of such a deep dense global layer. Here, we investigate the consequences of an alternative mechanism for BMO freezing, reactive crystallization, on the initial condition of solid-state mantle convection and long-term planetary evolution.Based on scaling relationships, we show that any cumulates, which crystallize from the BMO (e.g., due to initial cooling or reaction) are readily entrained by mantle convection. Once the BMO-mantle boundary is exposed, the BMO reacts with the mantle to form reactive cumulates. Reaction is driven by disequilibrium between mantle rocks and the BMO, a situation that is inevitable independent of BMO initial composition. As reactive cumulates are continuously entrained by mantle convection, the BMO continues to freeze by reactive crystallization. Based on lower-mantle mineral-melt phase equilibria3, we calculate the compositional evolution of the BMO, and the chemistry of the BMO cumulate package. We demonstrate that for a wide range of BMO initial compositions, the cumulate package consists of two discrete layers: the first is pure bridgmanite close to the MgSiO3 end-member; the second is mostly bridgmanite+ferropericlase that is moderately enriched in FeO and incompatibles, i.e. similar in composition to FeO-enriched pyrolite. The mass or thickness of the cumulate package depends on reaction kinetics, but is significantly larger than that of the BMO. The bridgmanitic layer is expected to be entrained by mantle convection due to its intrinsic buoyancy, but resist efficient mixing due to its intrinsic strength, thereby potentially providing an explanation for seismic scatterers/reflectors and ancient geochemical reservoirs4. The moderately FeO-enriched layer is expected to stabilize thermochemical piles, providing a candidate origin for the seismically-observed large low shear velocity provinces (LLSVPs)5.These results have implications for the long-term (thermal) evolution of planets in general. Earth-sized terrestrial (exo-)planets and super-Earths should also initially host a MgSiO3-rich layer as well as a moderately FeO-enriched layer. In contrast, small terrestrial planets such as Mars may host a more strongly Fe-rich deep dense global layer as long as no BMO is stabilized in their histories.[1] Ballmer+, G-cubed 2017; [2] Labrosse+, Nature 2007; [3] Boukar&#233;+, JGR Solid-Earth 2015; [4] Ballmer+, Nat.Geosci. 2017; [5] Ballmer+, G-cubed 2016.&#160;

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