Evolution of deep mantle reservoirs after the magma ocean: the influence of melt extraction

Evolution of deep mantle reservoirs after the magma ocean: the influence of melt extraction

Laura Lark, ChEd Boukaré, James Badro, Henri Samuel

 

Earth’s magma ocean stage and aftermath likely produced a reservoir of iron and trace element enriched silicate melt at the base of the mantle, termed a “basal magma ocean” (BMO) (Boukaré et al., 2025; Labrosse et al., 2007). As the BMO crystallized, its cumulates would likely be buoyant both because iron would behave somewhat incompatibly and because melt under extreme pressure is compressed to similar (or even higher) density than crystal of the same composition (Caracas et al., 2019). Consequentially, BMO crystallization would have been self-limiting, in that heat loss is necessary for crystallization to progress, but crystallization forms a layer of cumulates which insulate the BMO, reducing heat loss. Therefore, the evolution of the cumulates of the BMO interacting with convection in the overlying mantle is extremely important for the thermal evolution of the deep planet, with implications for BMO longevity and core dynamo generation.

 

We investigate the co-evolution of the BMO, its cumulates, and the overlying mantle with the fluid dynamics code Bambari (Boukaré, 2025) which incorporates melting, melt-crystal fractionation, and melt migration into a mantle convection model with coupled core (0-D heat reservoir). We are exploring the evolution of cumulates from the freezing BMO and how this affects BMO heat loss. For example, we vary the initial concentration of heat-producing elements in the BMO vs. solid mantle (γ) and observe that piles form preferentially in models with a more strongly heated basal magma ocean. At the base of piles, melting and drainage of iron-rich melts results in overall depletion of iron from piles. The lower density reinforces piling behavior, which strengthens melting and iron drainage (Figure 1). We are continuing to evaluate regimes of piling and implications for heat loss and interaction with the overlying mantle.

Figure 1. Snapshots of model mantle composition, cropped to show deep mantle only. Piles in convecting mantle overlie freezing basal magma ocean (white above melt fraction of 0.9). Model with more strongly heated BMO (higher ) shows more depleted upwellings within piles (yellow arrows).

 

References

Boukaré, C.-É., Badro, J., & Samuel, H. (2025). Solidification of Earth’s mantle led inevitably to a basal magma ocean. Nature, 640(8057), 114–119. https://doi.org/10.1038/s41586-025-08701-z

Caracas, R., Hirose, K., Nomura, R., & Ballmer, M. D. (2019). Melt–crystal density crossover in a deep magma ocean. Earth and Planetary Science Letters, 516, 202–211. https://doi.org/10.1016/j.epsl.2019.03.031

Labrosse, S., Hernlund, J. W., & Coltice, N. (2007). A crystallizing dense magma ocean at the base of the Earth’s mantle. Nature, 450(7171), 866–869. https://doi.org/10.1038/nature06355