After the formation of the Moon, its internal temperature was high, and its mantle was entirely molten. As this magma ocean cooled, it underwent fractional crystallization, starting with the formation of olivine and then continuing with the formation of pyroxenes and plagioclases. Finally, towards the end of solidification, ilmenite, a dense mineral rich in iron and titanium, crystallized along with plagioclases and pyroxenes. The accumulation of ilmenite and pyroxene would have formed a dense layer above the less dense cumulates formed earlier, resulting in a gravitational instability leading to a mantle overturn. This process has been chemically and dynamically modeled and could explain the asymmetric distribution of incompatible elements (KREEP) and the high TiO₂ content in some lunar basalts. This phenomenon likely had significant consequences on the distribution of chemical elements in the Moon, providing constraints on accretion and evolution scenarios. A recent study using thermodynamic, geophysical, and geodetic data showed that a dense layer is currently present at the base of the lunar mantle. It could correspond to ilmenite-rich cumulates like those formed during the lunar mantle overturn.
Such a dynamic, including a mantle overturn, has been proposed for Mars and Mercury, suggesting that this process could be common during the differentiation of magma oceans on terrestrial planets.
The ANR X-MAT (eXploring Mantle density Anomalies in Terrestrial planets) project aims to answer several questions: (1) What is the composition of the basal cumulate, and what is its impact on the geochemical evolution of terrestrial planets? (2) Is there a more complex geometry than a one-dimensional model (concentric layers), and can it be detected by modeling tidal deformation? (3) Geodynamically, how did the density anomalies form and get preserved in the mantle of terrestrial planets?
- Team: Clément Ganino, Agnès Fienga, Anthony Mémin, Nicolas Coltice and Alex Guinard
- Duration: 36 months 2025-2027
