Impact-Driven Redox Stratification of Earth's Mantle

Planetary formation involves highly energetic collisions, the consequences of which set the stage for the ensuing planetary evolution. During accretion, Earth's mantle was largely molten, a so-called magma ocean, and its oxidation state was determined by equilibration with metal-rich cores of infalling planetesimals through redox buffering reactions. We test two proposed mechanisms (metal layer and metal droplets) for equilibration in a magma ocean and the resulting oxidation state (Fe³⁺/ΣFe). Using scaling laws on convective mixing, we find that the metal layer could promote oxidation of a magma ocean, but this layer is too short-lived to reproduce present-day mantle Fe³⁺/ΣFe (2%–6%). Metal droplets produced by the fragmentation of impactor cores can also promote oxidation of a magma ocean. We use Monte Carlo sampling on two possible accretion scenarios to determine the likely range of oxidation states by metal droplets. We find that equilibration between silicate and metal droplets tends toward higher mantle Fe³⁺/ΣFe than presently observed. To achieve present-day mantle Fe³⁺/ΣFe and maintain the degree of equilibration suggested by Hf-W and U-Pb systematics (30%–70%), the last (Moon-forming) giant impact likely did not melt the entire mantle, therefore leaving the mantle stratified in terms of oxidation state after main accretion completes. Furthermore, late accretion impacts during the Hadean (4.5–4.0 Ga) could generate reduced domains in the shallow upper mantle, potentially sustaining surface environments conducive for prebiotic chemistry.