Experimental constraints on the oxidation state of early magma on the Earth

The link between a planetary interior and its surface is a key to understanding the formation process of the surface environment of the planet. The distribution of ferrous (Fe²⁺) and ferric (Fe³⁺) iron in the mantle of rocky planets controls the oxidation state of the mantle and influences volcanic gas composition and the storage capacity of volatiles in the mantle, including life-essential elements, such as hydrogen and carbon. Thus, elucidating the distribution of Fe²⁺ and Fe³⁺ in the mantle just after this formation provides key insights into the surface environment before the rise of life and the origin of habitable planets. 

In a previous study, we showed that the Earth’s magma ocean was more enriched in Fe³⁺ than the present upper mantle, and therefore, highly oxidizing (Kuwahara et al., 2023, Nat. Geosci.). How was the upper mantle’s oxidation state reduced to the current state? To answer this question, we examined the possibility of the incorporation of Fe³⁺ into the lower mantle during the crystallization of the magma ocean.

The results show that crystallization of bridgmanite, the most dominant lower mantle mineral, does not preferentially incorporate Fe³⁺ compared to coexisting magma. This suggests that the early Earth’s upper mantle was also highly oxidized if the Earth’s magma ocean was rich in Fe³⁺. The atmosphere formed by the degassing of volatiles from such a highly oxidizing mantle would have been rich in CO₂ and SO₂, thereby forming a Venus-like surface environment. Because the magma ocean crystallization process cannot reduce the upper mantle’s oxidation state, the authors have proposed the reduction of the upper mantle by metallic iron contained in late-accreting materials after the formation of the Earth. Indeed, the amount of metallic iron delivered by late accreting materials constrained by the abundance of highly siderophile (iron-loving) elements in the Earth’s mantle is comparable to that required to reduce the upper mantle’s oxidation state to the present. Further geological constraints on the oxidation state of the mantle are necessary to test this hypothesis.