“The Significance of Magma’s Oxidation in Shaping Earth’s Evolution”

In the context of Earth’s early history, the highly oxidized nature of our planet’s mantle raises intriguing questions about its potential similarity to Venus. Furthermore, the current state of the upper mantle seems to have been influenced by the introduction of metallic iron subsequent to Earth’s initial formation.

Understanding how the atmosphere and mantle oxidation state of Earth have evolved over time is a pivotal pursuit. The interplay between a planet’s interior and its surface holds the key to unraveling the mysteries of planetary formation. The distribution of ferrous (Fe2+) and ferric (Fe3+) iron within the mantles of rocky celestial bodies dictates the mantle’s oxidation state. This, in turn, has far-reaching consequences, affecting the composition of volcanic gases and the mantle’s ability to store vital elements like hydrogen and carbon, essential for life.

Therefore, gaining insights into the distribution of Fe2+ and Fe3+ in the mantle immediately following Earth’s formation provides critical information about the conditions on our planet’s surface prior to the emergence of life and the development of habitable environments.

Previous scientific investigations have already shed light on the fact that Earth’s early magma ocean was more enriched in Fe3+ than the current upper mantle. This suggests a highly oxidized environment during that era. This raises the question: How did the oxidation state of the upper mantle decrease to its present levels? To address this query, researchers explored the possibility of Fe3+ integration into the lower mantle during the crystallization phase of the magma ocean.

The results of these investigations have unveiled a significant revelation. It appears that the crystallization of bridgmanite, the predominant mineral in the lower mantle, did not preferentially incorporate Fe3+ when compared to coexisting magma. This implies that the upper mantle of the early Earth was also highly oxidized if the magma ocean possessed significant Fe3+ content. The resulting atmosphere, shaped by the outgassing of volatiles from such a highly oxidized mantle, would have been rich in carbon dioxide (CO2) and sulfur dioxide (SO2), thus resembling the conditions observed on Venus.

However, the process of magma ocean crystallization alone cannot account for the reduction in the upper mantle’s oxidation state to its current state. To bridge this gap, the researchers have proposed a fascinating hypothesis. They suggest that the reduction in the upper mantle’s oxidation state was facilitated by the introduction of metallic iron from late-accreting materials that occurred after Earth’s initial formation. Remarkably, the quantity of metallic iron brought in by these late-accreting materials, as constrained by the abundance of highly siderophile elements in Earth’s mantle, aligns with the reduction needed to attain the present oxidation state.

Nonetheless, this hypothesis demands further validation through additional geological constraints on the mantle’s oxidation state. The intricate relationship between Earth’s interior and its surface continues to be a subject of captivating research, providing valuable insights into our planet’s history and the conditions that shaped its evolution.

Reference: “Partitioning of Fe2+ and Fe3+ between bridgmanite and silicate melt: Implications for redox evolution of the Earth’s mantle” by Hideharu Kuwahara and Ryoichi Nakada, 25 May 2023, Earth and Planetary Science Letters. DOI: 10.1016/j.epsl.2023.118197

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More about Earth’s Mantle Oxidation

• Earth and Planetary Science Letters
• Kuwahara et al., 2023, Nat. Geosci.
• DOI: 10.1016/j.epsl.2023.118197