Hadean mantle oxidation inferred from melting of peridotite under lower-mantle conditions

The early Earth's mantle is believed to have been highly reducing when it formed from a magma ocean equilibrating with the core. Conversely, some geological evidence suggests that the Hadean upper mantle was oxidized to a similar degree as or even more than that of today. Previous work on the high-pressure melting of andesite demonstrated the disproportionation of Fe2+ to Fe3+ plus Fe-0 in the melt, suggesting the magma ocean became more oxidized as Fe-0 was removed into the core. Here we present results of experiments melting peridotite at pressures to 28 GPa, that of the uppermost lower mantle. We show the Fe3+ content of Earth's magma ocean was an order of magnitude greater than that of the present upper mantle if the magma ocean reached the lower mantle, which quantitatively explains the geological evidence for a highly oxidized Hadean mantle. This Hadean great mantle oxidation should have ended with the cessation of the huge impacts that sustained a deep magma ocean. The subsequent reduction in Fe3+ content and oxidation state of the upper mantle may be attributed to the accretion of reduced materials by small impactors in late Hadean and early Archaean eons. The early Earth's mantle rapidly oxidized during the Hadean because of iron disproportionation and core segregation, according to experiments melting peridotite under deep-mantle conditions.

Automated summary

The formation of the early Earth's mantle is believed to have been highly reducing, but geological evidence suggests it was oxidized. Experiments melting peridotite under deep mantle conditions show that the Fe3+ content of Earth's magma ocean was significantly higher than that of the present upper mantle, explaining the evidence for a highly oxidized Hadean mantle. This oxidation likely ended with the cessation of huge impacts, and the reduction in Fe3+ content and oxidation state of the upper mantle could be attributed to accretion of reduced materials by small impactors during the late Hadean and early Archaean eons.