A study of cerium in zircon minerals has allowed an assessment of the redox conditions that prevailed when Earth's earliest magmas formed. The results suggest that the mantle became oxidized sooner than had been thought. See Letter p.79
A prime goal of petrologists has been to assess the redox conditions of magmas produced throughout Earth's evolution, because magmas are known to affect various major phenomena — such as the composition of volcanic emanations, which are widely believed to affect the composition of the atmosphere1. But research efforts have been hampered by the lack of rocks from the Hadean eon, which encompasses nearly the first half-billion years of Earth's existence. On page 79 of this issue, Trail et al.2 report their analysis of the sole mineral survivors of the Hadean, zircon samples more than 4 billion years old. Their findings allowed them to determine the 'fugacity' of oxygen in Hadean magmatic melts, a quantity that acts as a measure of magmatic redox conditions. Unexpectedly, the zircons record oxygen fugacities identical to those in the present-day mantle, leading the authors to conclude that Hadean volcanic gases were as highly oxidized as those emitted today.
The continuous reshaping of the surface and deep interior of our dynamic planet has inevitably resulted in the loss of evidence of Earth's ancient composition. In this respect, Earth contrasts sharply with other planetary objects — particularly the Moon and Mars, which are largely unaffected by large-scale geodynamics. Thanks to the Apollo programme and to meteorites inferred to be of Martian origin, we have rock samples that provide direct testimonies of the Moon and Mars's very early past. But the same cannot be said of Earth. Nevertheless, it is generally assumed that all of these bodies were chemically reduced when they first formed and then underwent either a gradual or stepwise oxidation3.
One of the main arguments for a reduced start is that, during or soon after accretion, Earth must have entered a magma-ocean stage, when a sizeable part of the planet was molten because of the energy released by accretion processes3. A massive segregation process must then have occurred that separated metals from silicates, thus allowing the formation of the metallic core and the silicate-containing mantle. The formation of Earth's core is thought to have occurred 30 million to 60 million years after the beginning of the Solar System. The coexistence of silicate and metal liquids in the magma ocean would have made conditions severely reduced, with oxygen fugacity (fO2) 5 to 8 log units below the current value for the upper mantle (Fig. 1).
Assuming that such a reduced starting point is inescapable, how did Earth reach its modern oxidized state, and how fast did this happen? Persistent oxidizing conditions at Earth's surface started when the planet was around 2.3 billion to 2.4 billion years old, a phenomenon known as the Great Oxidation Event1 (GOE). Rock analyses4 suggest that the mantle's redox state has been similar to its current state from as early as 3.8 billion years ago. Trail et al.2 wanted to look even further back in time, so they developed a technique for studying Hadean zircons, which are the only solids dating from the first 500 million years of Earth's history that are known to have survived unchanged to the present day. By analysing the cerium content of the zircons and developing a method to calibrate their results, the authors were able to determine fO2 for Hadean magmas.
Their findings extend the mantle's oxidized realm to almost 4.4 billion years ago. Although somewhat tenuous, this is the first direct evidence of the redox state of the earliest Earth. If the zircons analysed by the authors are representative of the Hadean eon, this result shrinks the duration of the reduced era of Earth's mantle to less than 150 million years. It also increases the lag time between the oxidation of the mantle and the subsequent oxidation of the atmosphere (the GOE; Fig. 1), which might seem to make it difficult to establish a direct connection between the two events. However, redox changes upon the release of gas from magmas5 and changes in the pressure of volcanic degassing6 could explain why the atmospheric GOE happened so long after the mantle's oxidation. But what was the cause of the mantle's great oxidation event?
Planetary-scale geodynamics is often cited as a mechanism for bringing oxidized material into the deep, reduced mantle, and water is thought to be responsible for oxidation7. This concept is deeply entrenched among geologists, although the details of the redox mechanisms involved remain unknown. Although a matter of strong debate8,9, Earth's current budget of volatile compounds (including water) seems to have been reached no later than 100 million years after core formation, and so volatiles may have fuelled the mantle's great oxidation at about that time (Fig. 1). However, it is known that terrestrial magmas do not necessarily have a high oxidation state, even if volatiles are abundant10,11, and that the Moon's reduced lavas contain a substantial amount of volatiles12. Taken together, these facts argue against the existence of an unequivocal relationship between high fO2 and the abundance of volatiles.
But the accretion of volatiles into Earth's bulk is not the only possible mechanism for oxidation of the mantle. Experiments have shown13 that, at the pressure of the lower mantle, iron(II) oxide is converted into iron metal and iron(III) oxide — which means that large bodies such as Earth can self-oxidize their mantle, whereas smaller ones cannot (or do so to a lesser extent). In other words, a nominally dry mantle can be oxidized. This mechanism, however, would require that wholesale mantle convection and mixing occurred within a short period of time (less than 100 million years).
Alternatively, changes in the mantle's redox state could have occurred if the early magma ocean crystallized and degassed, because of the redox effect of degassing5 and the greater affinity of iron(III) for liquid relative to minerals14 (the iron(III) would have become concentrated in the residual liquid, thus increasing the oxidation state of the liquid). Such processes would probably have affected the entire mantle and would not have required large-scale convection.
Clearly, many parts of Earth's early evolution are still obscured by poor preservation of the geological record and by our limited knowledge of the mechanisms that drive magmatic redox patterns. Even so, the zircon record revealed by Trail et al.2 firmly anchors one of the first redox steps of the infant Earth. This extraordinary tale is certainly not finished, and future work will augment the existing data — perhaps the discovery of other zircon samples will allow fO2 to be determined for even more highly reducing conditions at earlier periods of Earth's history, thereby refining our knowledge about what fO2 represents. Also much needed are experimental or modelling studies aimed at understanding the mechanisms of redox processes in magma, and the extent to which redox state reflects either the origins of magmas or later processes that affect them.