The finding that an unusual iron oxide forms at extremely high pressures suggests that hydrogen and oxygen — two elements that strongly influence Earth's evolution — are generated in the mantle. See Letter p.241
Hydrogen greatly affects the properties of many materials. It is thought that most of the hydrogen in modern Earth is in water molecules, many of which are found in water-bearing minerals. It is therefore crucial to understand the stability and circulation of such hydrous minerals in Earth's interior, and this need has led to numerous studies of hydrous minerals under high-pressure and high-temperature conditions. In this issue, Hu et al.1 (page 241) cast fresh light on the hydrogen-circulation issue. They report that an oxygen-rich iron oxide, FeO2, is stabilized at pressures greater than about 76 gigapascals, and that this material might enable previously unknown hydrogen and oxygen cycles to occur in Earth's mantle.
Earth's core is mainly made of metallic iron, whereas the major minerals in the upper mantle contain mostly ferrous iron (Fe2+). The most abundant form of iron on Earth's surface is haematite (Fe2O3), which contains ferric iron (Fe3+) and is the main constituent of iron ore. Most of this ferric iron is thought to have formed by the oxidation of ferrous or metallic iron by the modern, oxygen-rich atmosphere.
On the basis of the distribution of ferric, ferrous and metallic iron from the surface to the core, it is thought that Earth's redox state becomes increasingly reducing with depth, so that the amount of ferric iron in the lower mantle would be limited. High-pressure laboratory experiments2,3 revealed that, when olivine, (Mg,Fe2+)2SiO4 (the most abundant mineral in the upper mantle) is subjected to conditions corresponding to those of the lower mantle, it changes into a mixture of two other minerals, bridgmanite, (Mg,Fe2+)SiO3, and ferropericlase, (Mg,Fe2+)O. However, aluminium ions are also found in the mantle. When these are added, bridgmanite containing a large amount of ferric iron forms, together with ferropericlase and some metallic iron4. More than 60% of the total iron in bridgmanite can be ferric iron.
Hu et al. now add to this picture by investigating what happens when haematite is compressed in oxygen and heated to generate the pressure and temperature conditions that correspond to the deep lower mantle (78 GPa and 1,800 kelvin). The authors used X-ray diffraction to study the sample, but the diffraction patterns obtained were quite 'spotty' and the sample existed neither as a powder nor as a single crystal. In such cases, the crystal structures of materials cannot be determined in detail. The researchers therefore used a method called multigrain crystallography5 to analyse the spotty patterns, and concluded that the sample is an aggregate of at least 33 single crystals in which the haematite has changed into FeO2, an iron oxide that has the same atomic structure as pyrite (FeS2).
This finding might suggest that Fe4+ — normally a metastable form of iron — had formed under the extreme experimental conditions, and that its charge is balanced by two O2− ions. However, Hu et al. found that the oxygen–oxygen (O–O) bond in FeO2 is only 1.937 ångströms in length; by comparison, the ionic radius of O2− is 1.40 Å (ref. 6), corresponding to an O–O bond length of 2.8 Å or greater. The observed bond length does, however, agree with the typical O–O bond length for a peroxide ion (O22−). If the sample contains peroxide ions, then the iron must be ferrous, to balance the charge of those ions; in other words, the iron has been reduced from Fe3+ in haematite to Fe2+. Such a reaction is possible only at very high pressures, because the FeO2 has a smaller volume than a mixture of haematite and oxygen, and the smaller volume becomes energetically favourable under extreme high pressures.
Hu and colleagues went on to show that the mineral goethite, FeOOH, also forms FeO2 at 2,050 K and a pressure of 92 GPa by releasing hydrogen. Goethite commonly forms from the reaction of haematite and water at Earth's surface. The authors further demonstrated that the FeO2 formed in this way becomes unstable, and probably breaks down into ferrous oxide (FeO) and oxygen when the pressure is reduced.
These findings present new possibilities for how hydrogen and oxygen form and circulate inside Earth. When goethite (or a mixture of haematite and water) is carried deep into the lower mantle by subduction processes, then hydrogen and FeO2 are formed (Fig. 1). Hydrogen is extremely mobile and will spread upwards, eventually escaping into space, whereas heavy FeO2 will sink to the bottom of the lower mantle. But if the FeO2 is lifted to the upper part of the lower mantle by, for example, an upwelling plume of hot rock, it will become unstable and release oxygen gas on the way. This means that large amounts of hydrogen and oxygen might occasionally be produced in the lower mantle.
No such possibility has previously been considered. As the authors claim, this process could have acted as an additional or alternative oxygen source for the Great Oxidation Events — the periods in Earth's history when the atmosphere became oxygenated. Until now, it was thought that the oxygen was supplied by biological activity alone.
If hydrogen is released by the subduction of goethite, how will it behave at great depths? Unfortunately, hydrogen is invisible to X-rays and to electron microscopy, which makes its behaviour difficult to study at the atomic scale. Neutron diffraction is a powerful probe for directly observing hydrogen, and a facility7,8 that enables this technique to be used at high pressures and temperatures has successfully tracked the movement of hydrogen within materials9. Such techniques could be used to study the behaviour of hydrogen in Earth's interior.
And what is the fate of FeO2 when it sinks to the bottom of the lower mantle? Just like the finding10 that bridgmanite adopts an unexpected dense phase at pressures greater than 120 GPa, Hu and colleagues' work suggests explanations for the structural complexity of the region called the D′′ layer near the core–mantle boundary. Further studies are required to address this issue, and to work out how hydrogen and oxygen circulate in the deep Earth.
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Nature Communications (2018)