Water-mediated iron oxides in the Earth's lower mantle
Abstract
Knowledge of water in the Earth's interior, presumably at least several times the size of the world's oceans, is fundamental importance to understanding the dynamics, structure, and evolution of Earth. Meanwhile, oxygen and iron are Earth's most abundant elements by number of atoms and by mass, respectively. They form compounds dictating major chemistry of our planet. The generally accepted view believed that O2 dimer only existed on Earth's highly oxidized surface, and that oxygen anion existed in the invariable 2- valence state in minerals throughout the deep interior, where the redox states were controlled by the 3d transition element Fe which could vary between two valance states, ferric Fe3+ and ferrous Fe2+. The oxygen fugacity decreases with increasing depth as defined by a series of iron oxides with O/Fe stoichiometry from the end-member ferric iron oxide Fe2O3, through Fe5O7, Fe3O4, Fe4O5, Fe5O6 to the other end-member ferrous iron oxide FeO at the highly reducing core-mantle boundary. Interestingly, iron superoxides (FeO2Hx with x = 0 to 1) with an O/Fe ratio of 2.0 form in the presence of water (hydrous materials) under deep lower mantle conditions. Here we studied pressure-induced changes on O, Fe, and H and their interactions in FeO2Hx. We found many unexpected chemical behaviors. Iron remains in the reduced, spin-paired ferrous state in spite of its unprecedentedly high O/Fe ratio. The valence state of oxygen is not constant at 2- as commonly known in other oxides, but varying around 1-. It forms interactive dimer with an O-O bond length 30% longer than commonly considered as interactive dimer, but still shorter than non-interactive dimer. Hydrogen becomes weakly bonded in the structure, and its amount does not affect the valence of iron. Their altered behaviors in the middle Earth certain have major impacts to our planet, suggesting a broad chemical paradigm change in the middle Earth.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMDI33A..07L
- Keywords:
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- 1025 Composition of the mantle;
- GEOCHEMISTRYDE: 3621 Mantle processes;
- MINERALOGY AND PETROLOGYDE: 7208 Mantle;
- SEISMOLOGYDE: 8124 Earth's interior: composition and state;
- TECTONOPHYSICS