Earth’s oxygen-hydrogen cycles

Figure caption: a, A two-dimensional XRD image showing the newly developed sharp spots of FeO<sub>2</sub>. b, A microphotographic image of FeO<sub>2</sub> through diamond culets. c, Structural representation of the pyrite-type FeO<sub>2</sub>.
Figure caption: a, A two-dimensional XRD image showing the newly developed sharp spots of FeO2. b, A microphotographic image of FeO2 through diamond culets. c, Structural representation of the pyrite-type FeO2.

Iron and oxygen are two of the most geochemically important elements on Earth. The core is rich in iron and the atmosphere is rich in oxygen, and between them is the entire range of pressures and temperatures on the planet. Iron, in its multiple oxidation states, controls the oxygen fugacity and oxygen budget. Hydrogen has a key role in the reaction of Fe and O, causing iron to rust in humid air. Recent experiments at HPCAT identified a highly stable, pyrite-structured iron oxide (FeO2) at 76 GPa and 1,800 K that holds an excessive amount of oxygen. The “rust” mineral goethite, FeOOH, decomposes under the deep lower-mantle conditions to form FeO2 and release H2. The reaction could cause accumulation of the heavy FeO2-bearing patches in the deep lower mantle, upward migration of hydrogen, and separation of the oxygen and hydrogen cycles. This process provides an alternative interpretation for the origin of seismic and geochemical anomalies in the deep lower mantle. (Q. Hu, et al., Nature 534, 241-244 2016)