Geochemistry: Hydrogen and oxygen in the deep Earth.

Abstract

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 First-principles calculations and direct experiments are used to identify a highly stable, pyrite-structured iron oxide at pressures and temperatures relevant to Earth's deep lower mantle. Ho-Kwang Mao and colleagues show that the mineral goethite (FeOOH), ubiquitous in nature as 'rust' and found in large quantities as bog iron ore, decomposes under such 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. The authors conclude that the process provides an alternative interpretation for the origin of seismic and geochemical anomalies in the deep lower mantle, as well as a sporadic oxygen source for the Great Oxidation Event over 2 billion years ago that created the present-day oxygen-rich atmosphere