A new form of iron oxide controls hydrogen and oxygen cycles in the Earth interior - Dr. Ho-Kwang Mao

JUNE 9, 2016

We breathe oxygen every day, but we might not know the Earth interior keeps approximately one million times oxygen more than the atmosphere. A team of HPSTAR scientists led by Ho-Kwang "Dave" Mao has discovered a new form of iron oxide - FeO₂ that holds unprecedentedly large amount of oxygen, forming when the subducting plates carry the common “rust” (FeOOH) down into the deep mantle and controlling the flow of hydrogen and oxygen cycles in the Earth interior. Their findings are published in the June 9, 2016 issue of the journal Nature.

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 lays the entire range of pressures and temperatures on the planet.

“Interactions between oxygen and iron dictate Earth’s formation, differentiation—or the separation of the core and mantle—and the evolution of our atmosphere, so naturally we were curious to probe such reactions would change under the high-pressure conditions of the deep Earth,” said Ho-Kwang “Dave” Mao, the director of HPSTAR.

The research team — Qingyang Hu, Duck Young Kim, Wenge Yang, Liuxiang Yang, Yue Meng, Li Zhang, & Ho-Kwang Mao harness the laser-heated diamond anvil cell to mimic the extreme conditions in the Earth interior. They heat hematite (a-Fe2O3) in oxygen in a high-pressure diamond-anvil-cell chamber, getting a previously unknown, highly stable, pyrite-structured FeO2, called the P-phase at above 1,800 Kelvins (1,500 Celsius) and 750,000 times atmospheric pressure.

Their further study shows that the mineral goethite (FeOOH), exists ubiquitously as ordinary “rust” in the nature, decomposes under the deep lower mantle conditions (from 237,000 times atmospheric pressure (24 gigapascals) and ~1600 Kevin to 1.3 million times atmospheric pressure (136 gigapascals) and ~2500 K), also forming this new P-phase and releasing hydrogen.

FeOOH is found in iron ore deposits that exist in bogs, so it could easily move into the deep Earth at plate tectonic boundaries, as could samples of ferric oxide, Fe2O3, which along with water will also form the pyrite-like iron oxide under deep lower mantle conditions.

“This 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”, Mao said. “This process could have started in Earth’s infanthood and give us an alternative interpretation for the origin of seismic and geochemical anomalies in the deep lower mantle, as well as a sporadic O2 source for the Great Oxidation Event over two billion years ago that created the present oxygen-rich atmosphere”.

Caption: Hydrogen cycling in the lower mantle. Image courtesy of Qingyang Hu.

Adappted from Carnegie Institution for Science.