Deep Mantle Rust Could Have Been the Source of Early Earth’s Oxygen

June 10, 2016 | Joanne Kennell

Diagram of the decomposition of FeOOH
Photo credit: Courtesy of Ms. Xiaoya.

Iron is the most abundant element on Earth.

Iron and oxygen are two of the most abundant and chemically important elements on Earth. Why? The core is rich in iron while the atmosphere is oxygen rich. Since oxygen is a highly reactive gas that combines readily with iron, due to the huge range of temperatures and pressures between the atmosphere and the core, there are many different forms and combinations of each element.

A team of scientists from the Carnegie Institute for Science, led by Ho-Kwang “Dave” Mao, mimicked the conditions found deep inside the Earth and, in doing so, identified a form of iron oxide that could explain seismic and geothermal signatures found deep within Earth’s mantle — the 1,860-mile-thick layer of hot silicate rocks between the crust and the core.

"Interactions between oxygen and iron dictate Earth's formation, differentiation -- the separation of the core and mantle -- and the evolution of our atmosphere, so naturally we were curious to probe how such reactions would change under the high-pressure conditions of the deep Earth," said Mao in a Carnegie Science press release.

SEE ALSO: In a Surprise Discovery, Early Earth’s Atmosphere May Not Have Been Thicker Than Today’s

The team brought ordinary rust (FeOOH) to a temperature of 3,200 degrees Fahrenheit, and stressed it to 900,000 times normal atmospheric pressure. The result was a form iron oxide, FeO2, an element that resembles pyrite, also known as fool’s gold. This reaction also gave off hydrogen in the form of H2.

Rust, since it is found in iron ore deposits, could easily travel deep into the Earth due to plate tectonics. If water is also present, the two can combine to form the pyrite-like iron oxide.

What’s more, the H2 released during this reaction could work its way to the surface, reacting with other materials along the way. On the other hand, the pyrite would sink deep into Earth’s mantle, forming reservoirs of oxygen. However, if some iron oxide happened to move upward to the middle part of the mantle, it could separate into iron and O2.

"Pools of free oxygen under these conditions could create many reactions and chemical phases, which might be responsible for seismic and geochemical signatures of the deep Earth," Mao explained.

Interestingly, the team notes that their results could offer an alternative explanation for what’s known as the Great Oxygenation Event — when Earth’s atmosphere acquired a large amount of free oxygen between 2 and 2.5 billion years ago.

Previous hypotheses for the source of this oxygen include: bacteria undergoing photosynthesis, which releases oxygen as a byproduct; and the formation of the continents and movement of plate tectonics. However, the release of oxygen due to rising of FeO2 from the deep mantle could provide a new physical, rather than biological, explanation.

The study is published in Nature.

Read next: Ancient Space Dust Is Challenging Established Views on Earth’s Early Atmosphere

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