Diamonds and rust at the core-mantle boundary

Diamonds and rust at the core-mantle boundary

Iron-carbon alloys react with water in diamond-anvil cells under the high-pressure and high-temperature conditions associated with Earth’s deep mantle.Credit: Arizona State University

Steel rusts from water and air on the earth’s surface. But what about deep inside the Earth?

The Earth’s core is the largest carbon store on Earth – about 90% is buried there. Scientists have shown that the oceanic crust that sits on top of tectonic plates and falls into the interior by subduction contains water-bearing minerals that can sometimes descend all the way to the core-mantle boundary. The core-mantle boundary is at least twice as hot as the lava, and hot enough that water can be released from hydrous minerals. Therefore, chemical reactions similar to those of rusted steel may occur at the Earth’s core-mantle boundary.

Byeongkwan Ko, who recently received his Ph.D. from Arizona State University.graduate student, his collaborators publish their findings on the core-mantle boundary geophysical research lettersThey performed experiments at Argonne National Laboratory’s Advanced Photon Source, where they melted the iron-carbon alloy by compressing it together with water to the pressures and temperatures expected at the Earth’s core-mantle boundary.

The researchers found that water and metals react and produce iron oxides and hydroxides, much like rust on the surface of the earth. However, they found that under core-mantle boundary conditions, carbon emerges from the liquid iron metal alloy and forms diamond.

“At a depth of 3,000 km, the boundary temperature between the silicate mantle and the metallic core reaches about 7,000 degrees Fahrenheit, which is sufficient for most minerals to lose H2O is trapped in its atomic-scale structure, said Dan Shim, a professor in ASU’s School of Earth and Space Exploration. “In fact, the temperature is high enough that some minerals should melt under these conditions.”

Since carbon is an iron-loving element, a large amount of carbon is expected to be present in the core, while the mantle is thought to have relatively low carbon. However, scientists found that there is far more carbon in the mantle than expected.

“At the pressures expected at the Earth’s core-mantle boundary, the alloying of hydrogen with the iron-metal liquid appears to reduce the solubility of other light elements in the core,” Shim said. “Thus, the solubility of carbon that may be present in the core decreases locally where hydrogen enters the core from the mantle (by dehydration). Under pressure-temperature conditions at the core-mantle boundary, the stable form of carbon is diamond. So Carbon escaping from the liquid outer core becomes diamond when it enters the mantle.”

“Carbon is an essential element of life and plays an important role in many geological processes,” Ko said. “The new discovery of the carbon transfer mechanism from the core to the mantle will help to understand the carbon cycle deep inside the Earth. This is even more exciting considering that diamond formation at the core-mantle boundary may have occurred from Earth It’s been billions of years since the subduction started.”

Ko’s new research suggests that carbon leaking from the Earth’s core into the mantle through this diamond-forming process may provide enough carbon to explain the elevated levels of carbon in the mantle. Ko and his collaborators also predict that diamond-rich structures may exist at the core-mantle boundary, and that seismic studies may detect these structures because seismic waves should travel unusually fast for these structures.

“The reason seismic waves should travel unusually fast in diamond-rich structures at the core-mantle boundary is that diamond is extremely incompressible and less dense than other materials at the core-mantle boundary,” Shim said.

Ko and team will continue to study how this reaction changes the concentrations of other light elements in the core, such as silicon, sulfur and oxygen, and how these changes affect the mineralogy of the deep mantle.

Heavy iron isotopes leaking from Earth’s core

More information:
Byeongkwan Ko et al., Water-induced diamond formation at the Earth’s core-mantle boundary, geophysical research letters (2022). DOI: 10.1029/2022GL098271

Courtesy of Arizona State University

Citation: Diamonds and Rust at the Earth’s Core-Mantle Boundary (31 Aug 2022) Retrieved 1 Sep 2022 from -boundary.html retrieved

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