Diamonds and rust border the Earth’s mantle

The iron-carbon alloy reacted with water at the high pressure and high temperature conditions related to the deep Earth mantle in the diamond anvil cell. Credit: Arizona State University

Steel rusts with water and air on the surface of the earth. But what about the depths of the earth’s interior?

The Earth’s core is the largest carbon store on Earth – nearly 90% is buried there. Scientists have shown that oceanic crust that sits atop tectonic plates and falls inland, through subduction, contains hydrated minerals and can sometimes descend all the way to the core mantle boundary. The temperature at the core-mantle boundary is at least twice that of lava, high enough that water can be released from the hydrated minerals. Therefore, a chemical reaction similar to steel rusting can occur at the boundaries of the Earth’s mantle.

Byungkwan Koo, recently received a Ph.D. from Arizona State University. He and his collaborators produce their findings about the primary mantle boundary in Geophysical Research Letters. They conducted experiments at the Advanced Photon Source at Argonne National Laboratory, compressing iron-carbon alloys and water together to the expected pressure and temperature at Earth’s core boundary, melting the iron-carbon alloy.

The researchers found that water and metals react and make iron oxides and iron hydroxides, just as they do with rust on Earth’s surface. However, they found that for core-mantle boundary conditions, carbon exits from iron-metal alloys and forms diamonds.

“The temperature at the boundary between the silicate mantle and the mineral core at a depth of 3,000 km reaches nearly 7,000 F, which is high enough that most minerals H2“O has been captured in their atomic structures. In fact, the temperature is high enough that some metals melt under such conditions,” said Dan Shim, a professor in Arizona State University’s School of Earth and Space Exploration.

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

“Under the expected pressures of the Earth’s core boundary, mixtures of hydrogen with a liquid metallic iron appear to reduce the solubility of other light elements in the core,” Shim said. “Therefore, the solubility of carbon, potentially in Earth’s core, decreases locally as hydrogen enters the core from the mantle (through dehydration). The constant form of carbon at the pressure and temperature conditions of Earth’s core boundary is diamond. So the carbon escaping from the outer core is diamond. The liquid becomes diamond when it enters the mantle.”

“Carbon is essential to life and plays an important role in many geological processes,” Koo said. “The new discovery of the mechanism of carbon transfer from the core to the mantle will shed light on understanding the carbon cycle in the deep Earth. This is all the more exciting given that diamond formation at the core-mantle boundary may have been going on for billions of years since the start of subduction on the planet.”

Kue’s new study shows that carbon seeping from the core into the mantle through this diamond-forming process may provide enough carbon to explain the high amounts of carbon in the mantle. Koe and his collaborators also speculated that diamond-rich structures could exist at the core-mantle boundary and that seismic studies might detect the structures because seismic waves must travel at an unusual speed for the structures.

“The reason seismic waves must propagate at exceptional speed through diamond-rich structures at the core-mantle boundary is because diamond is very incompressible and less dense than other materials at the core-mantle boundary,” Shim said.

Ko and his team will continue to investigate how the reaction can also change the concentration of other light elements in the core, such as silicon, sulfur and oxygen, and how these changes might affect the mineralogy of the deep mantle.

Isotopes of heavy iron leaking from the Earth’s core

more information:
Byeongkwan Ko et al, Water-induced diamond formation at the Earth’s boundary-mantle mantle, Geophysical Research Letters (2022). doi: 10.1029/2022GL098271

Provided by Arizona State University

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