Researchers Discover a “Diamond Factory” Deep Inside Earth



Steel rusts on the surface of the Earth as a result of water and air. But what about the Earth's innards deep within?

The Earth's core, where 90% of the carbon is stored, is the planet's largest carbon reservoir. Scientists have demonstrated that hydrous minerals can occasionally reach the boundary between the core and the mantle in the oceanic crust, which sits on top of tectonic plates and sinks into the interior. In order for water to escape from the hydrous minerals, the temperature at the core-mantle boundary must be at least two times higher than that of lava. As a result, a chemical reaction resembling rusting steel may take place close to the core-mantle barrier of the Earth.

In the journal Geophysical Research Letters, Byeongkwan Ko, a recent Ph.D. graduate from Arizona State University, and his colleagues recently published their research on the core-mantle boundary. At the Advanced Photon Source at the Argonne National Laboratory, they conducted experiments in which they compressed and heated water and an iron-carbon alloy to temperatures that were comparable to those at the Earth's core-mantle boundary, melting the iron-carbon alloy.

In a diamond-anvil cell, the iron-carbon alloy reacted with water under high pressure and high temperature circumstances similar to those in the deep mantle of the Earth. Arizona State University, source

Similar to how metal rusts on the surface of the Earth, scientists found that water and metal react to generate iron oxides and iron hydroxides. They did note, however, that diamonds are formed when carbon separates from the liquid iron-metal alloy at the core-mantle boundary conditions.

According to Dan Shim, a professor at ASU's School of Earth and Space Exploration, the temperature at the boundary between the silicate mantle and the metallic core at 3,000 km depth reaches around 7,000 F, which is high enough for most minerals to lose H2O trapped in their atomic-scale structures. In fact, the temperature is so great that under these circumstances some minerals should melt.

As an element that loves iron, carbon is predicted to be in considerable amounts in the core while being relatively insignificant in the mantle. Scientists have discovered that the mantle contains a lot more carbon than they had anticipated.

Shim stated that hydrogen alloying with iron metal liquid "appears to restrict the solubility of other light elements in the core at the pressures envisaged for the Earth's core-mantle boundary." "As a result, carbon's solubility, which is probably present in the Earth's core, declines locally where hydrogen enters the core from the mantle (through dehydration). Diamond is the stable form of carbon at the Earth's core-mantle boundary's pressure-temperature conditions. As a result, when carbon enters the mantle from the liquid outer core, it turns into diamond.

Carbon is a necessary component of life and is crucial to numerous geological processes, according to Ko. The understanding of the carbon cycle in the deep interior of the Earth will be improved by the recent finding of a carbon transfer pathway from the core to the mantle. Given that the diamond creation at the core-mantle interface may have been ongoing for billions of years since the planet's subduction began, this is even more interesting.

According to Ko's latest research, the diamond formation process, which causes carbon to escape from the core into the mantle, may provide enough carbon to account for the elevated levels of carbon in the mantle. Additionally, Ko and his associates hypothesized that diamond-rich structures would be present near the core-mantle border and that seismic studies might be able to find them since seismic waves from the structures should move extremely quickly.

Shim explained that because diamonds are incredibly incompressible and less dense than other materials at the core-mantle barrier, seismic waves should go through diamond-rich formations at the core-mantle border extraordinarily quickly.

Ko and the group will keep looking into how the reaction can alter the concentration of other light elements like silicon, sulfur, and oxygen in the core and how such changes can affect the deep mantle's mineralogy.

By ARIZONA STATE UNIVERSITY 

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