Wednesday, October 5, 2022

‘Diamond Factory’ May Have Existed in Earth’s Interior for Billions of Years: Study

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Carbon plays a vital role in geological processes occurring in the Earth’s interior. While most carbon on our planet exists in its core, whether or not the core carbon can be added to the mantle is unclear due to the lack of knowledge of possible carbon transfer mechanism at Earth’s core-mantle boundary. In new research, a team of geoscientists led by Arizona State University and Michigan State University conducted experiments by reproducing the extreme pressure and temperature conditions of the core-mantle boundary. Their results show that water can react with the metallic iron core and liberate carbon as diamond, suggesting an important relationship between Earth’s water and carbon cycles. In addition, their results predict possible existence of diamond in some regions of the deepest mantle.

The iron-carbon alloy reacted with water at high pressure and high temperature conditions related to the Earth’s deep mantle in a diamond-anvil cell. Image credit: Ko et al., doi: 10.1029/2022GL098271.

“Carbon is an essential element for life and plays an important role in many geological processes,” said Dr. Byeongkwan Ko, a postdoctoral researcher at Michigan State University.

“The new discovery of a carbon transfer mechanism from the core to the mantle will shed light on the understanding of the carbon cycle in the Earth’s deep interior.”

“This is even more exciting given that the diamond formation at the core-mantle boundary might have been going on for billions of years since the initiation of subduction on the planet.”

In their experiments, Dr. Ko and colleagues compressed iron-carbon alloy and water together to the pressure and temperature expected at the Earth’s core-mantle boundary, melting the iron-carbon alloy.

They found that water and metal react and make iron oxides and iron hydroxides, just like what happens with rusting at Earth’s surface.

However, they found that for the conditions of the core-mantle boundary carbon comes out of the liquid iron-metal alloy and forms diamond.

“Temperature at the boundary between the silicate mantle and the metallic core at 3,000 km depth reaches to roughly 3,900 degrees Celsius (7,000 degrees Fahrenheit), which is sufficiently high for most minerals to lose water captured in their atomic scale structures,” said Professor Dan Shim, a researcher at Arizona State University.

“In fact, the temperature is high enough that some minerals should melt at such conditions.”

Because carbon is an iron loving element, significant carbon is expected to exist in the core, while the mantle is thought to have relatively low carbon. However, geoscientists have found that much more carbon exists in the mantle than expected.

“At the pressures expected for the Earth’s core-mantle boundary, hydrogen alloying with iron metal liquid appears to reduce solubility of other light elements in the core,” Professor Shim said.

“Therefore, solubility of carbon, which likely exists in the Earth’s core, decreases locally where hydrogen enters into the core from the mantle (through dehydration).”

“The stable form of carbon at the pressure-temperature conditions of Earth’s core-mantle boundary is diamond.”

“So the carbon escaping from the liquid outer core would become diamond when it enters into the mantle.”

The current study shows that carbon leaking from the core into the mantle by this diamond formation process may supply enough carbon to explain the elevated carbon amounts in the mantle.

The authors also predict that diamond rich structures can exist at the core-mantle boundary and that seismic studies might detect the structures because seismic waves should travel unusually fast for the structures.

“The reason that seismic waves should propagate exceptionally fast through diamond-rich structures at the core-mantle boundary is because diamond is extremely incompressible and less dense than other materials at the core-mantle boundary,” Professor Shim said.

The findings were published in the journal Geophysical Research Letters.

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Byeongkwan Ko et al. 2022. Water-Induced Diamond Formation at Earth’s Core-Mantle Boundary. Geophysical Research Letters 49 (16): e2022GL098271; doi: 10.1029/2022GL098271

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