The Earth’s core is the largest carbon storage on Earth — roughly 90% is buried there. Scientists have shown that the oceanic crust that sits on top of tectonic plates and falls into the interior, through subduction, contains hydrous minerals and can sometimes descend all the way to the core-mantle boundary. The temperature at the core-mantle boundary is at least twice as hot as lava, and high enough that water can be released from the hydrous minerals. Therefore, a chemical reaction similar to rusting steel could occur at Earth’s core-mantle boundary.
Researchers from the Arizona State University conducted experiments where they 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. The researchers 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 ( Geophysical Research Letters).
They found 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. They also predicted 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, as per a release.
“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,” says Dan Shim from Arizona State University in the release.
