Saed News: Researchers have discovered rust and diamonds at the boundary between the Earth's core and mantle.
According to the analytical news website Saed News, quoting Fars and the SciTechDaily website, the Earth’s core is the largest carbon reservoir on the planet, with 90% of carbon buried within it. Scientists have found that oceanic crust, which lies atop tectonic plates and subducts into the Earth, contains hydrous minerals and sometimes reaches the boundary between the core and the mantle.
At the core–mantle boundary, temperatures are at least twice that of lava—so high that water is released from these hydrous minerals. As a result, a chemical reaction comparable to rusting steel can occur near the core–mantle boundary.
Researchers from Arizona State University, aiming to study this boundary, conducted experiments at the Advanced Photon Source at Argonne National Laboratory in the U.S. They compressed and heated water, an iron–carbon alloy, to simulate the conditions at the core–mantle boundary and melt the alloy.
They discovered that water reacts with the metal to form iron oxides and iron hydroxides—just like rusting on Earth’s surface. They also observed that under these extreme conditions, carbon separates from the molten iron alloy and forms diamonds.
According to scientists, the temperature at the boundary between the silicate mantle and the metallic core, around 3,000 kilometers beneath Earth’s surface, reaches approximately 7,000°F (about 3,870°C). This is hot enough for most minerals to lose water bound in their atomic structures—and in fact, hot enough for some of those minerals to melt entirely.
Because carbon is an iron-loving element, it is expected that large amounts of carbon reside in the Earth’s core, whereas the mantle was previously believed to contain relatively little. However, scientists have now found that the amount of carbon in the mantle is far greater than previously thought.
They suggest that under the high pressures at the core–mantle boundary, hydrogen alloyed with molten iron reduces the solubility of other light elements in the core. Therefore, when hydrogen enters the core from the mantle via dehydration, it locally reduces the solubility of carbon—causing the carbon, likely present in the core, to separate.
Diamond is the stable form of carbon under the extreme pressure and temperature at the core–mantle boundary. So, when carbon escapes from the Earth’s outer (liquid) core and enters the mantle, it transforms into diamond.
Carbon is an essential element for life and plays a major role in many geological processes. This new discovery of a carbon transfer mechanism from the core to the mantle helps improve our understanding of the deep-Earth carbon cycle. Interestingly, the formation of diamonds at the core–mantle boundary likely continued for billions of years after subduction began on Earth.
The new study suggests that carbon leakage from the core to the mantle—via diamond formation—may be the reason for the unexpectedly high carbon content in Earth’s mantle. Researchers predict that diamond-rich structures exist at the core–mantle boundary and could potentially be detected through seismic studies.
Seismic waves are expected to travel very quickly through these diamond-rich structures because diamonds are extremely incompressible and have lower density compared to other materials at the core–mantle boundary.
The research team also plans to investigate how these reactions might alter the concentrations of other light elements such as silicon, sulfur, and oxygen in the core, and how such changes could affect the mineralogy of the deep mantle.
