Rusting Earth’s core may release diamonds at the edge of the mantle

(ORDO NEWS) — The iron-nickel alloy of the earth’s core contains a large amount of dissolved carbon. Scientists have identified a process that can lead to the release of this carbon into the mantle and the formation of diamonds from it.

The largest reservoir of carbon on Earth is not the biosphere at all, but the earth’s core . It consists of an iron-nickel melt, in which light elements are dissolved, which tend to pass from the rock to the composition of iron.

First of all, these are sulfur, silicon and carbon, as well as oxygen and hydrogen. It is estimated that the core contains about nine-tenths of the Earth’s carbon.

It was believed that carbon was present in the earth’s core from the very moment of its formation and was securely sealed in it.

The density of the metal melt of the core is twice that of the minerals of the Earth’s lower mantle, and mixing does not occur – as between water and wax that has solidified on top of it.

Scientists from Arizona State University in the United States (Arizona State University), led by Byeongkwan Ko (Byeongkwan Ko) set up an experiment in which they reproduced the interaction of materials at the boundary of the lower mantle and the core. They have discovered a process that can “extract” carbon from the core and move it into the mantle.

The researchers proceeded from the assumption that free water (in the form of a supercritical fluid ) may be present in the Earth’s lower mantle . During subduction , the oceanic crust, saturated with water, sinks into the mantle, and its fragments can reach the core boundary.

With temperatures in the lower mantle reaching nearly four thousand degrees Celsius, even the most stable hydrated minerals lose their ability to retain water in their composition. It diffuses through the rocks, “leaks” to the boundary of the core and comes into contact with iron.

The researchers simulated this “meeting” by placing water and iron carbide Fe 3 C (metallurgists know it as cementite, which is part of cast iron) in a diamond anvil and subjecting its contents to pressure and temperature corresponding to the boundary of the earth’s core.

They analyzed the resulting reaction products using X-ray diffraction at a synchrotron source at the Argonne National Laboratory, determining crystal structures and identifying chemical compounds by structures, like fingerprints.

Rusting Earths core may release diamonds at the edge of the mantle 2
An image of a grain of cementite in an aquatic environment inside a cell with diamond anvils. Arrows indicate “rusted” areas in which X-ray diffraction showed the presence of microscopic diamonds

It turned out that iron reacts with water, but the “rusting” of the earth’s core is different from the corrosion of metal structures on the earth’s surface. The chemistry of high pressures and temperatures can be very different from what we are used to.

On the surface, rust consists of iron oxides and hydroxides in a high oxidation state (+3). In the core, the oxygen and hydrogen of water are completely included in the composition of iron, turning it into FeO oxide with a lower oxidation state (+2), and FeH x hydrides.

Under normal conditions, oxides and hydrides are usually incompatible with each other: they react in the opposite direction, forming the original metal and water.

In the products of this reaction, carbon dissolves much less than in the iron itself, and it is forced out. During normal rusting, carbon in steel oxidizes and turns into carbonates, but at the core boundary, in the absence of oxygen, it is released in a free form. Huge pressure and temperature turn it into a diamond.

The authors of the study suggest that the mechanism of carbon extraction from the core into the mantle that they have identified can make a significant contribution to the carbon cycle in the Earth’s interior and explain its increased content in the mantle.

In addition, they hope that the accumulation of diamonds at the edge of the Earth’s core can be detected in seismic data.

The speed of sound in diamond, at 12 kilometers per second at atmospheric pressure, is more than twice the speed of sound in rocks, and diamond-enriched rocks can be seen on seismograms as areas of abnormally high seismic wave velocities.

At the core boundary, however, the difference in sound speeds may be smaller, since the speed of sound increases as the material is compressed, and rocks compress better than diamond. The speed of longitudinal waves in ordinary rocks of the lower mantle reaches 14 kilometers per second.

In addition, geologists have so far discovered only areas of reduced seismic wave velocity at the boundary of the core, and most of the diamonds found come from the upper and middle mantle.

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