Stable Iron Carbonates Survive Journey to Earth’s Interior

Super-deep diamonds that formed more than 670 kilometers below Earth’s surface sometimes contain pieces of carbonates, compounds with a carbon atom attached to three oxygen atoms. Carbonate minerals are collectively the largest carbon reservoir in Earth’s crust owing to high oxygen levels in the atmosphere, but exactly how these compounds survive the trip into the mantle and become encased in diamonds is unknown.

In a new paper in the journal Nature Communications [1], Valerio Cerantola (European Synchrotron Radiation Facility, France), a member of the DCO Extreme Physics and Chemistry Community, and colleagues, report that carbonates may enter Earth’s deep subsurface in a highly stable compound that forms from iron carbonate. Cerantola worked with fellow DCO members Catherine McCammon (University of Bayreuth, Germany), Ilya Kupenko (now at University of Münster, Germany), Marco Merlini (University of Milano, Italy), Leyla Ismailova (Skolkovo Institute of Science and Technology, Russia), Alexandr Chumakov (European Synchrotron Radiation Facility, France), and Leonid Dubrovinsky (University of Bayreuth, Germany) to investigate the behavior of iron carbonate at the high pressure and temperature conditions present in the deep mantle. They showed that under these extreme conditions, carbonate molecules can reorganize so that the carbon carries an extra oxygen atom, forming a tetrahedral shape. The researchers detected two new compounds created at high temperature and pressure, with one “tetracarbonate” having the potential to survive travel deep into the lower mantle, where it may play a role in diamond formation.

“We were able to report for the first time the structure of the new high-pressure compounds, orthocarbonate and tetracarbonate, and show that the tetracarbonate is stable at temperature and pressure conditions of the deep lower mantle,” said Cerantola.

Carbonates formed in Earth’s oxygen-rich surface environment enter the subsurface through subduction, when continental crust descends into the mantle, a highly reduced environment where carbon is more likely to exist in diamond or methane than carbonate. 

Researchers had thought iron might play a role in stabilizing carbonates in the mantle [2], in part, because of a change in iron atoms at extreme conditions. Previous experiments also led scientists to the knowledge that a magnesium and iron carbonate mineral called ferromagnesite [3-4] forms high-pressure carbonates in the deep mantle, but the details were not clear.

In the current study, the researchers grew iron carbonate crystals that were too small to see with the naked eye, measuring just 10 to 15 microns across. They placed individual crystals into a diamond anvil cell, an apparatus that can exert great pressure by squeezing a sample between two diamonds. Using a laser, the researchers heated the crystal and squeezed it to simulate the wide range of pressures and temperatures in Earth’s mantle. Then they analyzed the contents of the high-pressure cells using advanced analytical techniques available through the European Synchrotron Radiation Facility and the Advanced Photon Source. Through single-crystal x-ray diffraction, which measures how the atoms in the crystal scatter the beam of x-rays, they determined the compound’s atomic structure. With synchrotron Mössbauer spectroscopy, they could measure tiny changes in the energy levels of iron atoms as they absorb gamma rays before and after the reactions took place, which is essential to determine the mechanisms of formation of the new high-pressure carbonates.

"It’s just like you have a car sitting there and immediately all the iron turns to rust.”

The researchers observed that under intense heat and pressure, the iron carbonate self-oxidizes, reducing some carbonate to diamond, and rearranges the remaining carbonate to form a new, more compact crystal structure with carbon tetrahedra.

“This is a profound oxidation of iron,” said McCammon. “In one of the structures, all of the iron goes from its reduced form to its oxidized form. It’s just like you have a car sitting there and immediately all the iron turns to rust.”

The results of the diamond anvil cell experiments indicate that one of the newly discovered crystal structures can exist at temperatures and pressures throughout the mantle, suggesting that it may carry carbonate down to the deep mantle where it can contribute to diamond formation. “This confirms that the deep carbon cycle can extend its roots deep inside our planet, transporting carbon from the surface all the way down to the core-mantle boundary,” said Cerantola, who has been deeply involved with the DCO Early Career Scientist Network.

In future work, the group plans to investigate the effects of magnesium in iron carbonate compounds, which are a more accurate representation of Earth’s mantle minerals. This work will be part of CarboPaT, a German Science Foundation initiative to investigate properties of carbonates in the deep Earth. McCammon wants to test if the presence of carbonates can be detected by analyzing the seismic waves that travel through Earth’s layers after earthquakes. These waves may offer clues that corroborate the existence of such stable deep carbonate compounds.

This image, taken through a microscope, shows the inside of a diamond anvil cell. At the center of the metallic gasket is a single crystal of iron carbonate and a ruby sphere for pressure calibration. Credit: Valerio Cerantola
The crystal structure of iron carbonate changes dramatically at high pressure and high temperature. The low-pressure form on the left contains carbon (black) and oxygen (red) atoms arranged in triangular planes between iron octahedra (orange), while in the high-pressure form on the right, carbon and oxygen form tetrahedra. Credit: Catherine McCammon 
Iron carbonate occurs on the Earth’s surface in the form of angular crystals. The mineral specimen is roughly 5 centimeters long.
Credit: RRUFF database, sample number 7584, from the University of Arizona Mineral Museum


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