Carbon Dioxide Stays Solid Under Deep Mantle Conditions

Researchers showed that under the intense temperatures and pressures that exist close to the core-mantle boundary, carbon dioxide can exist as a covalently bonded, extended crystalline form called phase V. These findings run contrary to similar experiments, where carbon dioxide split into diamond and oxygen under deep mantle conditions.

On the surface, carbon dioxide moves in complex cycles between living organisms and the physical world. In the subsurface, carbon dioxide plays equally important roles, but tracking its movements through the crust and mantle is a much harder task. Theoretical models have predicted that carbon dioxide exists as a solid crystalline structure deep into the mantle, but high-temperature and high-pressure laboratory experiments have found that carbon dioxide dissociates to form oxygen and diamonds under these conditions.  

A new study from DCO Extreme Physics and Chemistry Community members Roberto Bini (Universita di Firenze, Italy), Kamil Dziubek and Demetrio Scelta (European Laboratory for Non-linear Spectroscopy, Italy), along with colleagues at the University of Vienna and the European Synchrotron Radiation Facility (ESRF), demonstrates that carbon dioxide can exist in a crystalline form under conditions simulating the core-mantle boundary. The researchers squeezed carbon dioxide up to 120 GPa (more than 1 million times greater than the pressure at sea level) and heated it to about 2400 degrees Celsius, which caused the carbon dioxide to rearrange into a tetrahedrally coordinated crystalline form called phase V (CO2-V). Their findings, published in a new paper in Nature Communications [1], suggest that carbon dioxide is more stable in the deep mantle than expected. 

“The results were amazing,” said Bini, who serves on the Extreme Physics and Chemistry Community Scientific Steering Committee. “Theoretical predictions claim that there is no dissociation into diamond and oxygen, and phase V is stable well above 100 GPa. That is exactly what we found.”

This is not the first time that Bini and colleagues have synthesized CO2-V, but it’s the first time they have observed the molecule at pressures and temperatures found in the lower deep mantle. “We already studied and synthesized this high pressure and temperature form some years ago, but what is really new and interesting is that we could synthesize this form above 100 GPa. So far the synthesis of phase V from pure carbon dioxide was possible only at 70 to 80 GPa,” said Bini, due to certain physical properties of the carbon dioxide that made it difficult to heat while under higher pressures.

Earth rasterized
While carbon dioxide exists as a gas at the temperatures and pressures at Earth’s surface, the molecule enters a crystalline structure called phase V under the pressure and temperature conditions of the deep lower mantle. Credit: Roberto Bini

Bini and his colleagues used a trick to heat the carbon dioxide to such high temperatures, by adding a pinch of magnesium carbonate. First, they compressed the carbon dioxide and magnesium carbonate in a diamond anvil cell, which is a high-pressure device that squeezes a material between two diamonds. At room temperature, the carbon dioxide changes into an unstructured form that is much like solid glass. Then the researchers heated the sample to more than 2400 degrees Celsius using a laser from the ID27 high-pressure beamline at the ESRF, set to a wavelength that magnesium carbonate specifically absorbs. The magnesium carbonate heats the carbon dioxide without causing any by-products. 

The researchers verified that the carbon dioxide had transformed into its crystalline CO2-V form through X-ray diffraction, a technique that uses the scattering of X-rays by the crystal to determine its structure. They also performed Raman spectroscopy, which involves shining a laser at a sample and detecting the diffused light to determine the molecular structure, which further confirmed the results. In this crystalline form, the carbon and oxygen atoms take on a tetrahedral shape and form a dense, interconnected network, with each carbon atom attached to four oxygen atoms.

In previous studies, researchers have observed carbon dioxide breaking down into diamond and oxygen under similar conditions, but that result may have been a side effect of the experimental setup, said Bini. He and his co-authors suggest that in these experiments, the metal ring used to heat up the sample may have catalyzed the carbon dioxide’s dissociation. Such reactions are difficult to avoid, and the current experiment also generated some side products in the sample near the gasket.  

Carbon dioxide enters the mantle through the subduction of one tectonic plate beneath another. If carbon dioxide survives as phase V all the way into the deep mantle, then it could impact the locations and conditions under which rocks melt to form plumes in the mantle and the formation of super-deep diamonds. Additional high-temperature and pressure experiments that include other mantle materials may help illuminate carbon dioxide’s behavior in the deepest parts of the mantle.

“Now we are in good shape from an experimental point of view to test exactly this issue,” said Bini. “It could be very interesting to see how the phase V form behaves with respect to silicate, carbonate, and other systems that we know are in the lower most mantle.”

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