A Deep Dive Into the Carbon-Hydrogen Cycle

Earth feels immobile beneath our feet, but deeper down are swirls of solid rock. These slow-moving, vertical loops, called mantle convection, bring hot mantle material up to the surface to grow new seafloor, while dragging the edges of tectonic plates back down into the mantle. As part of this recycling process, mantle convection shuttles carbon dioxide and water from the surface into the mantle, which may encourage melting and help continue this underground circulation.

DCO Extreme Physics and Chemistry Community member Eglantine Boulard with Deep Life Community member François Guyot (both at Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, France), and colleagues, have discovered a new way that hydrogen, bound up in water, may move within the deep lower mantle. In the lab, the researchers reacted carbon dioxide with goethite, a common iron mineral (FeOOH), under high temperatures and pressures. Above 2,000 degrees Celsius, the mixture formed a tetrahedral-shaped carbonate compound (Fe4C3O12) and water. Their findings, published in a new paper in the journal National Science Review [1], suggest that deep carbon and hydrogen cycles may be more complex and more interconnected than previously thought.

During subduction, when the edges of tectonic plates sink into the mantle, they carry with them minerals rich in carbonates (carbon) and hydrates (water). Recently, several DCO researchers have published studies describing stable, high-pressure forms of carbonates and hydrates, suggesting that subduction may transport carbon and hydrogen (as water) deep into the mantle. Many researchers have studied the deep cycling of carbon and hydrogen separately, but Boulard and her colleagues think these cycles may be linked. “Here, the idea was to see if a hydrous phase and carbon-rich phase will interact with each other and to see if their respective structural changes under the extreme conditions of Earth’s mantle will have an effect on the recycling of elements,” said Boulard.

To test this idea, the researchers combined goethite crystals and carbon dioxide gas in a diamond anvil cell, a device that exerts intense pressures on a sample by squeezing it between two diamonds. They heated the cell using lasers and applied pressures up to 107 GPa, which is more than 1 million times the atmospheric pressure at sea level, to mimic the extreme conditions in the lower mantle. By using X-ray diffraction, which reveals the crystal structure of the compounds inside the diamond anvil cell, and Raman spectroscopy to identify those compounds, the researchers observed that as the temperature rose, the goethite (FeOOH) first converted into a high-pressure form, FeO2Hx. When the sample reached 2,000 degrees Celsius, reflecting temperatures in the lower mantle, the carbon dioxide reacted with the iron compound to form a compact, tetrahedral-shaped iron carbonate compound (Fe4C3O12) and water.

“It’s a new process for dehydrating materials in deep Earth, and it’s releasing hydrogen inside the mantle,” said Boulard. This release of water may trigger partial melting of mantle materials, since water lowers the melting temperatures of silicate minerals that make up the mantle.

The findings provide additional evidence for the idea that iron carbonates act as carriers, ferrying carbon deep into the lower mantle. “We always ask ourselves what kind of phase or species hosts carbon in the mantle,” said Boulard. “Of course, it can be reduced carbon, like diamonds, but in terms of oxidized carbon, the iron carbonates look more and more like really good candidates.”

In future experiments, Boulard plans to mix goethite with a carbonate compound, to see if carbonate behaves differently from carbon dioxide. Researchers still disagree over whether carbonates or carbon dioxide are more likely to exist under mantle conditions. Eventually, she plans to include silicate minerals in the reaction chamber, which would more closely simulate the silicate-rich mantle environment.

Boulard also hopes that going forward, researchers will continue studying the deep carbon cycle along with the deep hydrogen cycle. “Studies on multiple elemental cycles at the same time, are becoming increasingly important as we try to understand deep Earth,” said Boulard.

 

At conditions that occur at the core-mantle boundary, the mineral geothite (FeOOH) transforms into FeO2Hx (right side of Earth). In the presence of carbon dioxide it forms a tetrahedral-shaped carbonate (Fe4C3O12) and water (left side of Earth), which demonstrates a potential link between the deep carbon and deep hydrogen cycles. Credit: Eglantine Boulard

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