In Earth’s Magma Ocean, Carbon Chemistry Got Complicated

Computer simulations of the magma ocean on early Earth suggest that the carbon chemistry was surprisingly complex at high pressure. Iron likely sequestered much of the carbon into the metallic core while diamonds may have formed from clusters of carbon.

In the first 100 million years after the formation of the solar system, another planetary body collided with the early Earth, blasting them both into a huge disk of hot gases and melted rock. Earth and the Moon condensed from this disk into two balls of molten magma. Though Earth has long since cooled, the chemistry of that early magma ocean left its mark on the planet. The carbon compounds that formed within the magma billions of years ago likely determined how much carbon ended up on the surface compared to the core, and what forms carbon assumes deep in the mantle. 

Early Eaarth
Schematic illustration showing the differentiation process of Early Earth. Credit: Natalia Solomatova.

In a new paper in Nature Communications [1], DCO researchers use a computer simulation method called ab initio molecular dynamics (MD) to predict how carbon atoms would interact with other components of the magma ocean. Modeling by Natalia Solomatova, Extreme Physics and Chemistry Community members Razvan Caracas (both at CNRS, École Normale Supérieure de Lyon, France), and Craig Manning (Chair of the DCO Executive Committee and the Extreme Physics and Chemistry Community; University of California Los Angeles, USA) showed that carbon has a tendency to bond with iron, which may have carried much of the carbon into the metal-rich core. Carbon atoms also formed complex clusters and chains, especially at higher pressures, which potentially seeded the growth of diamonds in the deep mantle. 

MD simulations offer a way to study the behavior of carbon at higher temperatures and pressures than researchers can recreate in the lab. The simulation starts out with a collection of atoms and then calculates the various ways they could move and interact over a set time period. The researchers used these simulations to model early Earth’s magma ocean.  

“Imagine that you take the entire mantle and the crust – everything that is inside Earth except the core. Put it together, and that’s the average composition of our simulations,” said Caracas. They also included different amounts of carbon, either as carbon monoxide (CO) or carbon dioxide (CO2). Then they ran the simulations to see how the carbon behaved under the intense temperature and pressure conditions that would have existed in the magma ocean. The researchers use the GENCI supercomputers to run the simulations because they are so complex that they would take 5 million hours to run on a desktop computer.

These were the first simulations to look at the behavior of carbon in the presence of iron under high-pressure magma ocean conditions and they revealed that carbon and iron readily form clusters. “Our study suggests that during these early stages, carbon would preferentially be dragged deep into Earth,” said Solomatova. “It’s reasonable to assume that there is a hidden reservoir of carbon in the deep interior.”

Deep Earth compounds
Simulations suggest that at the high pressures and temperatures that would have occurred when early Earth had a magma ocean, carbon forms complex compounds with iron, silicon, and oxygen. Credit: Natalia Solomatova

The researchers point out that MD simulations can’t be used to quantify how much carbon would have ended up in the core compared to the mantle, but they do illuminate some surprisingly complicated carbon chemistry. Carbon not only bonds to iron, but also replaces oxygen to bind directly to silicon. Although this finding surprised the researchers, it confirmed a DCO-supported study from 2013 that reported the same result when researchers mixed silicon, carbon, and oxygen in the lab at high temperatures, but under normal atmospheric pressure [2]. 

As the researchers added more carbon to the simulations, they saw that the atoms link up into chains. With enough carbon and under the right conditions, these polymers could grow into diamonds or metal carbides. However, depending on the abundance of oxygen, carbon might take the form of carbonate minerals. 

“The fact that we see this unique and colorful behavior of carbon warrants additional studies,” said Solomatova. She already has begun simulations that include water, to see how the presence of hydrogen affects the chemistry of carbon in the early magma ocean.

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