Earth’s Core-Mantle Boundary May Be Decorated in Marble and Diamonds

Scientists agree that during subduction, the edge of one tectonic plate sinks beneath another, moving surface carbon in the form of carbonates into Earth’s mantle, which accounts for more than 80% of Earth’s volume and represents a giant carbon reservoir. But whether those carbonates sink all the way through the mantle to the core-mantle boundary, or completely melt and break apart somewhere higher up, is still a controversial question. Answering this question would impact our understanding of what materials make up the mantle, and how much carbon it holds.

A new study published in the journal Earth and Planetary Science Letters [1] finds that carbonates (CO3) may reach the deep lower mantle in the form of high-pressure marble rich in calcium carbonate. DCO Extreme Physics and Chemistry Community members Susannah Dorfman (Ecole Polytechnique Federale de Lausanne, Switzerland, now at Michigan State University, USA), James Badro (Institut de Physique du Globe de Paris (IPGP), France and Ecole Polytechnique Federale de Lausanne, Switzerland), and colleagues simulated the transport of carbon to the lower mantle by combining dolomite, a sedimentary mineral composed of calcium magnesium carbonate (CaMg(CO3)2), with iron under high pressure and temperature. The researchers discovered that while the magnesium carbonate part of the mineral broke down to yield diamonds and other compounds, the calcium carbonate portion remained stable. The study suggests that slabs of subducted surface material may reach the core-mantle boundary bearing diamonds and marble.

Through the DCO and grants from the Swiss National Science Foundation, UnivEarthS Labex program at IPGP, and Michigan State University, Dorfman and her colleagues have been performing petrological experiments to estimate more accurately the amount of carbon that enters the deep mantle through subduction. “The uncertainty on that estimate has been overwhelmingly huge,” said Dorfman. ”If we want to know what role the deep Earth plays in the carbon cycle then we have to know whether carbon is even getting down there.”

To begin answering this question, the researchers simulated what would happen to a subducting slab as it reached the lower mantle. Dorfman and colleagues used a diamond anvil cell, which compresses a small sample between two diamonds to generate tremendous pressure, and heated the apparatus with a laser. Inside, two slices of dolomite, which makes up an important carbon reservoir in subducting slabs, sandwiched a layer of iron foil, which represents iron in the deep mantle and at the core-mantle boundary. The researchers heated the sandwich to between 1,500 and 2,200 degrees Celsius and 51 to 113 GPa pressure, which is between 0.5-1 million times the pressure at sea level and corresponds to most of the range of pressures that occur in the lower mantle.

Once the sandwich returned to surface pressure and temperature, the researchers used transmission electron microscopy and energy-dispersive X-ray spectroscopy to visualize and identify the reaction products. Their analysis revealed that the magnesium component of the dolomite reacted with iron to create diamond, iron carbide (Fe7C3), and magnesium iron oxide ((Mg,Fe)O). The calcium component however, persisted as a high-pressure form of marble, suggesting that it may travel all the way to Earth’s core-mantle boundary. “This is the endpoint of subduction,” said Dorfman. “If carbonates from the surface go all the way down to the base of the mantle, we can also think about how they interact with the core.”

Dorfman points out that it’s also possible that the calcium carbonate component will react with silicate minerals in the mantle, if they are nearby, which other DCO researchers have demonstrated previously. If this occurs, then all the carbonate in the slab would likely end up as diamond instead.

The amount of oxygen in the subsurface, which can be measured as oxygen fugacity, will also impact whether carbonates survive into the deep lower mantle. Scientists generally think that oxygen levels in the mantle decrease with depth. “Oxygen fugacity is a way of putting a number on the amount of oxygen, just like pH is a way of putting a number on the acidity of a system,” said Dorfman.

In the current study, the iron foil in the sandwich keeps the oxygen fugacity in the reaction low, to better simulate the deep lower mantle. The results suggest that calcium carbonate is less sensitive to oxygen fugacity than magnesium carbonate. In the mantle, as oxygen disappears, magnesium carbonate will become diamond first, while calcium carbonate converts to diamond only at lower oxygen levels.

Dorfman recently received a National Science Foundation CAREER grant to continue working on the question of whether carbonates reach the core-mantle boundary. She and her colleagues plan to complicate their system by adding silicate minerals to better simulate the mantle environment that surrounds a subducting slab. They also plan to investigate further how different oxygen fugacities affect the survival of carbonates in the mantle. These findings may help explain fluctuations in atmospheric oxygen over time, and how Earth maintains the current balance of oxygen that makes the planet habitable.

 

By the time a subducted plate reaches the core-mantle boundary, its carbonate minerals may have reacted with iron and converted into diamonds and a high-pressure form of marble. Credit: Susannah Dorfman
 
This energy-dispersive X-ray map shows the results of the reaction inside the dolomite-iron sandwich cooked at 66 GPa and 2000 °C. The elements are color coded as follows: red = iron, blue = calcium, green = magnesium, white = carbon. Mw represents magnesiowüstite ((Mg,Fe)O). Credit: Dorfman, et al. 

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