Patches Along Core-Mantle Boundary May Be Deep Hiding Place for Oxygen

Almost 3,000 kilometers below the surface, mysterious patches sit in between the rocky mantle and Earth’s iron-rich core. Scientists know of these blotches because they significantly slow passing seismic waves generated by earthquakes. Since the discovery of these so-called “ultralow-velocity zones” (ULVZs) in the mid-1990s, researchers have proposed several possible explanations, but no one has been able to fully account for these subsurface anomalies.

DCO members Jin Liu, Wendy Mao (both at Stanford University, USA), and Ho-Kwang (Dave) Mao (Carnegie Institution of Washington, USA) may have come up with an explanation for what’s inside ULVZs. They and their colleagues report their findings in a new paper in Nature [1]. Through high-pressure and high-temperature lab experiments, the researchers show that a new form of iron peroxide (FeO2Hx), created from water and iron under conditions found in the mantle, can slow sound waves, much like ULVZs in Earth. If correct, these findings suggest that ULVZs may help cycle water and other volatile materials through the subsurface, providing a deep hiding place for oxygen.

“Something strange is going on in these regions,” said Wendy Mao, who serves as a co-chair of the scientific steering committee for the Extreme Physics and Chemistry Community. “We can’t sample from the core-mantle boundary, so in the lab, people have been trying to mimic the conditions and look at different materials to see what might be consistent with low-velocity zones.”

Qingyang Hu (Center for High Pressure Science and Technology Advanced Research, China), a co-author of the study and former postdoctoral researcher in Wendy Mao’s lab, recently discovered that this iron peroxide compound likely occurs under mantle conditions when iron combines with the oxygen and hydrogen in water [2].

To discover more of the iron peroxide’s properties, the group investigated how it affects sound waves. They created the compound by placing iron foil and water into a pressure vessel called a diamond anvil cell, and compressed the contents to about 900,000 times the pressure at sea level. With a laser, the researchers heated the sample up to 2,200 degrees Celsius. Finally, they used an intense beam of x-rays generated by the synchrotron at Argonne National Laboratory in Illinois to measure how rapidly seismic waves could travel through the material. The investigators concluded that the iron peroxide slowed the waves in a way that is consistent with what occurs in ULVZs.

When the iron peroxide forms deep in the mantle, the iron likely comes from the core, but the source of the water is less clear. It may be primordial water, trapped in the deep mantle during Earth’s formation, or it could be surface water in the form of water-bearing minerals lowered into Earth by subduction, when the edge of one tectonic plate sinks beneath another into the mantle.

If the mysterious patches at the core-mantle boundary are made of iron peroxide, then they likely represent a giant underground stockpile of oxygen. One odd characteristic of the material is that, compared to iron oxides found at the surface, it is made up of a greater proportion of oxygen. The iron peroxide breaks down under intense heat, so when ULVZs heat up, they have the potential to release a burst of oxygen to the surface.  Such an incident may have contributed to the Great Oxygenation Event, when oxygen levels in the atmosphere skyrocketed about 2.4 billion years ago. 

“This discovery could tie in with a lot of other pieces of evidence suggesting that Earth’s deep material is interacting with the surface,” said Mao, noting that some studies have proposed that ULVZs are the source of hot mantle plumes that bring melted mantle rocks to the surface. This form of iron peroxide “could play a role in cycling those materials back from the bottom to the top.”

Next, the researchers plan to investigate how the addition of common elements, such as magnesium, or carbon-rich fluids, would affect the material’s properties. One big question is where the hydrogen from the water is ending up. It is difficult to measure hydrogen in this type of lab setup, but they think that some hydrogen becomes incorporated into the iron peroxide crystal structure, while some may be released.

“There are a lot of different threads to follow,” said Mao. “There are probably many other phases and interesting phenomena left to be discovered.”

 

In this artistic representation of the formation of ultralow-velocity zones, the blue water released from water-bearing minerals in the subducting slab reacts with the yellow, iron-rich outer core to form the orange hydrogen-bearing iron peroxide patches along Earth’s core-mantle boundary. Credit: Qingyang Hu

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