Extreme heat and pressure may have shaped diamond production in the zone where Earth's core meets the mantle for billions of years. (Photo by Johan Swanepoel/Alamy Stock Photo)

Update on Giant blobs in the Earth’s mantle

Giant blobs in the Earth’s mantle may be powering a “diamond factory” near the planet’s core.

Extreme chemical reactions could explain why there is so much carbon in the Earth’s middle layer.

Extreme heat and pressure may have shaped diamond production in the zone where Earth's core meets the mantle for billions of years. (Photo by Johan Swanepoel/Alamy Stock Photo)
Extreme heat and pressure may have shaped diamond production in the zone where Earth’s core meets the mantle for billions of years. (Photo by Johan Swanepoel/Alamy Stock Photo)

The rocky middle layer between the Earth’s molten metal core and mantle could be a diamond factory.

A new laboratory experiment discovers that the combination of iron, carbon, and water — all potential ingredients found at the core-mantle boundary — can form diamonds under extreme temperatures and pressures. If this process occurs deep within the Earth, it could explain some oddities of the mantle, such as why it contains more carbon than scientists expect.

The findings could also help to explain strange structures deep in the core-mantle boundary where earthquake waves slow dramatically. These “ultra low velocity zones” are associated with strange mantle structures, such as two giant blobs under Africa and the Pacific Ocean; they can be a few miles across or hundreds of miles across. Nobody is sure what they are. Some scientists believe they are 4.5 billion years old and made of materials from the very early Earth. However, new research suggests that some of these zones may be the result of plate tectonics, which likely began long after Earth’s formation, possibly 3 billion years ago.

“We’re adding a new idea that these aren’t entirely old structures,” said study lead author Sang-Heon Shim, an Arizona State University geoscientist.

Simulating the depths of the Earth
Liquid iron collides with solid rock where the core meets the mantle. Shim told Live Science that the transition is as dramatic as the rock-to-air interface on Earth’s surface. Strange chemistry can occur during such a transition, especially at high pressures and temperatures.

Furthermore, studies that use earthquake wave reflections to image the mantle have revealed that materials from the crust may penetrate to the core-mantle boundary, which is 1,900 miles (3,000 kilometers) below Earth’s surface. Tectonic plates push under one another at subduction zones, driving oceanic crust into the subsurface. Water is locked in the minerals of the rocks in this oceanic crust. As a result, Shim believes that water could exist at the core-mantle boundary and drive chemical reactions. (One theory for the mantle blobs beneath Africa and the Pacific is that they are made of distorted oceanic crust that has been pushed deep into the mantle, possibly carrying water with it.)

Giant blobs in the Earth's mantle
Diamonds form in high-temperature, high-pressure environments such as those found at the core-mantle boundary.

To put the theory to the test, the researchers gathered the ingredients found at the core-mantle boundary and pressed them together with diamond anvils, generating pressures of up to 140 gigapascals. (This is approximately 1.4 million times the pressure at sea level.) The samples were also heated to 6,830 degrees Fahrenheit (3,776 degrees Celsius).

“We watched what kind of reaction happened when we heated the sample,” Shim explained. “Then we found a diamond and an unexpected element exchange between the rock and the liquid metal.”

Producing diamonds
Water behaves very differently under the pressure and temperature of the core-mantle boundary, according to Shim, than it does on Earth’s surface. Hydrogen molecules are separated from oxygen molecules. Because of the high pressure, hydrogen gravitates toward iron, which constitutes the majority of the core. As a result, the oxygen from water remains in the mantle, while the hydrogen combines with the core.

When this occurs, hydrogen appears to push other light elements in the core aside, including, crucially, carbon. This carbon is expelled from the core and into the mantle. Diamond is the most stable form of carbon at the high pressures found at the core-mantle boundary.

Shim explained, “That’s how diamonds form.”

These aren’t the same diamonds that might sparkle in an engagement ring; most diamonds that make their way to the surface and eventually become someone’s jewelry form a few hundred kilometers, not a few thousand kilometers, below the earth’s surface. The core-mantle diamonds, on the other hand, are likely buoyant and could be swept through the crust, distributing carbon as they go.

According to the proportion of elements in stars and other planets, the mantle contains three to five times more carbon than researchers would expect. Shim speculated that the diamonds discovered in this layer of the Earth could explain the discrepancy. He and his colleagues calculated that if just 10% to 20% of the water in the oceanic crust makes it to the core-mantle boundary, it could produce enough diamonds to explain the crust’s carbon levels.

If this is the case, many of the low-velocity zones in the mantle could be areas of water-driven melt caused by oceanic plate churn deep within the planet.

The next challenge will be to demonstrate that this process occurs thousands of kilometers below the surface. Shim explained that there are several ways to look for evidence.

One approach is to look for structures within the core-mantle boundary that could be diamond clusters. Diamonds are dense and would transmit earthquake waves quickly, so researchers would need to find high-velocity zones in addition to the previously discovered slow-moving zones. Other researchers at Arizona State University are looking into this possibility, according to Shim, but their findings have not yet been published.

Another option is to investigate diamonds that may have originated deep within the Earth’s mantle. These diamonds can occasionally reach the surface with tiny pockets, or inclusions, full of minerals that can only form under extremely high pressure.

Even the famous Hope Diamond could have formed deep within the planet’s mantle. Shim explained that when scientists claim to have discovered very deep diamonds, their claims are frequently challenged, in part because the inclusions are so small that there is barely any material to measure. However, he believes it is worthwhile to look for core-mantle boundary inclusions.

“If someone could find evidence for that, that would be some kind of discovery,” he said.

The findings were published in the journal Geophysical Research Letters on August 11th.

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