The boundary zone between Earth’s molten metal core and the mantle, its rocky middle layer, could be a diamond factory.
A new lab experiment shows that under extreme temperatures and pressures, the combination of iron, carbon and water – all potential constituents found at the core-mantle interface – can form diamond. If this process also happens deep inside Earthit could explain some of the mantle’s odd quirks, including why it contains more carbon than scientists expect.
The findings could also help explain strange structures deep in the core-mantle boundary, where earthquake waves slow down dramatically. These regions, known as “Ultra Low Velocity Zones,” are associated with strange mantle structures, including two huge blobs beneath Africa and the Pacific Ocean (opens in new tab); They can be just a few kilometers wide or many hundreds. Nobody knows exactly what they are. Some scientists believe they are 4.5 billion years old and made of materials from very old Earth. But the new research suggests some of these zones may owe their existence plate tectonics (opens in new tab)which probably began long after the formation of the earth, perhaps 3 billion years ago.
“We’re adding a new idea that these aren’t entirely ancient structures,” the study’s lead author Sang-Heon Shim, a geoscientist at Arizona State University, told Live Science.
Related: Layers of the Earth: Exploring our planet inside and out
Simulation of the deep earth
Where the core meets the mantle, liquid iron rubs against solid rock. That’s as dramatic a transition as the rock-air interface at Earth’s surface, Shim told Live Science. In such a transition, especially at high pressures and temperatures, strange Chemistry (opens in new tab) can happen.
In addition, studies using the reflections of earthquake waves to map the mantle have shown that materials from the crust can advance to the core-mantle boundary, about 1,900 miles (3,000 kilometers) below Earth’s surface. at subduction zones (opens in new tab)tectonic plates slide under one another and push oceanic crust underground. The rocks in this oceanic crust have water trapped in their minerals. As a result, Shim said, it’s possible that water exists at the core-mantle boundary and can drive chemical reactions down there. (One theory about the pair of mantle blobs beneath Africa and the Pacific is that they consist of distorted oceanic crust that has been pushed deep into the mantle, possibly carrying water with it.)
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To test the idea, the researchers pulled together the ingredients available at the core-mantle boundary and squeezed them with diamond anvils, creating pressures of up to 140 gigapascals. (That’s about 1.4 million times the pressure at sea level.) The researchers also heated the samples to 6,830 degrees Fahrenheit (3,776 degrees Celsius).
“We monitored what kind of reaction was taking place as we heated the sample,” Shim said. “Then we discovered diamonds and an unexpected exchange of elements between the rock and the liquid metal.”
produce diamonds
Under the pressure and temperature (opens in new tab) At the core-mantle boundary, Shim said, water behaves very differently than it does on Earth’s surface. The hydrogen molecules separate from the oxygen molecules. Because of the high pressure, hydrogen is attracted to iron, the metal that makes up most of the core. So the oxygen from the water stays in the mantle while the hydrogen fuses with the core.
When this happens, the hydrogen appears to push aside other light elements in the core, most notably carbon. This carbon is booted out of the core and into the mantle. At the high pressures at the core-mantle interface, the most stable form of carbon is diamond.
“That’s how diamonds are made,” Shim said.
These are not the same diamonds that might sparkle in an engagement ring; Most of the diamonds that make their way to the surface and eventually become someone’s jewelry are formed a few hundred kilometers down, not a few thousand. But the core-mantle diamonds are likely buoyant and could be swept through the crust, dispersing their carbon as they go.
The mantle contains three to five times more carbon than researchers would expect based on the proportions of the elements in stars and other planets. The diamonds found in this layer of earth could explain the discrepancy, Shim said. He and his team calculated that even 10 to 20 percent of the water in the oceanic crust up to the core-mantle boundary could produce enough diamonds to explain the carbon content in the crust.
If that’s the case, many of the low-velocity zones in the mantle could be areas of water-driven melt triggered by the movement of oceanic plates deep within the planet.
Proving that this process occurs thousands of kilometers below the surface is the next challenge. There are several ways to look for evidence, Shim said.
One 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 alongside the already discovered regions where waves travel slowly. Other researchers at Arizona State University are studying this possibility, Shim said, but the work isn’t published yet.
Another option is to study diamonds, which may have come from very deep in the Earth’s mantle. These diamonds can sometimes come to the surface with tiny pockets or inclusions, full of minerals (opens in new tab) which can only form under very high pressure.
Even the famous Hope Diamond (opens in new tab) may have formed very deep in the planet’s mantle. When scientists claim to have discovered very deep diamonds, those claims are often disputed, Shim said, partly because the inclusions are so tiny there’s little material to measure. But it might be worth looking for inclusions at the core-mantle interface, he said.
“That would be kind of a discovery if someone could find evidence of it,” he said.
The researchers reported their findings Aug. 11 in the journal Geophysical Research Letters (opens in new tab).
Originally published on Live Science.
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