The boundary region between Earth’s molten mineral core and the mantle, the rocky middle layer, may be a diamond factory.
A new lab experiment has found that under extreme temperatures and pressures, a mixture of iron, carbon and water — all potential components found at the core-mantle boundary — can form diamond. If this process occurs in depth as well a landmay explain some of the mantle’s strange quirks, including why it contains more carbon than scientists expect.
The findings may also help explain the strange structures deep in the core-mantle boundary where seismic waves slow down dramatically. These regions, known as “ultra-low velocity regions”, are associated with peculiar mantle structures, including Two giant points under Africa and the Pacific Ocean (Opens in a new tab); It could be a few miles away or several hundred. Nobody knows exactly what they are. Some scientists believe it dates back 4.5 billion years and is made of materials from very ancient Earth. But the new research suggests that some of these regions may owe their existence tectonic plates (Opens in a new tab)which likely began after the formation of the Earth, perhaps 3 billion years ago.
“We add a new clue that these are not quite ancient structures,” study lead author Sang-Heon Shim, a geologist at Arizona State University, told Live Science.
Earth Depth Simulation
When the core meets the mantle, the liquid iron rubs against the solid rock. This is an exciting transformation like the rock-to-air interface at Earth’s surface, Shim told Live Science. In such a transformation, especially at higher pressures and temperatures, it is strange chemistry (Opens in a new tab) might happen.
Furthermore, studies using reflections of earthquake waves to image the mantle have shown that material from the crust may penetrate the core mantle boundary, about 1,900 miles (3,000 km) below the Earth’s surface. in subduction zones (Opens in a new tab), tectonic plates shove under each other, pushing oceanic crust deeper into the Earth’s interior. The rocks in this oceanic crust have water trapped in their minerals. As a result, Shim said, it’s possible that water is present at the core-mantle boundary and could trigger chemical reactions below. (One theory about a pair of blobs in the sub-African and Pacific mantle is that they are made up of deformed oceanic crust that has been pushed deeper into the mantle, likely carrying water with it.)
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To test the idea, the researchers combined the components available at the core-mantle boundary and compressed them together using a diamond anvil, generating pressures of up to 140 gigapascals. (This is 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 watched what kind of reaction occurred when we heated up the sample,” Shim said. “Then we discovered diamonds, and we discovered an unexpected exchange of elements between the rock and the liquid metal.”
diamonds come out
under pressure and temperature (Opens in a new tab) Shim said that water behaves very differently than it does on Earth. Hydrogen molecules separate from oxygen molecules. Because of the high pressure, the hydrogen is attracted towards iron, which is the metal that makes up most of the core. Thus, oxygen from the water remains in the mantle, while hydrogen fuses with the core.
When this happens, hydrogen appears to push aside the other light elements in the core, including, crucially, carbon. This carbon is ejected from the core into the mantle. At the high pressures found at the core and mantle boundary, the most stable form of carbon is diamond.
“This is how diamonds are formed,” Shim said.
These are not the same as diamonds that would sparkle in an engagement ring. Most of the diamonds that make their way to the surface and eventually become someone’s jewelry, are at a depth of a few hundred kilometers, not a few thousand. But diamond cores are likely buoyant and can sweep throughout the crust, distributing carbon as it travels.
The mantle contains three to five times more carbon than researchers would expect based on the proportion of elements in stars and other planets. Shim said diamonds found in this layer of the earth might explain this discrepancy. He and his team calculated that even if 10% to 20% of the water in the oceanic crust reached the core mantle boundary, it could produce enough diamonds to explain the crust’s carbon levels.
If so, many of the low-velocity regions in the mantle may be water-driven melt regions, caused by ripples of oceanic plates deep in the planet.
The next challenge is to prove that this process occurred thousands of kilometers below the surface. Shim said there are two ways to look for evidence.
The first is to look for structures within the core and mantle boundaries that could be diamond clusters. Diamonds are dense and can transmit earthquake waves quickly, so researchers will need to find areas of high speed alongside already detected areas where the waves are moving slowly. Shim said other researchers at Arizona State University are investigating this possibility, but the work has yet to be published.
Another option is to study diamonds that may come from great depths in the Earth’s mantle. These diamonds can sometimes come to the surface with small pockets or inclusions, full of minerals (Opens in a new tab) It can only form under very high pressure.
until the The famous diamond hop (Opens in a new tab) It may have formed very deep in the planet’s mantle. Shim said that when scientists claim to have discovered diamonds that are very deep, those assertions are often controversial, in part because the inclusions are so small that there is no material to measure. But he said it might be worth looking for boundary inclusions in the mantle core.
“It would be kind of a discovery, if someone could find evidence of that,” he said.
The researchers announced their findings August 11 in the journal Geophysical Research Letters (Opens in a new tab).
Originally published on Live Science.