Extreme chemical reactions could explain why there is so much carbon in the Earth's middle layer. Millions of years of heat and pressure may have created a 'production' (formation) zone for diamonds where the Earth's core meets the mantle. The boundary between the core and the mantle, the Earth's rocky middle layer, could be this diamond 'factory' or site of creation.
A new laboratory experiment at Arizona State University, USA, has revealed that under extreme temperatures and pressure, a combination of iron, carbon, and water—all potential ingredients found in the core-mantle boundary zone—can form diamonds under certain conditions. If such a process also occurs deep within the Earth, it could explain some unusual characteristics of the mantle, including why it contains more carbon than scientists previously predicted.
The findings could help explain strange structures deep within the core-mantle boundary, where earthquake waves slow down drastically. These areas are known as 'ultra-low velocity zones' and are associated with peculiar mantle structures, including two giant blobs beneath Africa and the Pacific Ocean; their diameter can range from a few miles to many hundreds of miles. No one knows exactly what these structures are. Some scientists believe they formed 4.5 billion years ago and are composed of very ancient material from the Earth's origins. New research suggests that some of these zones may have resulted from plate tectonics, which likely began well after the Earth was formed, possibly ~3 billion years ago.
Modelling the Earth's interior
Where the Earth's core meets the mantle, liquid iron encounters solid rock. This is just as radical a transition as the interface between rock and air on the Earth's surface, scientists explained to the publication Live Science. At such a transition, especially under high pressure and temperature, unusual chemical reactions can occur.
Furthermore, studies using seismic wave reflections to image the mantle have shown that materials from the Earth's crust can reach the core-mantle boundary at depths of approximately 3,000 kilometres below the Earth's surface. In subduction zones, tectonic plates press underneath one another, driving oceanic crust deep into the interior. Minerals in the rocks of the oceanic crust contain trapped water. Scientists estimate it is possible that water exists at the boundary of the core and the interior and may facilitate chemical reactions deep down. (One theory about the pair of mantle blobs beneath Africa and the Pacific Ocean is that they consist of deformed oceanic crust pushed deep into the mantle, possibly carrying water with it.)
Diamonds form under the high-temperature and high-pressure conditions that exist at the boundary between the Earth's core and mantle.
To test this idea, researchers combined the ingredients available at the core-mantle boundary and compressed them with diamond-tipped anvils, creating pressures of up to 140 gigapascals. (That is about 1.4 million times the pressure at sea level.) The researchers heated the samples to 6,830 degrees Fahrenheit (3,776 °C).
“We observed what reaction occurs when we heat the sample,” the researchers explained. “We then detected diamonds and discovered an unexpected exchange of elements between the rock and the liquid metal.”
Diamond formation
Under the pressure and temperature conditions at the core-mantle boundary, water behaves very differently than it does on the Earth's surface. Hydrogen molecules separate from oxygen molecules. Due to the high pressure, hydrogen gravitates towards iron, which is the metal that makes up the majority of the core. Thus, the oxygen in the water remains in the mantle, while the hydrogen merges with the core.
When this happens, the hydrogen appears to displace other light elements in the core, including, crucially, carbon. This carbon is pushed out of the core and into the mantle. At the high pressure found at the core-mantle boundary, the most stable form of carbon is diamond. “That is how a diamond is formed,” the scientists point out.
These are not the same diamonds that might sparkle in a piece of jewellery; most of the diamonds that reach the Earth's surface and eventually become jewellery form at depths of a few hundred kilometres, not a few thousand. The diamonds from the Earth's core are likely fluid, and they could be flushed up through the crust, thereby distributing carbon.
The mantle contains three to five times more carbon than researchers would expect based on the proportions of elements in stars and other planets. The diamonds found in this Earth layer could explain this discrepancy. Scientists have calculated that if even 10-20% of the water in the oceanic crust reaches the core-mantle boundary, it could release enough diamonds to explain the amount of carbon in the crust. If this is the case, then many low-velocity zones in the mantle could be water-induced melting zones triggered by the movement of oceanic plates into the depths of the planet. The next task is to prove that this process occurs thousands of kilometres below the surface.
There are several ways to look for evidence
One is to look for structures at the core-mantle boundary that could be clusters of diamonds. Diamonds are dense and transmit earthquake waves quickly, so researchers should look for high-velocity zones alongside the already discovered regions where waves move slowly. Other researchers at Arizona State University are investigating this possibility, but their work has not yet been published.
Another possibility is to study diamonds that might be obtained from deep within the Earth's mantle. Such diamonds can sometimes reach the surface with tiny inclusions full of minerals that can only form under extremely high pressure.
Even the famous Hope diamond may have formed very deep in the planet's mantle. When scientists claim to have discovered very deep-seated diamonds, these claims are often controversial, partly because the inclusions are so tiny that there is almost no material to measure in them. However, they believe it might be worth looking for inclusions from the core-mantle boundary. “It would be a discovery of sorts if someone could find evidence of that,” the Arizona State University scientists claim. The researchers reported their findings this August in the journal Geophysical Research Letters.