Extreme chemical reactions could explain why there is so much carbon in the Earth's middle layer.
A new laboratory experiment has shown that at extreme temperatures and pressures, a combination of iron, carbon and water – all potential ingredients found at the core-mantle interface – can form a diamond. If this process also occurs deep inside the Earth, it could explain some of the strange features of the mantle, including why it has more carbon than scientists expect.
The researchers reported their findings in the journal Geophysical Research Letters.
The findings could also help explain strange structures deep at the core-mantle interface, where waves from earthquakes slow down dramatically. These regions, known as “ultra-low velocity zones”, are associated with strange mantle structures, including two giant sunspots under Africa and the Pacific Ocean. They may be only a few miles across or many hundreds. Nobody knows exactly what they are. Some scientists believe that they date back to 4.5 billion years old and are composed of materials from a very ancient Earth. But a new study suggests that some of these zones may owe their existence to plate tectonics, which likely began long after the formation of the Earth, perhaps 3 billion years ago.
“We are adding a new idea that these are not exactly old structures. Where the core meets the mantle, liquid iron rubs against solid rock. This transition is as dramatic as the contact between rocks and air on the Earth's surface. With such a transition, especially at high pressures and temperatures, strange chemistry can occur,” said study lead author Sang-Heon Shim, a geologist at Arizona State University.
Moreover, studies that use earthquake reflections to image the mantle have shown that materials from the crust can penetrate to the core-mantle boundary about 3,000 km below the Earth's surface. In subduction zones, tectonic plates push each other, driving the oceanic crust into the bowels. In the rocks of this oceanic crust, water is enclosed in minerals.
As a result, Shim said, it's possible that water exists at the core-mantle interface and could trigger chemical reactions there. (One theory about a pair of mantle slicks beneath Africa and the Pacific is that they are composed of deformed oceanic crust that has been pressed deep into the mantle, potentially carrying water with it.) Diamonds form under conditions of high temperature and high pressure , similar to those present at the boundary between the core and the mantle.
According to Shim, under pressure and temperature at the interface between the core and the mantle, water behaves completely differently than on the surface of the Earth. Hydrogen molecules are separated from oxygen molecules. Due to the high pressure, hydrogen is attracted to iron, 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, the hydrogen seems to repel other light elements in the core, including, most notably, carbon. This carbon is pushed out of the core into the mantle. At high pressures at the core-mantle interface, diamond is the most stable form of carbon.
“This is how diamonds form,” Shim said, adding that diamonds from the core of the mantle are likely to be buoyant and able to move through the crust , distributing its carbon along the way. The mantle contains three to five times more carbon than researchers expected, based on the proportion of elements in stars and other planets.
He and his team calculated that even if 10% to 20% of ocean water crust gets to the core-mantle boundary, this could produce enough diamonds to account for the levels of carbon in the crust. If so, then many of the low-velocity zones in the mantle could be areas of water melting caused by the churning of oceanic plates deep within the planet.