Earth recycles ocean floor into salty diamonds

The diamond on your finger is most likely made of recycled seabed cooked deep in the Earth.


The most wonderful quality about diamonds is that they are one of the only types of physical matter on this earth that do not wear and tear. As a result, you can not tell the difference between a diamond worn 300 years ago from a diamond freshly mined.

Now, a new study reveals that most of the diamonds are made of recycled seabed cooked deep in the Earth. It suggests many diamonds begin life as sediment on the bottom of the ocean, before being swallowed up by the Earth’s mantle and forged into our favorite shiny stones.

Diamonds often receive attention and dominate other precious stones. It has been known to mankind for ages and has been used since ancient times. They are also symbols of status, wealth and romance, but are slowly starting to lose a little of their mystique in recent years, as scientists learn more about them. The gems are far more common than you might think, with a quadrillion tons of diamond newly discovered deep within the Earth, and there’s plenty of them floating around out in space.

In this study, scientists shed light on the lifecycle of the diamonds. Scientists studied the salt trapped in many diamonds and found that the stones are formed from ancient seabeds that became buried deep beneath the Earth’s crust.

In experiments recreating the extreme pressures and temperatures found 200 kilometres underground, Dr. Michael Förster, Professor Stephen Foley, Dr. Olivier Alard, and colleagues at Goethe Universität and Johannes Gutenberg Universität in Germany, have demonstrated that seawater in sediment from the bottom of the ocean reacts in the right way to produce the balance of salts found in diamond.

Lead author Michael said, “There was a theory that the salts trapped inside diamonds came from marine seawater, but couldn’t be tested. Our research showed that they came from marine sediment. We knew that some sort of salty fluid must be around while the diamonds are growing, and now we have confirmed that marine sediment fits the bill.”

Scientists tested the idea by placing samples of marine sediment into a chamber that would simulate the high pressure and high temperature deep within the Earth. Samples of peridotite, the most common type of rock in the mantle, was also added before the mix was cooked.

They found that salts similar to those found in diamonds began to form at pressures between four and six gigapascals, and temperatures between 800 and 1,100° C (1,472F and 2,012° F). These are the conditions you’d expect at depths between 120 and 180 km (74.5 and 112 mi) below the surface of the Earth.

Michael said, “We demonstrated that the processes that lead to diamond growth are driven by the recycling of oceanic sediments in subduction zones. The products of our experiments also resulted in the formation of minerals that are necessary ingredients for the formation of kimberlite magmas, which transport diamonds to the Earth’s surface.”

Scientists explained, “For this process to work, large sections of the seafloor would have to be swallowed up very quickly, and drop to a depth of more than 200 km (124 mi). That’s because the pressure needs to work its magic first before the heat melts the material down. That might sound unlikely, but it happens all the time – the Mariana Trench is an active subduction zone that’s sending huge amounts of seawater and sediment down into the mantle.”

The research is presented in the journal of Science Advances.


See stories of the future in your inbox each morning.