Diamonds are made from recycled seabed
Discolouration in industrial diamonds reveal their high-pressure origin. Andrew Masterson reports.
Experiments recreating the extreme pressures and temperatures found 200 kilometres underground have confirmed that diamonds are made of recycled seabed.
The research, led by Michael Förster of Australia’s Macquarie University, focused on fibrous diamonds, which are generally used for industrial purposes, such as tipping drill bits.
Much more common than the familiar crystal-clear “gem” diamond, they have chemical inclusions muddying up their otherwise pure carbon structure, lending them a cloudy appearance.
The additives include sodium and potassium. These, Förster and colleagues reasoned, could reveal information about the environment and conditions in which they were they formed.
Creating analogues of possible start-points, however, was challenging. The majority of the diamonds found near the Earth’s surface were created deep in the planet’s mantle, at depths below 120 kilometres, under more than 40,000 times the surface atmospheric pressure, and at temperatures between 800 and 1100 degrees Celsius.
Their journey upwards is typically propelled by a special kind of magma, called kimberlite.
And while that general mechanism is well supported by evidence, a question has always remained regarding the origin of the elements inside the fibrous variety.
“There was a theory that the salts trapped inside diamonds came from marine seawater, but couldn’t be tested,” says Förster. “Our research showed that they came from marine sediment.”
The research lends weight to the suggestion that diamond formation is triggered when a section of seabed at the edge of a tectonic plate gets sucked beneath a neighbouring plate, at a depth greater than 200 kilometres. This process occurs very quickly, ramping up both pressure and heat.
The marine sediment then comes into close contact with a type of rock common in the mantle, known as peridotite.
In the lab work, at a depth corresponding to between 120 and 180 kilometres below the surface, the sediment and the peridotite reacted to form alkali chlorides, which contain exactly the same ratio of potassium to sodium as the inclusions in fibrous diamonds.
At higher temperatures – greater than 1100 degrees Celsius – the chlorides became unstable, shifting between solid, melt and fluid phases, explaining why, the researchers concluded, the same highly saline fluids are found in both the diamonds and the surrounding kimberlite magmas.
“We demonstrated that the processes that lead to diamond growth are driven by the recycling of oceanic sediments in subduction zones,” says Förster.
“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.”
The research is published in the journal Science Advances.