Even more bling beneath the Earth's surface – and scientists think they know why

Scientists have found a possible reason why a cache of "super-deep" blue diamonds – like the famous Hope diamond – originate up to four times deeper in the Earth than other types of diamonds.

These diamonds, considered rare on the Earth's surface, can be found a whopping 660 – 750km beneath it.

Fin24 recently reported that more than a quadrillion tons of diamonds, or one thousand times more than a trillion, had been found buried in the earth, some 145 to 240km deep.

Now, a new study has probed how a much deeper layer of blue diamonds got there – and why they have their intriguing colour, despite lying far from abundant stores of the element that supplies it.

The study, titled "Blue boron-bearing diamonds from Earth's lower mantle" published in the latest issue of the journal Nature, argues that these super-deep blue diamonds are different from other kinds of diamonds due to the shifting and sinking of oceanic tectonic plates.

Blue diamonds, the study argues, grew in the presence of rocks that were "essentially once part of the ocean floor".

They get their colour from boron, an element predominantly found in the crust close to the surface of the earth, which suggests they somehow made it all the way down to the super-deep high-pressure environment where the diamonds are formed.

Ancient oceans

According to the study, blue diamonds "have a connection to deeply recycled seawater from ancient oceans".

Oceanic tectonic plates are pushed down into the mantle at subduction zones – which science website LiveScience describes as "the biggest crash scenes on Earth", due to the fact that they are formed where two tectonic plates collide, and, put very simply, one plate eventually slides under the other. 

The study was led by Evan Smith of the Gemological Institute of America and included UCT Professor of Geology Stephen Richardson on the team. "The extent to which Earth’s surface materials are recycled into the deep mantle via these subduction zones has until now been difficult to evaluate," UCT said in a statement on Friday.

"The study sheds light on the issue by examining blue diamonds."

Largest and most valuable

Exactly how or where these diamonds crystallise within the mantle has remained unknown to date, UCT said.

However, scientists figured out that enormous and irregularly shaped diamonds stand out "because they are so pure". This gave rise to an acronym, CLIPPIR: Cullinan-like, Large, Inclusion-Poor, relatively Pure, Irregular shaped and Resorbed.

Richardson explained: "A couple of years ago we found that the world’s largest and most valuable gem diamonds, like the 3 106-carat Cullinan, formed from metallic liquid deep in the Earth’s mantle transition zone."

This led researchers to speculate – based on the elements found in the gems – that the rare blue diamonds, which also tend to be large, could be of the same kind as the CLIPPIR diamonds, and also originate from somewhere very deep.

The new study is also overturning the belief that super-deep diamonds are typically small and never of gem quality, UCT said.

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