Crown Jewels and Hope diamonds originated "super deep" in the Earth
There’s plenty of mystique surrounding diamonds, and while much of it is manufactured (thanks, De Beers), some earn it. Researchers have now found evidence that some of the world’s most impressive gems, such as the Hope Diamond, originated much deeper in the Earth’s mantle than previously thought.
Most diamonds form in the upper mantle, just beneath the continental crust, at depths of between 150 and 200 km (93 to 124 mi). In rarer cases diamonds are known to form further down, at estimated depths between 360 to 750 km (224 to 466 mi). But, the team says, no actual diamonds had so far been confirmed to originate towards the lower end of this range.
Now, researchers at the Gemological Institute of America (GIA) say they’ve found that evidence. The team studied two large diamonds – a 20-carat type IIb blue diamond discovered in South Africa, and a 124-carat CLIPPIR diamond from Lesotho. Using a spectrometer, they found the remains of a mineral called bridgmanite, which is a smoking gun for a super-deep formation.
“Bridgmanite doesn't exist in the upper mantle, or at the surface,” says Evan Smith, of the GIA. “What we actually see in the diamonds when they reach surface is not bridgmanite, but the minerals left when it breaks down as the pressure decreases. Finding these minerals trapped in a diamond means that the diamond itself must have crystallized at a depth where bridgmanite exists, very deep within the Earth.”
Bridgmanite is very common in the lower mantle, below a depth of 660 km (410 mi), suggesting that the two types of diamond studied formed at least that far down.
But perhaps the most interesting part of the study is the implications it has for some of the most famous gems in the world. The Hope Diamond, on display in the Smithsonian, is a type IIb blue diamond. And the Cullinan diamond, pieces of which are set into the Crown Jewels in the Tower of London, is a CLIPPIR diamond. That means both of these majestic marvels are likely "super deep" diamonds.
"Discovering the deep mantle origin means that the material in these diamonds undergoes a remarkable journey,” says Smith. “We believe that the boron, which give the Hope diamond it's characteristic blue color, originates from the bottom of the oceans. From there, plate tectonics drags it hundreds of kilometers down into the mantle, where it can be incorporated into diamond. It shows that there is a gigantic recycling route that brings elements from Earth's surface down into the Earth, and then occasionally returns beautiful diamonds to the surface, as passengers in volcanic eruptions.”
The research was presented at the Goldschmidt Conference this week.