Super-deep Diamond Reveals Mineral From Earth's Mantle Never Before Seen by Human Eyes

Scientists have found a never-before-seen mineral in a diamond formed deep in the Earth's mantle. Thought be the planet’s fourth most abundant mineral, calcium silicate perovskite is usually buried 400 miles below the surface.

The diamond provides “fundamental proof” of the long-theorized idea that slabs of oceanic crust that sink deep within the Earth are recycled into the lower mantle, researchers said.

A study of the diamond was published online Wednesday in the journal Nature.

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Unyielding diamond container

164528_web A super-deep diamond from the Cullinan Mine, similar to the one that was found trapping calcium silicate perovskite. Petra Diamonds

Researchers found the trapped mineral in a diamond from South Africa’s Cullinan Mine. This mine has yielded some of the most expensive diamonds in the world, including two of the largest diamonds in the British Crown Jewels.

It's also the source of some of the most scientifically valuable diamonds, Graham Pearson, a professor at the University of Alberta and one of the study’s authors, said in a statement.

The rocks can shed light on the deepest parts of Earth’s core. “The only possible way of preserving this mineral at the Earth’s surface is when it’s trapped in an unyielding container like a diamond,” Pearson explained.

Diamonds are a geologist’s best friend

Calcium silicate perovskite is one of the most important minerals comprising Earth’s lower mantle. This rare diamond trapped some of the mineral when it formed some 400 miles below the planet’s surface—about 275 miles deeper than most of the glittering rocks.

The diamond—which was discovered about half a mile deep—would have sustained more than 24 billion pascals of pressure during its formation, Pearson said.

Rock-hard evidence

Analysis of the diamond provided rock-hard evidence that material from the oceanic crust—which stretches just six miles below the sea—is recycled into the lower mantle, where the gem was formed.

“The specific composition of the perovskite inclusion in this particular diamond ... provides fundamental proof of what happens to the fate of oceanic plates as they descend into the depths of the Earth.”

Until now, this crustal recycling was mostly the stuff of theory, inferred from seismology and other scientific research. This diamond discovery, Pearson said, is “a nice illustration of how science works.”

“You build on theoretical predictions—in this case, from seismology—and … once in a while you’re able to make a clinching observation that really proves that the theory works.”

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