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To form pink diamonds, build and destroy a supercontinent

The world’s largest supply of pure diamonds — and of greater than 90 p.c of all pure pink diamonds discovered thus far — could have shaped as a result of breakup of Earth’s first supercontinent, researchers report September 19 in Nature Communications.

The diamond-bearing rocks of the Argyle mine in Western Australia in all probability shaped about 1.3 billion years in the past, the evaluation reveals, alongside a rift zone that sundered the supercontinent Nuna. The discovering means that exploring historic rift zones for diamond troves could also be extra worthwhile than beforehand thought.

On Earth’s floor, carbon atoms are likely to type gentle, uninteresting graphite. However down within the forge that’s the higher mantle, excessive circumstances mould the factor into onerous, dense gem stones (SN: 9/14/20). These diamonds can escape their chthonic womb by hitching a experience in quickly ascending magmas (SN: 1/18/12). Close to the floor, the molten materials solidifies as vertical tubes of volcanic rock, referred to as kimberlite pipes. Most diamonds are present in these formations.

However this basic story doesn’t clarify the Argyle formation, nor its pink diamonds. To make a diamond blush, one thing extra highly effective than mere mantle circumstances should contort its sturdy crystal construction, altering the way it absorbs and transmits gentle.

One other wrinkle lies in Argyle’s diamondiferous pipes. They aren’t kimberlite, however quite lamproite pipes, that are usually thought to type at shallower depths, says geologist Maya Kopylova of the College of British Columbia in Vancouver, who was not concerned within the new examine.

Lamproites’ shallow origins could clarify why they normally lack kimberlites’ wealthy diamond hundreds. The exception is Argyle — by some means, its lamproites raised treasures from the deep.

Argyle lamproite sample
This 15-centimeter-wide Argyle lamproite pattern comprises darkish, diamond-bearing volcanic clasts distributed inside a lighter grey rock.Andreas Zametzer

The eccentricities of the Argyle formation have lengthy puzzled geologists. Chemical analyses carried out within the Eighties urged it shaped roughly 1.2 billion years in the past. However that courting was questionable; the mineral that was analyzed could have been chemically altered by fluids within the Earth, doubtlessly yielding a too-young age. What’s extra, the outcomes did little to clear up Argyle’s mysterious origins, says Hugo Olierook of Curtin College in Perth, Australia. “Nothing was actually occurring [geologically] in Australia on the time.”

So Olierook and colleagues dated resilient grains of the minerals apatite and zircon that fell into the lamproite when it was nonetheless molten. The researchers additionally analyzed titanite, a mineral that seems to have crystallized barely later than the remainder of the lamproite.

By measuring the portions of radioactive components and their decay merchandise inside every mineral, the group discovered that the lamproite shaped about 1.3 billion years in the past, roughly 100 million years sooner than beforehand urged.

“Once I first bought the age, I believed, this doesn’t make any sense in any respect,” Olierook says. However whereas he was biking residence a few hours later, it clicked. “That’s when the primary supercontinent was breaking apart,” he says.

Argyle sits inside an historic continental suture, the place two plates as soon as collided to type a part of Nuna, roughly 1.8 billion years in the past (SN: 1/11/17). “It’s the smashing of these continents collectively; that’s what made these diamonds pink,” Olierook speculates.

About 500 million years later, when Nuna rifted aside, that suture cut up like a reopened wound. Which will have opened conduits for lamproite magmas laden with rosy gems to rise by way of, Olierook says.

For many years, it has been thought that tectonic processes like rifting destroyed diamonds, Kopylova says. However this analysis helps a current paradigm shift, she says. Rifting “could be a set off to get diamonds from deep within the mantle to the floor.”

Nonetheless, she says, it stays a thriller why Argyle’s lamproite pipes are the one ones recognized that contained minable portions of diamonds. In late 2020, the diamond mine there stopped manufacturing after exhausting the diamonds that had been economically possible to extract.

It could be the case that extra gem-studded lamproites are awaiting discovery, Olierook says. Maybe someplace, one other Argyle lies hidden within the ruins of an historic rift.

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