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Scientists have discovered that the world’s largest source of natural diamonds was forged by the breakup of an ancient supercontinent some 1.3 billion years ago, a finding that provides tantalizing clues about where to find similar sources of rare diamonds made by similar events, reports a new study. The results reveal the true age of the Argyle diamond deposit, located in the remote far north of Australia, which became one of the world’s biggest sources of diamonds, as well as the origin of about 90 percent of all pink diamonds, following its discovery in 1979. Though the mine at the site closed in 2020, scientists continue to study Argyle to better understand its geological history and the origin of its immense trove of gems.
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Now, a team led by Hugo Olierook, a research fellow and geochemist at Curtin University in Perth, have discovered that Argyle is about 1.3 billion years old, making it about 100 million years older than previous estimates. The updated timeline reveals that “diamond deposits created by (super)continental breakup may be prevalent but hitherto under-recognized in rift zones at the edges of ancient continental blocks,” according to the team’s study, which was published on Tuesday in Nature Communications.Olierook and his colleagues were inspired to investigate Argyle’s age and origin by a workshop organized by study co-author Murray Rayner, principal geologist at the mining company Rio Tinto, focused on sharing the legacy of the mine with undergraduate students interested in geology.“It was from this workshop, seeing the rocks and diamonds first-hand, and a chat between [study co-author] Denis Fougerouse and I, where we realized we could constrain the true age of the Argyle volcano with modern dating techniques, and figure out what the real trigger was for Argyle,” Olierook told Motherboard in an email. “And, if we could do that, maybe, just maybe, it could help us to search for another pink diamond trove, now that Argyle had closed.”Argyle was initially estimated to be 1.2 billion years old by the geologist Bob Pidgeon, though Pidgeon was never fully satisfied with this timeline, according to Olierook. While the origins of the trove have remained unclear for decades, the researchers knew that its vast stores of pink diamonds were made by carbon that was buried at least 100 miles underground and was exposed to extreme pressures and volcanic activities associated with continental collisions.
“Pink diamonds are damaged diamonds,” Olierook explained. “The pink color is imparted by the diamonds actually getting bent and twisted, and this deformation causes the pink color. But, push a little too hard, and the diamonds turn brown. This is also how pink synthetic diamonds can be made today.”“Argyle was the first discovered diamond deposit, and one of only a handful, that was discovered not in the middle of an ancient continent (where the crust is thicker, and you’re more likely to have diamond rather than graphite) but at the juncture of where two continents (the Kimberley block and northern Australia) collided some 1.85 billion years ago, creating a large mountain belt,” he continued. “What we really didn’t know was what triggered Argyle to erupt—how did these pink diamonds get to the surface?”To answer this question, Olierook and his colleagues applied geochronology and dating techniques to a drill core of Argyle rock. To their astonishment, the results suggested that the deposit dates back 1.3 billion years, when this region was being pulled apart by the breakup of an ancient supercontinent called Nuna.“We actually had a bit of a betting pool going as to what the age of Argyle was, and all of us were wrong,” Olierook said. “Safe to say, we were pretty surprised. An age of approximately 1.3 billion years pushed the age back by 100 million years older than was previously thought by Prof. Pidgeon, and the rest of the scientific community.”
“It was not until later that day that I appreciated the significance of that,” he added. “1.3 billion years ago, all the continents had been part of what is the first well-established supercontinent for about 300 million years (from 1.6 to 1.3 billion years ago), a time when almost all the continents were joined in a single landmass with one giant ocean around it. At 1.3 billion years ago, all these continents started to break up. And while the mountain belt that created Argyle, now eroded to a flat plain, didn’t break up the Kimberley and northern Australia, it certainly would have stretched. And it’s this stretching that allowed a little bit of magma to shoot from deep down in the bowels of the Earth to the surface, bringing their pink diamonds with them.”In other words, subterranean diamonds that were created by continental collisions were then pushed upward through Earth’s crust by volcanism fueled by the disintegration of Nuna. By the early Cretaceous period, the deposit had risen to within about a mile from Earth’s surface; erosive forces have since exposed it at mineable depths in our modern geological time.This rewritten history of Argyle is fascinating on its own merits, but it could also serve as a roadmap to other diamond deposits formed by supercontinent breakups, which are far less common than deposits found in the middle of ancient continents.“If I was a diamond explorer and looking for pink diamonds, I’d focus my attention at any of these now-eroded ancient mountain belts that surround the cores of ancient continents,” Olierook said. “And every one of our continents on Earth has these—with prevalent countries including Canada, Brazil, Russia, South Africa and Australia, amongst others. I would look for rock types that coincide with the breakup of (super)continents, as it’s these rocks that might host new pink diamond deposits.”Finding these undiscovered troves involves all sorts of challenges, but the team hopes the new study can at least narrow down the search, while also revealing new details about the global carbon cycle.“How does carbon get transferred from the deepest parts of the Earth to the surface, and back again, and what does this cycle tell us about global Earth processes?” Olierook concluded. “In any carbon story, diamonds form an integral part as, most commonly, the carbon deep down would be in the form of diamond.”