In popular culture, diamonds aren’t often associated with breakups. But that’s not the case at the Argyle diamond mine in Western Australia, once the world’s most prolific source of pink diamonds. Researchers have discovered that the Argyle deposit was formed when one of the world’s oldest supercontinents began to break apart 1.3 billion years ago. The study, published in Nature Communications, shows how the separation of continents could be responsible for spraying diamonds to the surface, and may indicate where other such deposits might be found.
Diamonds have long been a feature of the stories told by the Miriwoong and Gija people of Western Australia. It wasn’t until 1979 that Western geologists discovered the region’s diamonds. Argyle shipped its first diamonds in 1983, quickly doubling world diamond production and producing 865 million carats in its 37-year history. The mine closed in November 2020 after it was deemed no longer economically viable.
The deposit was always an anomaly. Most of the world’s diamond deposits are found in carbon dioxide-rich kimberlite, whereas Argyle is embedded in water-rich, mantle-derived lamproite. Most diamond deposits are found within ancient continental blocks, but Argyle sits on the edge of a rift zone. Most importantly, Argyle’s pink and brown diamonds stand out from most colourless varieties.
A pink diamond might be Barbie’s dream, but a gemologist would consider it defective. “Pink only comes from deformation,” says Hugo Olierook, a geochemist at Curtin University in Australia and lead author of the new study.
It takes an enormous amount of force to twist the atomic arrangement of the diamond’s crystals and trigger a colour change. Twist a little and the diamond turns pink. Twist a little more and it turns the colour of brown sugar.
Olierook and his colleagues believe that Argyle’s diamonds changed colour when continental blocks broke apart to form Nuna, an ancient supercontinent and precursor to Pangaea, about 1.9 billion years ago. When the landmass split again 1.3 billion years ago, the weakened rifts generated the volcanism necessary to release the diamonds from their subterranean birthplaces and rush them to the surface like bubbles under a champagne cork, Olierook said.
Previous studies put the Argyle diamond deposit at 1.2 billion years old, but that was a relatively quiet time in the region. Olierook and his colleagues dated minerals from the rocks containing the diamonds and found that they were actually 1.3 billion years old. That’s about the time Nuna split. Deep-rooted volcanism at the dividing rift zone explains Argyle’s existence.
“Correlation doesn’t necessarily mean causation, but it certainly seemed like a very elegant explanation,” says Olierook.
There’s support for the theory, he noted. Another recent paper on kimberlites also pointed to continental break-ups as the trigger for diamond deposits to rise to the surface. “It was really nice to see that our two scientific studies, done independently by completely different teams, came to essentially the same conclusion,” Olierook said.
The volcanic cone that brought Argyle’s diamonds to the surface left a “mismatch of minerals”, Olierook explained. Some of these minerals formed after the diamonds, during the eruption, while others had formed long before.
To work out which crystals formed when, the researchers used a 30-micrometre-wide laser to bombard tiny amounts of individual mineral grains within the lamproite and measure their isotopes. They targeted titanite, a mineral formed shortly after the volcanic eruption, and compared the amounts of radioactive uranium and its stable daughter product, lead, present. This provided a minimum possible age for the diamond eruption: 1.26 million years ago.
To find a maximum age, the researchers focused their laser on single crystals of zircon and apatite, minerals that were present in the lamproite before the eruption. This put the rise of the diamonds to the surface at around 1.3 billion years ago, in line with the breakup of Nuna.
The researchers have been “really careful with the dating,” says Maya Kopylova, a geochemist at the University of British Columbia who wasn’t involved in the research. That’s admirable, she said; other studies have analysed whole rock samples, a technique that averages an age over all the minerals present, rather than giving the age of individual grains.
The researchers’ theory that the cycle of supercontinents propelled Argyle’s diamonds to the Earth’s surface is “probably right on the money,” said Steve Shirey, an isotope geochemist at Carnegie Science who was not involved in the study.
The study has given scientists a better idea of how such a prolific diamond deposit formed, which could give them a clue as to where other such finds might be hiding. But even if the paper’s theories are correct, it’s unlikely to lead to another pink diamond bonanza. Rift zones aren’t buoyant and are usually buried under kilometres of sediment. Again, Argyle is an anomaly: the deposit just happened to be in the right geological place at the right geological time for economic mining. If pink diamonds exist in other rift zones, they’re probably so deep that it’s not economically viable to mine them, or even to start looking for them, Kopylova said.
