Argyle pink diamond deposit formed when ancient supercontinent Nuna broke up: study
Pink and red diamonds have been subject to enough pressure to change their colour, but not so much that they turn brown. (Getty Images: Bloomberg)
Some 1.3 billion years ago, when a chunk of an ancient supercontinent started splitting apart, hot rock rich with pink diamonds violently blasted through the rift in an explosion bigger than any human has witnessed.
Key points:
- The now-closed Argyle mine in Western Australia produced more than 90 per cent of the world's pink diamonds
- Researchers calculated the age of the Argyle deposit coincided with the break-up of a supercontinent
- Similar diamond deposits are likely present on the edges of ancient continents, but finding them is easier said than done
The magma within the volcano cooled, locking the diamonds inside. And it eventually became the site of the multi-billion-dollar Argyle diamond mine in north-east Western Australia, famed for its pink diamonds.
This is according to a new study by researchers from Curtin University, mining company Rio Tinto, and international colleagues, who calculated when and how the rocks from the Argyle diamond deposit left their birthplace, hundreds of kilometres below ground.
That point in time coincided with a supercontinent called Nuna breaking apart, which the team suggests was crucial to delivering the diamonds to the surface.
The work was published in the journal Nature Communications.
Knowing exactly how such diamond deposits formed could help prospectors narrow down potential sites, Hugo Olierook, Curtin University geologist and study lead author, said.
"I think using that sort of [diamond-producing] recipe, I think we'll be able to find more of these pink diamonds, but it's not going to be easy."
Diamond's high-pressure birth
The Argyle mine, located on Miriwoong and Gija country, sits in a valley in the Matsu Ranges in the east Kimberley.
In 2020, after 37 years of churning out more than 90 per cent of the world's pink diamonds, the mine's owner and operator Rio Tinto closed it.
Its accessible stores were, by then, depleted.
But when the mine's trove of diamonds was born, almost 2 billion years ago, Earth was a very different place.
Animals hadn't evolved yet. Neither had plants. Life consisted of bacteria living in the oceans. And all the land was squashed up together in the Nuna supercontinent.
Even the continent we know as Australia today was, back then, a handful of smaller "continental blocks".
More than 150 kilometres below the surface, under immense pressure, carbon in the form of graphite — the soft, grey mineral used in pencils — crystallised into colourless diamonds.
Around 1.8 billion years ago, two continental blocks — what we know of as the Kimberley and Northern Australia — smashed together and stuck.
"And it's this collision that turned what were probably originally colourless diamonds into pink ones,"Dr Olierook said.
Why pink diamonds are pink
Pure, colourless diamond is made of pure carbon. The carbon atoms are arranged in a super stable and strong structure.
Trap tiny traces of nitrogen or boron into the carbon crystal, and you can get yellow and blue diamonds.
The Argyle diamond mine produced a multitude of coloured diamonds, but some, like blue diamonds, get their hue from trace impurities like boron. (Supplied: Murray Rayner)
But pink diamonds aren't pink because of trace elements. They get their rosy hue when extra pressure — say, from the collision of two continents or in a machine in a lab — slightly deforms their orderly 3D structure, which changes the colours of light allowed to pass through the crystal.
"If you press hard enough, those colourless diamond crystals slip a little bit, and you get a pink colour," Dr Olierook said.
"Press a little harder, and it goes darker pink, then briefly goes red.
"And then, press harder again, and it turns brown."
Journey from (not quite) the centre of the Earth
Despite being one of the strongest-known materials, diamonds are not necessarily forever (regardless of what advertisements claim).
If a diamond ascends to the surface too slowly, it will revert to its soft, grey graphite state, said Solomon Buckman, a geologist at the University of Wollongong who was not involved in the study.
So for a diamond in Earth's depths to remain a diamond on Earth's surface, it needs to come up fast — and the fastest way is to be propelled by a volcanic eruption.
To find out when the Argyle diamonds were brought to the surface, Dr Olierook and his team analysed material supplied by Rio Tinto from the mine, which included crystals trapped in thin slices of volcanic rock.
The dating techniques, Dr Buckman said, were widely used to date rocks with similar minerals, and could nail accurate age ranges.
The Argyle diamond mine was an open pit until 2010, and operations went fully underground in 2013. (Supplied: Murray Rayner)
Their calculation — that the volcano heaved the diamonds to the surface around 1.3 billion years ago — was around 100 million years older than an analysis conducted a few decades ago.
Crucially, the diamond deposit's revised age coincided with the supercontinent Nuna starting to break up.
As it did so, the researchers suggest that the Kimberley and Northern Australia continental blocks, which had collided a few hundred million years before, started pulling apart again.
This weakened the crust along the collision zone and let a jumble of magma, diamond-bearing rock and other minerals below to punch through kilometres of rock at 70 kilometres an hour, forming a volcanic pipe called a diatreme.
"Because the volume of the magma increases as it rises, diatremes end up being a carrot shape,"Dr Buckman said.
"Then when it gets to the surface, the magma solidifies and everything inside [the diatreme] is essentially frozen solid, including the diamonds."
Over the aeons, as the continents slowly shuffled around the globe, the top of the diamond-rich "carrot" weathered, letting a few diamonds loose.
For thousands of years, the gemstones were woven into Dreamtime stories.
And over the past 40 years, Rio Tinto extracted more than 865 million carats of rough diamonds from that single site.
Pink diamonds: not so rare after all?
Despite geologists now knowing how pink diamonds are formed and how "continental sutures" can bring them to the surface, finding similar deposits won't be easy.
Over hundreds of millions or even billions of years, the seams joining continental blocks can be covered by sand and soil, effectively disguising the top of diamond-bearing deposits that might be present.
The diamond-bearing "carrots" can also be quite narrow, maybe a few hundreds or tens of metres across at the widest point.
Still, Dr Olierook suspects pink diamond deposits are likely to be found anywhere that continental blocks have smooshed together, and not just during Nuna's existence.
"There's been at least three supercontinents we know of," he said.
"So any rocks that were deposited at the time they broke up, those are the ones I'd be looking at."