Ellen Phiddian
More than 1.3 billion years ago, two continents collided at modern-day Argyle in Western Australia, causing pressures so intense that it forced carbon deep underground to form diamonds with glittering pink, red and brown hues.
Then, when the earth stretched again at the 1.3 billion year mark, a fissure opened and the diamonds rocketed to the surface, creating the largest known deposit of diamonds in the world.
Or, at least, that’s the theory proposed by a study in Nature Communications, done by an Australian-based team of researchers.
The new formation theory could help geologists spot more pink diamond deposits elsewhere in the world.
The researchers used radiometric dating with lasers to figure out when pink diamonds, from the Argyle diamond mine, came to the surface, which allowed them to figure out how they formed.
“It’s actually very difficult to date how long a diamond’s been growing. That growth has to happen really deep in the earth: 150 kilometres plus,” says lead author Dr Hugo Olierook, from the John de Laeter Centre at Curtin University.
“But we figured out how they got from deep down there to the Earth’s surface in a matter of hours, days, weeks – rapidly, anyway.”
Previously, the Argyle diamond deposit was thought to be 1200 million years (or 1.2 billion years) old based on dating done in the 1980s. The researchers’ more accurate method moved this date back 100 million years.
“That timing of about 1300 million years ago, when Argyle came up, was coincident with the breakup of the first supercontinent: a supercontinent called Nuna,” says Olierook.
“The world was undergoing cataclysmic change – you had all the continents before that together in one landmass, and now you’re breaking it all apart. So you’re stretching bits of the crust, and by stretching it, you’re allowing these magmas from deep down to shoot up to creating little vertical pathways through the crust and to the Earth’s surface.”
This means Nuna’s breakup likely triggered a “diamond-bearing eruption” at Argyle.
“It would have been a fairly catastrophic event. And I imagine if that happened today in a built-up area it would be quite devastating,” says Olierook. He adds that an eruption like this hasn’t happened in the last million years.
The age of the diamonds helps explain the location of the Argyle diamond mine, which produced more than 90% of the world’s pink and red diamonds prior to its closure in 2020.
“Argyle is pretty unique in that it was the very first diamond deposit ever known that was sitting not in the middle of one of these continents, but at the juncture or the suture, if you like, of two,” says Olierook.
Other diamond deposits have been in the centres of ancient continents, but this discovery opens the door to finding more Argyle-like sites.
“Trying to find them is not going to be easy,” says Olierook.
“These junctures of continents are now often covered by sand and soil, so you have to look for the diamonds under all that dirt. And that’s a challenge.”
Even Argyle, rich as it was, yielded only a few grams of diamond per tonne of rock – and most of the diamonds were brown, rather than that coveted pink shade.
“The pinks, reds and browns are damaged diamonds,” explains Olierook.
“They’ve been subjected to such intense pressures that those diamonds have actually deformed – they’ve slipped a little along a particular plane. Those crystals slip and slide and twist and bend, and if you push just hard enough, they go pink. You push a little bit too hard, they briefly go red, and then they turn brown.”
Nevertheless, the researchers have some tips for diamond-seekers – like mining company Rio Tinto, which provided the Argyle gemstones for researchers to date.
“Where I’d look, if I was the one doing the exploration, is along these ancient mountain belts that are surrounding these ancient continents, in any rock types that have been affected by continental breakup,” says Olierook.
