Western Australia’s Argyle mine was among nature’s preeminent treasure troves for nearly 40 years. At its peak, Argyle produced more coloured diamonds than anywhere else on Earth and earned an especially sparkling reputation for its unparalleled cache of pink diamonds.
Researchers have spent decades trying to unravel the origins of Argyle’s glimmering gems. Now, by dating minerals in the mine’s volcanic rock, scientists think they may have finally pieced together the process that created the deposit around 1.3 billion years ago. In a paper published on Tuesday in Nature Communications, the team posits that the breakup of an early supercontinent lifted Argyle’s salmon-coloured stones from crushing depths toward Earth’s surface.
Located 2,200 kilometers northeast of Perth, Australia, in the country’s rugged Kimberley region, Argyle mine once covered an area the size of 94 football fields. Between its opening in 1983 and closure in 2020, when mining the gems there was no longer economically viable, Argyle produced more than 865 million carats of rough diamonds. Most of these stones come in pale shades of yellow or brown. But a small percentage of the site’s diamonds radiate rich pinks, purples or reds. More than 90 percent of the world’s pink diamond supply including the nearly 13 carat Pink Jubilee has come from Argyle.
The pink hue of Argyle’s most lavish diamonds is linked to damage they underwent deep within the earth. According to Hugo Olierook, a geologist at Curtin University in Perth and lead author of the new study, these diamonds start out colourless. But immense tectonic pressure from colliding continents can alter the stones’ crystal structure, unlocking the potential colours hidden within. “The diamonds are being forced to bend and twist,” Olierook says. “If they’re twisted just a little bit, it will turn some of these diamonds pink.” Further twisting makes them become brown.
Argyle’s diamonds took on their pink and brown tints around 1.8 billion years ago, when a piece of what is now western Australia smashed into the northern Australian plate and warped the region’s rock. But this only explains part of Argyle’s origin story. When the continents collided, the area’s diamonds were buried in the mantle, hundreds of kilometers below Earth’s surface. If the crystals had been closer to the surface, their carbon atoms would have been compressed into a different structure, transforming them from shimmering diamonds to lumps of dark gray graphite.
A volcano was necessary to bring the molten diamonds up from our planet’s mantle. “You need some sort of tectonic trigger to bring them up to the surface,” Olierook says. As the melt rises, carbon dioxide and steam expand, sparking an eruption that he compares to popping a champagne cork. At Argyle, this eruption likely occurred at a beach, where sand and seawater interacted with volcanic rock called lamproite.
To determine when the eruption occurred, the team sliced two thin sections of Argyle’s volcanic rock and polished them down to a minuscule width. Analyzing the sample’s mineral makeup under a microscope, the researchers were able to pinpoint sand grains from Argyle’s ancient beach and to date them with the help of radioactive elements they contained. By dating the youngest sand grains, the scientists were able to estimate when the beach was buried in lava. They also used tiny lasers to determine the ages of titanite minerals, which formed in the rock when the magma melded with quartz in the beach sand.
Comparing the ages of the youngest sand grains and the oldest titanite crystals allowed the researchers to estimate that the eruption at Argyle occurred between 1.3 billion and 1.26 billion years ago. This age range was older than previous estimates, which surprised Olierook and his colleagues. “We had a betting pool going, and nobody got 1,300 million years,” he says. “That was one of those glass shattering moments.”
That eruption timing corresponds to a volatile period in Earth’s tectonic history when one of the first supercontinents, called Nuna, was splintering apart. The team posits that this instability may have reopened a seam along the continental boundary where Argyle is now situated. This in turn sparked the volcanic activity that brought the diamond-bearing melt toward the surface, creating Argyle’s expansive diamond deposits.
The new time estimates add crucial context for understanding the volcanic eruption at Argyle, says Evan Smith, a researcher at the Gemological Institute of America, who researches the geology of diamonds but was not involved in the new study. “The previous age constraint for Argyle was younger, and it was a lot less clear how to frame the eruption in a broader geological context,” Smith says. He thinks the new study adds exciting evidence that these “eruptions are related to bigger processes that affect whole continents rather than being isolated, random burps of magma.”
Olierook thinks similar events may have occurred at other continental boundaries around the world. Most diamond-bearing deposits are found in the middle of continental plates where rock is exposed. This makes Argyle an outlier. When the mine was first discovered, most geologists thought that searching for diamonds along continental plate boundaries which are often uplifted by ancient mountain belts and buried beneath soil and sand was futile.
Though gem mining in these regions remains difficult, Olierook believes there are plenty of diamonds to be found in the rough. “I think all of them will host some sort of coloured diamonds,” he says. “They may all be brown, but with a little bit of luck, there could be a few pinks in there.”
Source: Jack Tamisiea Scientificamerican
