Not bad at all. I do think that a kimberlite pipe is mostly carbon when it erupts and of course avsuper liquid and supersonic. The carbon is mostly consumed right after the eruption and is of course mostly missing.
At least we have now understood these eruption cycles are linked to to plate rifting.
that still puts them in the center of cratons which means deep roots where liquid carbon could form.
The one in Tanzania may have been triggered by the Pleistocene nonconformity which saw the whole crust shifted fifteen degrees. At least we no longer need to project a massive liquid carbon layer.
Why supersonic, diamond-spewing volcanoes might be coming back to life
Strange volcanos called kimberlites bring diamonds up from Earth's depths. Scientists have always struggled to understand why they switched off millions of years ago – but perhaps they didn't
19 March 2024
Armand Sarlangue
https://www.newscientist.com/article/mg26134830-100-why-supersonic-diamond-spewing-volcanoes-might-be-coming-back-to-life/
Twenty years ago, deep beneath Botswana’s Kalahari desert, Thomas Gernon found himself walking in what seemed like hell. The temperature soared as the sound of explosions echoed off the walls. “It was like a baptism of fire,” he says. It was his first trip into a kimberlite diamond mine.
The place teemed with cameras and Gernon, now at the University of Southampton in the UK, had been warned of trouble should any gemstones be found – accidentally or otherwise – on his person. But he wasn’t here to find his fortune. He wanted an answer to one of Earth’s greatest mysteries.
Diamonds are precious to many, but they hold a special place in the hearts of geologists. They were forged long ago in the fiery depths of Earth’s inaccessible mantle, brought to the surface by riding on supersonic jets of magma from bizarre volcanoes called kimberlites.
We don’t know much about precisely how diamonds formed, but we do know they are like time capsules that can teach us the secrets of our planet’s distant past. And perhaps the biggest question of all is why the kimberlites that propelled them to the surface seem to have gone extinct millions of years ago.
Now, almost two decades since that first visit to a diamond mine, Gernon and his fellow kimberlite detectives may finally have an all-encompassing model for how the volcanoes work, and with it a better understanding of their treasures. What’s more, this work has uncovered a tantalising prospect – that kimberlites might not be extinct after all.
Diamonds, contrary to popular belief, don’t come from compressed coal. Precisely how they are made isn’t clear, but geochemistry studies and laboratories that mimic the extreme temperatures and pressures of Earth’s depths have provided clues. Some are thought to germinate in the mantle, a solid-but-fluid layer where carbon-rich material becomes so jam-packed that excess carbon is squeezed out to grow as pure crystal prisms. Others could be made as a tectonic plate is pushed into the mantle. As it descends, it drags organic matter, including carbonate minerals from the ocean, with it. The rising temperatures and pressures make those carbonates chemically unstable, popping out the carbon and forging diamonds.
How are diamonds formed?
There are probably dozens of other ways to make diamonds, too. But no matter the process involved, they are usually primordial. The vast majority of diamonds are more than a billion years old. “The oldest we’ve analysed are like 3.5 billion [years old],” says Steve Shirey, a geochemist at the Carnegie Institution for Science in Washington DC.
Diamonds are survivors, capable of withstanding almost anything. This makes them deep Earth historians. Flecks of minerals and fluids known as inclusions that date from billions of years ago are imprisoned inside them when they form, preserving chemical records of what the primeval Earth was like. Thanks to this, we know, for example, that “the mantle has at least another ocean’s worth of water” scattered within it, says Yana Fedortchouk, an experimental petrologist at Dalhousie University in Halifax, Canada.
The only way diamonds see daylight is by riding along on a very strange sort of volcanic eruption. Until 1869, the gems were only recovered from sediments left behind by rivers. But that year, they were found inside magmatic rocks at various South African farms, including some in Kimberley. These rocks were named kimberlites, and they are the product of extreme volcanic events.
To date, around 6000 kimberlite formations have been discovered. Most are in southern Africa, but they have been found on every continent, including Antarctica. About 3 per cent contain sizable diamonds. All of them have volcanic origins. And compared with the classic sort of mountainous volcanoes, kimberlite volcanoes are extremely weird.
The Mir mine is a kimberlite diamond quarry in Mirny, eastern Russia
Aleksey Suvorov/Alamy
The first feature that sets them apart is their magma. In most volcanoes, bubbling molten rock is left to simmer within Earth’s crust, where it transforms into the complex lavas that erupt on Earth today. But kimberlite volcanoes skip that step. Their magma contains fluids and minerals – and even chunks of semi-intact rock – from deep within the mantle. “Kimberlites erupt deeper portions of Earth’s mantle, and they erupt more of it as well,” says Graham Pearson, a geochemist at the University of Alberta in Canada.
This means that kimberlite eruptions must happen quickly. As it ascends from the mantle into the crust, the carbon in the erupting material transforms into carbon dioxide gas, turning the magmas into a frothy missile. This is like “a rocket going off”, says Janine Kavanagh, a volcanologist at the University of Liverpool, UK. As the magma accelerates, it carries up diamonds and entire pieces of mantle rock. “You have stuff that is coming up supersonically, and at some point in the upper crust – kaboom,” says Andrea Giuliani, a kimberlite researcher at ETH Zurich in Switzerland.
Nobody has ever witnessed a kimberlite eruption, so geologists piece details together to come up with potential scenarios. Maybe the scorching-hot magma encounters some cool water, triggering violent, steamy blasts. Perhaps the trapped gas within its molten matter angrily bubbles up. Either way, one explosion leads to another, then another, then another.
If the eruption breaches the surface, it would make a crater that expands while the magma channel widens. “You’d have pyroclastic flows, you’d have a big eruptive plume,” says Kavanagh. Explosions over many hours open a wound that digs deeper and deeper into the surface. Eventually, you may get lava fountaining or oozing out. And when it is all over, you are left with a gaping crater, some splinters of solidified magma known as dikes, and a pipe extending into Earth’s crust choked with shattered volcanic rock and other debris.
Now, millions of years after their eruptions, what remains of kimberlite volcanoes isn’t mountains. In fact, their architecture is frequently subterranean. Their most prominent features are buried, vertical pipes of rock, sometimes several kilometres in length. These are thought to be the conduits that once furiously funnelled magma to the surface. Some are “more like champagne glasses, some look more like a carrot”, says Giuliani.
Most kimberlite volcanoes are mines for diamonds
But the most perplexing fact to geologists is that kimberlite volcanism seems to have completely shut down. They started to erupt about 2 billion years ago and reached a peak roughly 100 million years or so ago, when the dinosaurs ruled the world. For a long time, the youngest-known kimberlite was 30 million to 40 million years old. After that, they vanish from the geologic record.
This bewitches and confounds scientists in equal measure. They have a hard enough time decoding the behaviours and triggers of actively erupting volcanoes. Understanding long-dead kimberlites is considerably more difficult—the volcanological equivalent of archaeology. And it doesn’t help that the pipes they form are filled with a chaotic jumble of smashed-up evidence. “Most of the rocks are complete rubbish,” says Giuliani. “You can’t study them.” Altogether, this means that “there’s this whole debate over the driving cause of kimberlite magmatism”, says Philip Janney, a geochemist at the University of Cape Town in South Africa.
Are kimberlites extinct?
In an attempt to solve these puzzles, geologists like Gernon have spent a lot of time in diamond mines. While those working in other branches of geology have places they can revisit time and time again for field work, kimberlites are frequently blasted apart by prospectors hoping to find diamonds. The evidence Gernon needs is “now either on someone’s ring, or crushed up”, he says.
This is why, over the years, he has often descended into various chasms carved out of the landscape by miners in both Botswana and South Africa. Sometimes, he was driven into them along expansive, spiralling roads. In more claustrophobic mines, “you get in this cage, and you’re dropped down this lift shaft,” says Gernon. On occasion, “there’s loads of baboons running around”, he says.
It was only after he had completed his PhD, in 2007, that Gernon glimpsed his first diamond. He was wandering through a mine hidden beneath a frozen lake in the remote Canadian Arctic. This time, he was shivering at temperatures of -50°C. The contrast with the African mines “was supremely strange”, he says. Suddenly, the light from his torch glinted off something stuck in the wall. “It was this perfect little triangle. It was beautiful. And I knew exactly what it was,” he says: a 10-carat diamond. “I got very attached to it!” He wasn’t, of course, allowed to keep it.
But he wasn’t there for the diamonds. Instead, Gernon and his colleagues have examined the non-diamond minerals and chemistry associated with kimberlites sufficiently over the past few decades to build up a picture of the behaviour and potential origins of these volcanic eruptions. What we know for certain is that they plumb the depths of the mantle, are ancient and are packed with various forms of carbon.
And then there was the most surprising finding. As more and more kimberlites were dated, it became apparent that the eruptions didn’t happen continuously all over the world. “Kimberlites pulse through time,” says Pearson. There was a spike in such eruptions around 500 million years ago. Then another 370 million years ago. Then another when the dinosaurs reigned supreme. But between these fiery episodes, kimberlite volcanism was uncommon.
The dates of these eruptive epochs seem strange, at first. But geologists saw order in this chaos: kimberlitic fireworks coincide with the breakup of continents, including supercontinents, like Pangaea, atop which the dinosaurs roamed. Perhaps, some thought, the wounds that open as continents fragment could have created pathways through Earth’s innards for the kimberlites to exploit.
There was just one problem. Almost all kimberlites are found within cratons, the colossal, 200-kilometre-thick cores of continents, which don’t experience such rifts. Cratons are several billion years old. Even when supercontinents are broken, these cores remain intact. That meant kimberlite magmas picked the thickest, toughest parts of the continents to puncture through – the path of most resistance – something most eruptive activity tends to avoid.
This 0.55-carat diamond was found in a kimberlite in South Africa
Bjorn Wylezich/Alamy
This seemed impossible to explain – until last year, when Gernon and his colleagues had an epiphany that tied everything together. While comparing the emergence of kimberlites with the timing of various continental fragmentations, the team realised that kimberlite eruptions don’t happen immediately after rifts first appear; most erupt about 26 million years later. And while some explode near the continent’s new schisms, the majority emerge within the thick continental interiors.
Gernon and his team plugged these ideas into a computer simulation of Earth’s tectonic plates, one advanced enough to authentically recreate how the shifting plates stir tides in the mantle below. As they watched, an elegant cascade took place. Their results explain where kimberlites get the carbon that allows them to erupt so furiously from and – crucially – how and why they break through the cratons.
“When you stretch the continent, you create this space, and all the hot stuff gushes up to fill this space,” says Gernon. That upwelling causes swirling tempests of fluid in the lower, more malleable part of the mantle. That first tempest, below the rifting site, creates another instability next to it, then another, and another, starting to weaken the tough craton. “It’s like a chain reaction,” says Gernon. “That’s when we knew we were onto something.”
These maelstroms erode the continent’s rocky underbelly, carrying off some of its water-soaked rocks. The base of the continent’s interior happens to have an abundance of carbon-rich rocks. And here, at such great depths in the mantle, the extremely high-pressure environment cooks them in just the right way to produce that first carbon-rich batch of kimberlite magmas.
When enough batches of this are made, they rocket up through the chewed-up continental core, creating a dazzling array of kimberlite eruptions roughly 26 million years after the continent began to be torn apart.
Gernon doesn’t like to call this work his “eureka” moment. But there is perhaps no better way to sum up his team’s paper, published in Nature last year, which independent scientists enthusiastically received. “I think that goes as far as anybody to provide a universal picture [on the origins of kimberlites],” says Russell.
A “new” kimberlite eruption
But the story may not end there. For a long time, the youngest known kimberlite was about 30 million years old. But in 2012, scientists were poking around in a series of shattered volcanic rocks and small cones in Tanzania, comprising three eruptive sites known as the Igwisi Hills volcanoes. Nobody had burrowed below the surface or peered into the crust using ground-penetrating seismic waves. But when they did, they found that the chemistry of the lavas here were unmistakably kimberlitic. And some of the volcanic rocks were dated to just 10,000 years ago or so.
Suddenly, Igwisi Hills became the youngest kimberlite volcanoes on Earth – shockingly young, born in the geologic equivalent of yesterday. This baffled researchers, since the world, right now, is tectonically calm. We aren’t in a period of supercontinental annihilation.
But this revelation gels with the origin story of kimberlites as told by Gernon’s team. Tanzania sits on a craton, and it is also influenced by the East African Rift, a massive rupture between two divorcing tectonic plates that began 25 million years ago. What this means is that the Igwisi Hills volcanoes could represent the prelude to a new era of kimberlite volcanism in this region. “It seems a good test of the model,” says Gernon, smiling.
Perhaps, one day, someone walking along the shores of East Africa will see a fountain of fire erupt far inland, illuminating the night sky. Diamonds, trapped for billions of years deep below, will scatter across the landscape. And scientists will watch in awe, wondering what new secrets the planet has decided to unearth.
No comments:
Post a Comment