Geologists from the University of Adelaide think they know why cold eclogites vanished for 600 million years – and it’s all to do with the very first supercontinent.
“Cold eclogites mysteriously disappeared from the Earth’s rock record between 1.8 and 1.2 billion years ago,” says Derrick Hasterok, a geologist and co-author of the new study published in Geology.
These temperature-sensitive, unusually dense metamorphic rocks are formed in high-pressure environments. These environments are usually hot, but ‘cold’ eclogites can be created in cold conditions more than a hundred kilometres below the surface, where pressures can reach three gigapascals (about 30,000 times higher than the atmospheric pressure at sea level).
Such environments are found where tectonic plates collide – specifically in subduction zones, where an oceanic plate is diving under a continent.
“You take a rock that’s near the surface and you send it down into a subduction zone very rapidly, and because it’s with a bunch of cold rocks, it stays relatively cold, and it gets to really high pressures,” Hasterok says.
“Cold eclogites are one of the unambiguous records of subduction – so they’re telling us that there is plate tectonics operating somewhere on Earth at that time.”
This means they can help us understand how this dynamic planet has changed over time.
But eclogites aren’t always preserved, like the massive gap seen between 1.8 to 1.2 billion years ago.
Hasterok and colleagues from the University of Adelaide think they can now explain this gap – because during this time, the Earth’s first supercontinent, Nuna, was beginning to assemble.
“It was the first really large supercontinent that brought together all the little bits of content scattered across the planet,” Hasterok says. “When that amalgamated together, it created a large ‘blanket’ over the mantle, and that prevented the heat from escaping.”
As the heat built up, the eclogites “fell off” into the mantle and never returned to the surface. So although they were still being formed during this time, they never made it back where we could see them.
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The team came to solve the eclogite mystery by looking at something completely different. They were studying how the chemistry of granites around the world had changed during this same time period, which they suspected was indicative of temperature.
“So we’ve got two things that are indicating some change in temperature, specifically an increase in temperature, around two billion years [ago],” Hasterok says. “That’s where the story started to come together.”
Combined, the granites and the eclogites indicated large-scale heating of the continents around two billion years ago, when Nuna started to form.
Lead author Renee Tamblyn explains: “The Earth has generally been cooling since its formation but Nuna had an insulating effect on the mantle, rather like a thick blanket, which caused temperatures to rise beneath the continents and prevent the preservation of eclogites and change the chemistry of granites.”
But this isn’t the only time period when eclogites vanish. They also disappeared again about one billion years ago before reappearing 700 million years ago.
This coincides with the life of another supercontinent called Rodinia.
“It may be the same phenomenon, but they don’t disappear as long as they do after Nuna,” Hasterok says. “The reason might be that the interior of the Earth is generally cooling through time.”
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While Rodinia may have also trapped heat and prevented the preservation of eclogites, the Earth was overall even cooler, so eclogites didn’t disappear for as long.
This ‘cooling off’ idea is supported by the fact that later supercontinents – like Gondwana and Pangea – didn’t trigger similar disappearances.
“There is ample evidence for a nearly continuous geological record of cold eclogites over the past 700 million years, during which time two supercontinents formed and broke up,” Hasterok says.
This study doesn’t just tell us about how the Earth has changed through time. It could also tell us about how the geology of the planet is arranged right now.
Some of the trace elements that fundamentally changed during the assembly of Nuna are found in critical minerals, which are key for manufacturing phones, monitors, wind turbines, electric cars, solar panels and more.
“Your cell phone, just to give you an example, contains almost 90 elements,” Hasterok says. “A lot of the elements that give those really rich colours on the screen come from some of these trace elements that are formed in critical minerals.”
But some of these elements are mined only in a handful of locations across the planet, and sometimes they’re located in only a single country.
Knowing how the concentrations of these trace elements have changed since Nuna is useful, because it gives an indication of where we should look to find them now.
“So if we wanted to look for a specific element, we might choose to look at rocks that are younger than two billion years ago,” Hasterok says.
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Much of Western Australia, for example, is older than two billion years and contains some of the oldest rocks on the continent, while the eastern states are much younger.
“The rocks in the Northern Territory and northwest Queensland are a little older than the 1.8-billion-year mark, so [there] may be a place where we can continue our investigations into this mysterious geological case,” says Hasterok.