A unique rock formation in China holds clues that tectonic plates subducted, or passed under other plates, during the Archean eon (4 billion to 2.5 billion years ago), just as they do today, according to a new study.
This 2.5 billion-year-old rock, known as eclogite, is rare, forming when oceanic crust is pushed into the mantle (the layer between the crust and the core) at relatively low temperatures. This high-pressure, low-temperature rock type is “largely confined to subduction zones on present-day Earth,” study co-lead researchers Timothy Kusky and Lu Wang, Earth scientists at China’s University of Geosciences, told the BBC. Live Science in an email. .
The study reveals the oldest known eclogites from an ancient mountain belt found in Earth’s oceanic crust, the researchers said. The next oldest rocks of this type – 2.1 billion year old rocks in the Democratic Republic of Congo — that’s about 400 million years younger, the researchers said.
While this isn’t the oldest evidence of plate tectonics on record – a 2021 study, for example, dated plate tectonics to about 3.6 billion years ago — the new finding is a valuable data point that shows that tectonic plates subduced under each other in the “early days” of Earth, at least geologically speaking.
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Tectonic plates — the moving slabs that make up the Earth’s outer crust — are responsible for cycling materials and elements from the depths of the Earth to its oceans, surfaces and atmosphere. For decades, the research team worked to understand Earth’s early history and evolution, “from the time it formed and cooled from a molten ball of magma in space” to when it solidified, forming a rigid outer crust that evolved to the tectonic plate. system we have today, Kusky and Wang said.
Plate tectonics are crucial for warming the planet. Due to the movement of tectonic plates, “heat is lost from the interior, like bread floating and moving in a pot of hot stew boiling below,” they said. “If the transition to a tectonic plate Earth happened early, or whether the planet evolved through different stages dominated by different heat loss mechanisms, is one of the most unresolved and debated questions in Earth science today.”
That’s why, over the past 20 years, the research team has mapped rocks from Archean aeons spanning about 1,600 kilometers in northern China – “an ancient mountain belt, called the orogen, recording the location where two tectonic plates collided about 2 .5 billion years ago,” said Kusky and Wang.
Many features in these rocks indicate that this ancient mountain belt formed as tectonic plates interacted with each other. For example, fragments of oceanic crust called ophiolites are trapped in the former collision zone, as are highly deformed mixtures of rocks called mélanges (French for “mixtures”) that mark the points where plates collided, the researchers said. The team also found large folded structures, called nappes, that tectonic plates pushed from hundreds to thousands of kilometers away.
High pressure, low temperature
The discovery of eclogites within the mixture reveals that a tectonic slab of oceanic crust subducted under another plate, metamorphosing – that is, having its composition, texture or internal structure altered by heat and pressure – as it plunged deep into the mantle.
It is rare to find Archean eclogites, which “led to a claim that modern plate tectonics did not operate in the Archean,” Kusky and Wang said. “So finding eclogite, a key indicator of deep and cold subduction, is very significant.”
A laboratory analysis of the eclogites from the site revealed that they formed on an expanding oceanic ridge about 2.5 billion years ago, were transported across the ocean floor and then pushed into the mantle by subduction. Microstructures in the garnet and clinopyroxene minerals indicate that they reached temperatures between 1,458 and 1,634 degrees Fahrenheit (792 and 890 degrees Celsius) and high pressures between 287,000 and 355,000 pounds per square inch (19.8 and 24.5 kilobars).
These numbers suggest that eclogites subduced to at least 65 km depth, the researchers reported. Put another way, these findings are similar to data from minerals found in modern subduction zones, Kusky and Wang said.
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Eventually, pressure from the two colliding tectonic plates squeezed the dense rocks back to the surface, “like a watermelon seed between wet fingers,” Kusky and Wang said.
“I think it’s a very interesting study,” Nicolas Greber, a professor of geochemistry at the Geneva Museum of Natural History and the University of Bern’s Institute of Geological Sciences, who was not involved in the research, told Live Science. “These eclogites are important because they not only show that there were subduction zones at that time, but actually these subduction zones were quite steep.”
Still, the findings aren’t all that surprising or all that new, said Roberta Rudnick, an earth science professor at the University of California, Santa Barbara, who was not involved in the study. “It’s not really particularly new, in my opinion,” as other researchers have reported about ancient eclogite and eclogite minerals. trapped inside diamonds that came about through volcanic tubes that “have been very well studied over the decades,” she told Live Science.
Rudnick added that “the whole topic of when plate tectonics start is definitely an unresolved issue. But I think most of the community would have had no problem with plate tectonics in operation 2.5 billion years ago. [ago].”
The study was published online April 4 in the journal Proceedings of the National Academy of Sciences.
Originally published on Live Science.