by A unique rock formation in China contains clues that tectonic plates subducted or passed under other plates during the Archean Eon (4 to 2.5 billion years ago), just as they do ours days, according to a new study.
This 2.5 billion year old rock, known as eclogite, is rare and forms when oceanic crust is pushed deep into the mantle (the layer between the crust and the core) at relatively low temperatures. . This type of high-pressure, low-temperature rock is “largely confined to subduction zones on present-day Earth,” co-researchers Timothy Kusky and Lu Wang, Earth scientists at the University, told Live Science in an email. Chinese University of Geosciences. .
The study reveals the oldest known eclogites of an ancient mountain belt found in Earth’s oceanic crust, the researchers said. The oldest rocks of this type — 2.1 billion year old rocks in the Democratic Republic of the Congo – are around 400 million years younger, researchers say.
Although it is not the oldest evidence of plate tectonics on record – a study from 2021, for example, dated plate tectonics about 3.6 billion years ago – the new discovery is a valuable data point showing that tectonic plates were subducting under each other in Earth’s “early” days, at least geologically.
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Tectonic plates – the moving slabs that make up the outer crust of the Earth – are responsible for the cycle of materials and elements from deep within the Earth to its oceans, surfaces and atmosphere. For decades the research team worked to understand the history and evolution of Earth, “ever since it formed and cooled from a ball of molten magma in space” until it solidified, forming a rigid outer crust that evolved into plate tectonics. system we have today, Kusky and Wang said.
Tectonic plates are crucial for global warming. Due to shifting tectonic plates, “heat is lost from within, much like bread floating and moving over a pot of boiling stew below,” they said. “That the transition to plate tectonics Earth occurred 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 Archean aeonic rocks extending about 990 miles (1,600 kilometers) across northern China – “an ancient mountain belt, called orogen, recording where two tectonic plates collided about 2.5 billion years ago,” Kusky and Wang said.
Many features of these rocks indicate that this ancient mountain belt was formed when tectonic plates interacted with each other. For example, fragments of oceanic crust called ophiolites are trapped in the former collision zone, as are mixtures of highly deformed rocks called mixtures (French for “mixtures”) that mark the places where the plates collided, have said the researchers. The team also discovered large folded structures, called nappes, that plate tectonics pushed back hundreds to thousands of miles.
High pressure, low temperature
The discovery of eclogites in the mixture reveals that a tectonic plate of oceanic crust subducted under another plate, metamorphoses – that is, its composition, texture or internal structure is altered by heat and pressure – by plunging deep into the mantle.
Finding eclogites from the Archean eon is rare, which “led to assert that modern plate tectonics did not work in the Archean,” Kusky and Wang said. “So finding eclogite, a key indicator of deep, cold subduction, is very meaningful.”
A laboratory analysis of eclogites from the site revealed that they formed on an expanding ocean ridge about 2.5 billion years ago, were transported across the ocean floor, and then pushed in the mantle by subduction. Microstructures of 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 figures suggest that the eclogites subducted at least 65 km deep, the researchers reported. That is, these findings are similar to data on minerals found in modern subduction zones, Kusky and Wang said.
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Eventually, pressure from the two colliding tectonic plates brought the dense rocks to the surface, “much like a watermelon seed between your wet fingers,” Kusky and Wang said.
“I think it’s a very interesting study,” Nicolas Greber, professor of geochemistry at the Natural History Museum in Geneva and the Institute of Geological Sciences at the University of Bern, told Live Science. did not participate in the research. “These eclogites are important because they show not only that subduction zones existed at that time but in fact that these subduction zones could have been quite steep.”
Even so, the results aren’t too surprising or even that new, said Roberta Rudnick, a professor of earth sciences at the University of California, Santa Barbara, who was not involved in the study. “It’s not really new, in my opinion,” as other researchers have already pointed out, as are ancient eclogites and eclogite minerals. trapped inside diamonds which passed through volcanic pipes that “have been very well studied over the decades,” she told Live Science.
Rudnick added that “The whole topic of early plate tectonics is definitely an unresolved question. But I think most of the community would have no problem with plate tectonics being up and running by 2 .5 billion years. [ago].”
The study was published online April 4 in the journal Proceedings of the National Academy of Sciences.
Originally posted on Live Science.
