A handful of ancient zircon crystals discovered in South Africa hold the oldest evidence of subduction, a key component of plate tectonics, according to a new study published in the open-access journal Advances AGU.
These rare time capsules from Earth’s youth point to a transition some 3.8 billion years ago from a rigid rocky surface to the active processes that shape our planet today, providing a new clue in a heated debate over when plate tectonics was set in motion.
The earth’s crust and the upper layer of the mantle just below it are broken up into rigid, slowly moving plates above the viscous but moving lower layers of mantle rock. The heat of the Earth’s core drives this slow but inexorable movement, responsible for volcanoes, earthquakes and the lifting of mountain ranges.
Estimates of when this process accelerated and the modern crust formed range from over 4 billion years ago to just 800 million years ago. The uncertainty arises because geologic records of the Earth’s youth are sparse, due to the surface recycling effect of plate tectonics itself. Almost nothing remains of the Hadean Eon, Earth’s first 500 million years.
“Hadean Earth is this big mystery box,” said Nadja Drabon, a Harvard University geologist and lead author of the new study.
In an exciting step towards solving this mystery, in 2018 Drabon and his colleagues unearthed a time series of 33 microscopic zircon crystals from a rare, ancient block of crust in the Barberton greenstone belt in South Africa, which formed at different times during a critical period. 800 million years ago, 4.15 to 3.3 billion years ago.
Zircon is a relatively common accessory mineral in the Earth’s crust, but ancient representatives from the Hadean Eon, 4-4.56 billion years ago, are extremely rare, found in only 12 places on Earth, and generally fewer than three at each location.
Hafnium isotopes and trace elements preserved in greenstone belt zircons told a story about conditions on Earth at the time of their crystallization. Zircons 3.8 billion years old and younger appeared to have formed in the rock being squeezed and melted like modern subduction zones, suggesting that the crust may have begun to move.
“When I talk about plate tectonics, I’m specifically referring to an arc, when one plate goes under another and you have all this volcanism,” Drabon said, describing a classic example of subduction.
“At 3.8 billion years ago there is a dramatic change where the crust is destabilized, new rocks are formed and we see geochemical signatures becoming more and more similar to what we see in modern plate tectonics. “Drabon said.
In contrast, the older zircons retained evidence of a global “protocrust” cap derived from the remelting of mantle rock that had remained stable for 600 million years, the study found.
The new study found a similar transition to conditions resembling modern subduction in zircons from other places around the world, dating back around 200 million years from South African zircons.
“We see evidence of significant change on Earth around 3.8 to 3.6 billion years ago and evolution towards plate tectonics is a clear possibility.” Dragon says.
Although not conclusive, the results suggest that a global shift may have begun, Drabon said, possibly starting and stopping in scattered locations before settling into the effective global engine of constantly moving plates that we see today.
Plate tectonics shapes the Earth’s atmosphere as well as its surface. The release of volcanic gases and the production of new silicate rocks, which consume large amounts of carbon dioxide from the atmosphere, dampen the large variations in temperature caused by too much or too little greenhouse gases.
“Without all the recycling and the formation of new crusts, we could go back and forth between boiling hot and freezing cold,” Drabon said. “It’s a bit like a thermostat for the climate.”
Plate tectonics has, so far, only been observed on Earth and may be essential for making a planet habitable, Drabon said, making the origins of plate motions interesting for research into early planetary development. life.
“The record we have for the oldest Earth is really limited, but just seeing a similar transition in so many different places makes it really possible that it could have been a global change in the processes crustaceans,” Drabon said. “Some kind of reorganization was happening on Earth.”