These tiny crystals are time capsules of early plate tectonic activity.

(ORDO NEWS) — Tiny zircon crystals dated to 3.8 billion years ago contain the earliest geochemical evidence of plate tectonic activity on Earth.

Isotopes and trace elements preserved in the crystals indicate that they were formed under subduction conditions – when the edge of one tectonic plate falls under the edge of the neighboring plate, creating special conditions. This allows us to re-evaluate the time of occurrence of plate tectonics on Earth.

Since plate tectonics has played a key role in creating the conditions for life on Earth by changing the composition of the oceans and atmosphere, understanding when and how it originated is also important for understanding how we got here and what makes the planet habitable.

Understanding the geology of the early Earth presents a challenge. Our world’s crust has been quite dynamic for 4.6 billion years, and the only direct record of the Hadean eon – between 4.6 and 4 billion years ago – can be found in crystals of the mineral zircon.

These crystals appear to have survived the ravages of time, but rarely, with just 12 locations on Earth, ancient grains have been found, and most locations have three or fewer.

However, recently a group of geologists discovered an amazing treasure. A chronological series of 33 microscopic zircon crystals dating from 4.15 to 3.3 billion years ago have been discovered in an ancient crustal block found in the Barberton Greenstone Belt in South Africa.

This series provided a rare opportunity to explore the changing conditions of the early Earth from the Hadean era to the Eoarcheon era, which lasted from 4 to 3.6 billion years ago.

Mineral crystals can serve as a kind of time capsule containing information about the conditions under which they formed, and zircon crystals in particular can be extremely valuable for these scientific purposes. Hafnium metal isotopes and trace elements found in zircon can be used to make inferences about the rocks from which they crystallized.

A team led by geologist Nadia Drabon of Harvard University has studied Greenstone Belt zircons to reconstruct the chronology of the conditions in which they formed.

They found that, starting around 3.8 billion years ago, the crystals had signatures of hafnium and trace elements similar to modern rocks formed in subduction zones – at the edges of tectonic plates.

According to the researchers, this suggests that plate tectonics was active during the formation of these crystals.

When I say “plate tectonics”, I mean precisely the arc structure, when one plate goes under another and volcanism occurs – remember, for example, the Andes and the Ring of Fire,” said Drabon.

“At 3.8 billion years [ago], there is an abrupt shift in which the crust destabilizes, new rocks form, and we see geochemical signatures becoming more and more similar to what we see in modern plate tectonics.”

Surprisingly, the zircon crystals, which are over 3.8 billion years old, were not formed in a subduction zone, but probably in the Hadean “protocrust”, which was formed from remelted mantle material before the mantle was depleted of basaltic melt elements by tectonic processes. .

The team then compared their finds with zircon crystals from around the same time from around the world to make sure they weren’t just seeing a local phenomenon. These other zircons showed similar transitions.

It’s hard to say for sure whether all these tiny grains point to the evolution of our world towards plate tectonics, but the results definitely indicate that global changes were taking place.

“We’re seeing evidence of significant changes on Earth around 3.8 to 3.6 billion years ago, and evolution toward plate tectonics is one clear possibility,” Drabon said.

“The data we have on Earth’s earliest periods is very limited, but seeing a similar transition in so many different places makes it quite likely that it could be a global change in processes in the Earth’s crust. There was some sort of reorganization going on on Earth.”

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