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These tiny crystals are “time capsules” of the tectonic activity of the Earth’s early plates

Tiny zirconium crystals dating to 3.8 billion years ago contain the oldest geochemical evidence of plate tectonic activity here on Earth.

Isotopes and trace elements stored in the crystals indicate that they formed under subduction conditions – when the edge of one tectonic plate slides below the edge of an adjacent plate, creating specific conditions. This provides new constraints on when plate tectonics appeared on Earth.

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

Understanding the geology of early Earth is like a challenge. The crust of our world has been pretty dynamic during its 4.6 billion years of history, and the only direct record of the Hadean eon – 4.6 to 4 billion years ago – can be found in the crystals of the mineral zircon.

These crystals seem to have survived the ravages of time, but rarely: only 12 places on Earth provided ancient grains, in most places three or less.

Recently, however, a team of geologists discovered an amazing treasure. A chronological series of 33 microscopic zircon crystals, dating from 4.15 to 3.3 billion years ago, was found in an ancient crust block found in the Barberton Greenstone Belt in South Africa.

The series provided a rare opportunity to explore the changing conditions of the early Earth, from the Hadeans to the Eoarchaeon era, which ran 4 to 3.6 billion years ago.

Mineral crystals can act as a kind of time capsule that contains information about the conditions in which they formed, and zirconium crystals in particular can be extremely valuable for this scientific purpose. Hafnium metal isotopes and trace elements found in zircon can be used to draw conclusions about the rocks from which they crystallized.

A team of scientists led by geologist Nadja Drabon of Harvard University studied Greenstone Belt zircons to reconstruct the timeline of the conditions under which they arose. They found that about 3.8 billion years ago, the crystals had hafnium and trace elements similar to modern rocks formed in subduction zones – at the edges of tectonic plates.

This suggests that plate tectonics was active at the time these crystals formed, scientists said.

“When I say plate tectonics, I mean specifically the setting of the arc, where one plate falls below the other and you have all the volcanism – remember, for example, Andy and the Circle of Fire,” Drabon said.

“In 3.8 billion years [ago] there is a dramatic shift as the crust destabilizes, new rocks form, and we see that the geochemical signatures are increasingly similar to what we see in modern plate tectonics. “

Fascinatingly, zirconium crystals older than 3.8 billion years did not form in subduction zones, but probably crystallized in the Hadean “protocol”, which formed from remelted mantle material before the mantle was depleted by tectonic elements of the basalt melt. processes.

The team then compared their findings with zirconium crystals from around the same time from around the world to make sure they did not only observe a localized phenomenon. These additional zircons showed similar transitions.

It is difficult to know exactly whether all the small grains point to the development of our world towards plate tectonics, but the results definitely indicate that global change has taken place.

“We see evidence for a significant change on Earth about 3.8 to 3.6 billion years ago, and evolution towards plate tectonics is one of the clear possibilities,” Drabon said.

“The record we have for the oldest Earth is really limited, but just seeing a similar transition in so many different places really allows for a global change in the Earth’s crust. There has been some reorganization on Earth.”

The research was published in AGU Advances.