Plate tectonics are responsible for the current shape of our Earth. But when did it start? Researchers have now found evidence of this on the basis of tiny zirconium crystals that are between 3.3 and 4.15 billion years old. The geochemical composition of these crystals indicates that the oldest formed before the Earth’s crust began to move. However, zircon up to 3.8 billion years old appears to have formed in regions where one tectonic plate bulges under the other. Thus, crystals provide the oldest evidence to date of the beginnings of plate tectonics.
It’s rare to look back on Earth’s early days. Rarely has any material survived billions of years and can still be examined to this day. One of the reasons for this is the so-called “recycling” of the Earth’s crust. One tectonic plate slides under the other. The crustal material that is pushed to the depths in the Earth’s mantle is melting. Mantle material rises at mid-ocean ridges and forms new crust. But an exceptional mineral can survive even the harsh conditions in this recycling process: zircon, the oldest known mineral on Earth. Like time capsules, the crystals offer a way to draw conclusions about conditions on Earth about four billion years ago.
Find clues in crystals
A team led by Nadia Drapon of Harvard University in Cambridge examined a series of zircons that were discovered in 2018 during excavations in the Barberton Greenstone Belt in South Africa. The crystals, about the size of a grain of sand, formed at different time points between 4.15 and 3.3 billion years, that is, exactly at the time when, according to current knowledge, plate tectonics should have started. Using a chronological series of 33 zirconium crystals, the researchers were able to understand how the Earth’s crust evolved over the course of 800 million years.
They focused on three different geochemical features of the existing crystals: hafnium isotopes, oxygen isotopes and trace element composition. Each of these features gave them a different piece of the puzzle. Isotopes of hafnium gave clues to the formation and evolution of the Earth’s crust, isotopes of oxygen to see if there were oceans, and trace elements for the formation of the crust.
Troubles 3.8 billion years ago
The result: isotopes of hafnium and trace elements in the oldest zircons showed that they had formed in a global “protext” that had been stable for millions of years. In contrast, zircon, which is 3.8 billion years old or younger, appears to have formed in rocks that have undergone pressure and melting similar to recent subduction zones, regions where one plate slides under the other. “In 3.8 billion years, there is a major shift: the crust is destabilized, new rocks are forming, and the geochemical signals become more like what we see in modern plate tectonics,” Drapon says.
This indicates that about 3.8 billion years ago the Earth’s crust split into plates that then began to shift against each other. “In the case of oxygen isotopes, on the other hand, no significant change could be observed initially,” the researchers reported. Only for zircons that are 3.5 billion years old or younger do we see evidence that they formed in ancient parts of the crust that were altered by contact with liquid water. This may indicate the presence of more developed plate tectonic and volcanic activity in the island arcs and other plate boundaries bordering the sea.
The researchers also compared their results with data on ancient zircons from other parts of the world. “We are seeing evidence of significant change on Earth about 3.8 to 3.6 billion years ago, and evolution into plate tectonics is a distinct possibility,” Drapon says. “The record that we have of the closest planet to Earth is very limited, but seeing a similar shift in so many different places really makes it possible to imagine that it would have been a global change in the processes of the Earth’s crust. There was some kind of restructuring going on on Earth.”
Source: Nadia Drapon (Harvard University, Cambridge) et al, AGU Advances, doi: 10.1029/2021AV000520
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