Plate tectonics is 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 with an age of 3.3 to 4.15 billion years. The geochemical composition of these crystals suggests that the oldest formed before the earth’s crust began to move. However, zircons up to 3.8 billion years old seem to have formed in areas where one tectonic plate pushed under another. The crystals thus provide the earliest indication of the beginnings of plate tectonics.
Insights into the early days of the earth are rare. Hardly any material has survived the billions of years and can still be examined today. One of the reasons for this is the so-called “recycling” of the earth’s crust. One tectonic plate slides under another. The crustal material that is pushed into the depths is melted in the earth’s mantle. Mantle material rises at mid-ocean ridges and forms new crust. But an extraordinary mineral can survive even the extreme 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 around four billion years ago.
Searching for clues in crystals
A team led by Nadja Drabon from Harvard University in Cambridge has now examined a series of zircons that were discovered in 2018 during excavations in the Barberton Greenstone Belt in South Africa. The crystals, the size of a grain of sand, formed at different points in time between 4.15 and 3.3 billion years, i.e. exactly at the time when, according to current knowledge, plate tectonics must have started. Using a chronological series of 33 zirconium crystals, the researchers were able to understand how the earth’s crust developed over these 800 million years.
They focused on three different geochemical features of the crystals found: the hafnium isotopes, the oxygen isotopes and the composition of the trace elements. Each of these features gave them a different piece of the puzzle. The hafnium isotopes gave clues to the formation and development of the earth’s crust, the oxygen isotopes to whether there were oceans, and the trace elements to the composition of the crust.
Upheaval 3.8 billion years ago
The result: The hafnium isotopes and trace elements in the oldest zircons showed that they were formed in a global “protocrust” that was stable for millions of years. In contrast, zircons, which are 3.8 billion years old and younger, appear to have formed in rocks that have experienced similar pressure and melting as modern subduction zones, the areas where one plate slides under another. “At 3.8 billion years, there is a dramatic shift: the crust is being destabilized, new rocks are being formed, and the geochemical signatures are becoming more like what we see in modern plate tectonics,” Drabon says.
This suggests that about 3.8 billion years ago the earth’s crust split into plates that subsequently began to shift against each other. “In the case of the oxygen isotopes, on the other hand, no significant change can initially be observed,” the researchers report. Only for zircons that are 3.5 billion years old or younger do we see evidence that they formed in older parts of the crust that had been altered by contact with liquid water. This could indicate the existence of more developed tectonic plates and volcanic activity at island arcs and other plate boundaries bordering the sea.
Global Change
The researchers also compared their results with data on ancient zircons from other parts of the world. “We’re seeing evidence of major change on Earth around 3.8 to 3.6 billion years ago, and evolution into plate tectonics is a clear possibility,” says Drabon. “The record we have for the earliest Earth is very limited, but seeing a similar transition in so many different places makes it really conceivable that it could have been a global change in crustal processes. There was a kind of restructuring taking place on Earth.”
Source: Nadja Drabon (Harvard University, Cambridge) et al., AGU Advances, doi: 10.1029/2021AV000520