The earliest evidence of plate tectonics –

Plate tectonics is responsible for the present shape of our earth. But when did it start? Scientists have found evidence of this on the basis of tiny zircon crystals ranging in age from 3.3 to 4.15 billion years. The geochemical composition of these crystals suggests that the oldest were formed before the earth’s crust began to move. However, zircons up to 3.8 billion years old appear to have formed in areas where one tectonic plate has crashed under the other. The crystals therefore provide the earliest evidence for plate tectonics.

Insights in the early days of the earth are rare. Hardly any material has survived for billions of years and is still available for study. One of the reasons for this is the so-called “recycling” of the earth’s crust. One tectonic plate slides under the other. The crust material that is pushed into the depths melts into the mantle of the earth. The mantle material rises on the mid-ocean ridges and forms a new crust. But the unusual mineral can survive even extreme conditions in this recycling process: zircon, the oldest known mineral on earth. Like time capsules, crystals make it possible to draw conclusions about conditions on Earth about four billion years ago.

Looking for clues in crystals

A team led by Nadja Drabon of Harvard University in Cambridge has now investigated a series of cubic zirconia discovered in 2018 during an excavation in the Barberton Greenstone Belt in South Africa. The grain-sized crystals of sand formed at various time points between 4.15 and 3.3 billion years, which is exactly the time at which, according to current knowledge, plate tectonics must have started. By using a chronological series of 33 zircon crystals, scientists were able to understand how the earth’s crust developed over these 800 million years.

They focused on three different geochemical characteristics of the crystals found: hafnium isotopes, oxygen isotopes and trace element composition. Each of these traits gave them a different piece of the puzzle. Hafnium isotopes gave clues to the formation and development of the earth’s crust, oxygen isotopes to the existence of the oceans, and trace elements to the composition of the crust.

Coup 3.8 billion years ago

Result: hafnium isotopes and trace elements in the oldest zirconiums showed that they originated in a global “prototype” that has been stable for millions of years. In contrast, rhinestones, which are 3.8 billion years old and younger, appear to form in rocks that have experienced similar pressure and melting as modern subduction zones, areas where one plate slides beneath another. “After 3.8 billion years, a dramatic change occurs: the crust destabilizes, new rocks are formed, and the geochemical signatures increasingly resemble what we see in modern plate tectonics,” says Drabon.

This suggests that about 3.8 billion years ago, the earth’s crust divided into plates, which then began to shift relative to each other. “On the other hand, in the case of oxygen isotopes, no significant changes can be observed initially,” the researchers say. Only in the case of cubic zirconia that are 3.5 billion years old or younger, we see evidence that they were formed in older parts of the crust that were altered by contact with liquid water. This may indicate the existence of more developed tectonic plates and volcanic activity on the island arcs and other plate boundaries bordering the sea.

Global change

The researchers also compared their results with data from ancient zircons from other parts of the world. “We see evidence of major changes 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 imaginable that it could be a global change in the processes of the earth’s crust. There has been a kind of restructuring on Earth. “

Source: Nadja Drabon (Harvard University, Cambridge) et al., AGU Advances, doi: 10.1029 / 2021AV000520

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