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  (Source: universetoday.com)
Researchers say the recycling began around the same time as the start of plate tectonics

A researcher from England has found that the Earth started recycling its crust approximately 3 billion years ago at the start of plate tectonics.

Bruno Dhuime, an isotope geochemist at the University of Bristol in England, explained that a surface of new continental crust covered much of the Earth 3 billion years ago until our planet began recycling it.

Dhuime and his team made this discovery after taking a closer look at zircon minerals found in sediments. These sediments were extracted from North America, Australia, South America and Eurasia.

The team particularly focused on the isotopes inside the sediments, which are atoms of the same element but with different numbers of neutrons. By looking at the oxygen isotope composition, they were able to tell if the sediments had come from new crust that wasn't affected by plate tectonics or recycled crust.

According to Dhuime, sediment in newly formed crust would have a narrow range of oxygen-18 to oxygen-16 ratios while recycled crust would have a much broader range of oxygen-18 to oxygen-16 ratios. Recycled crust has a broader range because of the influence of factors such as life.

They concluded that the Earth's first 1.5 billion years had an average net new crust growth rate of 0.7 cubic miles per year, which was high. However, this rate slowed 3 billion years ago by about a third. Dhuime said this decrease was due to "a change in the way the continental crust was generated." These changes occurred around the time of the start of plate tectonics, where oceanic and continental plates were finally solid enough to start running into each other.

"The new approach we developed based on the chemical information in the mineral zircon enables us to predict in a very accurate way the volume of continental crust that was present throughout Earth's evolution," said Dhuime. "Our next challenge is to determine which tectonic regime shaped the Earth's crust back before 3 billion years in the Archean and Hadean eons. The scarcity of rocks older than 3 billion years preserved on Earth's surface may, however, constitute an obstacle in future research."

Sources: MSNBC, PhysOrg.com





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