Ancient volcanoes reveal the recycled earth's crust

According to scientists at the University of St Andrews, ancient volcanoes dating back several billion years could provide new information on how the Earth's surface is recycled.

A study, published today in Nature Communications, reveals the fate of the old earth crust and could help solve the mystery of the connection of the Earth's surface and its mantle.

The outermost layer of the Earth is composed of rigid tectonic plates that move and collide in regions called subduction zones.

In collision zones, crustal materials are transported into the deep mantle, and one of the great challenges of Earth sciences is to understand what is happening in this crust and how long it stays in the mantle.

On some volcanoes on the Earth, geologists can find traces of these ancient crustal materials in the erupting lava. Until now, most of this work has been concentrated on oceanic islands like Hawaii or the Canaries.

However, oceanic islands are only present on the surface of the Earth for a few million years before collapsing and re-entering the mantle. They can therefore only provide a brief overview of the recycling of the Earth's crust over the earth's four billion years of history.

The St Andrews team investigated a series of alkaline magmas that broke out into continental cleavages similar to those in modern East Africa.

Although these magmas have very unusual chemical compositions, they have many similarities with these oceanic lavas and, most importantly, are found in the geological records of the Earth.

The team focused on an alkaline province of southwestern Greenland using state-of-the-art isotope techniques to highlight ancient crustal materials at the source of these magmas by chemical fingerprinting.

Thanks to a combination of remote field work (by boat and helicopter) and extensive chemical analysis, the team was able to show that these magmas were based on an old crust wetted in the mantle 500 million years ago. hatching of volcanoes.

Once the team understood these processes in Greenland, she collected global data on the alkaline chemistry of magma and was surprised to find that the vast majority of them contained a recycled crust component. in their source of magma.

The lead author, Will Hutchison, of the School of Earth Sciences and the Environment, said, "Our main result is that isotopes of alkaline magmas are highly variable, suggesting that their sources recycled crust changed over geological time.

"The beauty of our global data set lies in the fact that it extends over two billion years, making these unique alkaline rocks an extremely powerful record for understanding the recycling of the Earth's crust. during the history of the Earth. "

"By carefully collecting the igneous and sedimentary isotope recordings, this could help us understand how changing crustal inputs are related to volcanic eruptions and better understand what happens to tectonic plates once they are in place. they are transported to the deep Earth. "