The mystery of the Earth’s vanished crust is solved

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The Earth’s crust is what we live on and is by far the thinnest layers of the Earth. The thickness varies depending on where you are on Earth, the oceanic crust being 5 to 10 km and the continental mountain ranges up to 30 to 45 km thick.But what goes on under this crust remains obscure, including the fate of the crust sections that disappear into the Earth.

Now, a team of geochemists based at the National High Magnetic Field Laboratory, headquartered at Florida State University, have found important clues to where these rocks were hiding.

New research has revealed new evidence that while most of the earth’s crust is relatively new, a small percentage is made up of old pieces that had long sank in the mantle and then resurfaced later. They also found that, based on the amount of this “recycled” crust, the planet had produced a crust regularly since its formation 4.5 billion years ago – an image that contradicts dominant theories.

Co-author Munir Humayun, geochemist at MagLab and professor in the Florida State Department of Earth, Oceans and Atmospheric Sciences (EOAS), said: “Like the salmon returning to its spawning grounds, an oceanic crust returns to its place of reproduction, the volcanic ridges where the fresh crust is born. We used a new technique to show that this process is essentially a closed loop and that the recycled crust is distributed unevenly along the ridges. “

An old recycled crust returns to ocean ridges. Credit: Caroline McNiel / National MagLab

Scientists have long estimated what happens to the subducted crust after it has been reabsorbed into the hot, high-pressure environment of the planet’s mantle. It can sink deeper into the mantle and settle there, or rise to the surface in tufts, or to the surface in plumes, or swirl through the mantle.

Scientists had already seen clues supporting the theory. Certain basalts taken from the mid-ocean ridges, called enriched basalts, have a higher percentage of certain elements which tend to seep from the mantle into the melt from which the basalt is formed; others, called depleted basalts, had much lower levels.

To highlight the mystery of the disappearing crust, scientists observed 500 basalt samples from 30 regions of ocean ridges. Some were enriched, some were exhausted, and some were in between.

Scientists have discovered that the overall germanium and silicon contents are lower in the melting of the recycled crust than in the “virgin” basalt emerging from the molten rock of the mantle. They therefore developed another strategy which had this proportion to distinguish a specific chemical imprint for the subducted crust.

They therefore developed a precise technique for measuring the ratio using a MagLab mass spectrometer. Then they calculated the numbers to see how these ratios differed among the 30 regions sampled, expecting to see variations that would shed light on their origins.

Scientists didn’t find anything at first. It was about scientists and they started to look at the problem from a broader perspective. Instead of comparing basalts from different regions, they compared enriched and depleted basalts.

After quickly tightening up the data, the scientists were delighted to see apparent differences between these groups of basalts.

The team had detected weaker relationships between germanium and silicon in the enriched basalts – the chemical imprint of the recycled crust – in all of the regions they sampled, pointing to its spread like a marble cake in the coat. Essentially, they have solved the mystery of the vanished crust.

Humayun said: “Sometimes you look too closely, with your nose in the data, and you can’t see the patterns. Then you step back and go, “Whoa!” “

By digging deeper into the models they found, scientists discovered more secrets. Based on the amounts of enriched basalts detected on global ridges in the middle of the ocean, the team was able to calculate that approximately 5 to 6 percent of the Earth’s mantle is made up of recycled crust. This figure sheds new light on the history of the planet as a crust factory. Scientists knew that the Earth crusts the crust at a rate of a few inches per year. But has it done so consistently throughout its history?

Their analysis, said Humayun, indicates that “The rates of crust formation cannot have been radically different from what they are today, which is not expected by anyone.”

Journal reference:
  1. Elementary constraints on the amount of crust recycled in the generation of mid-ocean ridge basalts (MORB) »Science Advances (2020). DOI: /10.1126/sciadv.aba2923



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