The Earth’s crust is what we live in and is by far the thinnest layers on Earth. The thickness varies depending on where you are on Earth, with the oceanic crust 5-10 km and the continental mountain ranges 30-45 km thick.
But what happens underneath this crust remains obscure, including the fate of the fading sections of crust on Earth.
Now, a team of geochemists based at the National High Magnetic Field Laboratory based at Florida State University have found some important clues as to where those rocks have been hidden.
A new study has come out with new evidence that while most of the Earth’s crust is relatively new, a small percentage is made up of ancient chunks that had long since sunk into the mantle and then re-emerged. They also discovered that, based on the amount of that “recycled” crust, the planet had been constantly producing crust since its formation 4.5 billion years ago, an image that contradicts prevailing theories.
Co-author Munir Humayun, a geochemist at MagLab and a professor in the Florida Department of Earth, Ocean and Atmospheric Sciences (EOAS), said: “Just as salmon return to their spawning grounds, part of the oceanic crust returns to its breeding ground, the volcanic ridges where the fresh crust is born. We used a new technique to demonstrate that this process is essentially a closed loop and that the recycled crust is unevenly distributed along the ridges. ”
Scientists have long estimated what happens to the subducted crust after it is reabsorbed in the hot, high-pressure environment of the planet’s mantle. It can sink further into the mantle and settle there, or rise back to the plumed surface, or to the plumed surface, or rotate through the mantle.
Scientists had already seen clues that supported the theory. Some basalts collected from the ridges of the mid-ocean, called enriched basalts, have a higher percentage of certain elements that tend to leak from the mantle into the melt from which the basalt forms; others, called depleted basalts, had much lower levels.
To highlight the mystery of the disappearing crust, scientists observed 500 samples of basalt collected from 30 regions of the ocean ridges. Some got rich, others ran out, and others stayed in the middle.
Scientists discovered that the overall expanses of germanium and silicon were lower in the melt of the recycled crust than in the “virgin” basalt that arose from molten rock in the mantle. So they created another strategy that had that ratio to distinguish a particular chemical chemical footprint for the subducted cortex.
Thus, they devised a precise technique to measure the ratio using a mass spectrometer in the MagLab. They then reduced the numbers to see how these proportions differed across the 30 sampled regions, hoping to see variations that would shed light on their origins.
Scientists first discovered nothing. This worried scientists, and they began to see the problem from a broader perspective. Rather than comparing basalts from different regions, they compared enriched and depleted basalts.
After quickly re-analyzing the data, the scientists were delighted to see apparent differences between those groups of basalts.
The team had detected lower ratios of germanium to silicon in enriched basalts, the chemical fingerprint of the recycled crust, in all of the sampled regions, noting their spread of marble cake across the mantle. Essentially, they solved the mystery of the fading crust.
Humayun said: “Sometimes you’re looking closely, with your nose in the data, and you can’t see the patterns. Then you step back and say “Whoa!”
Digging deeper into the patterns they found, the scientists discovered more secrets. Based on the amounts of enriched basalts detected on the global ridges of the mid-ocean, the team was able to calculate that about 5 to 6 percent of Earth’s mantle is made from recycled crust. This figure sheds new light on the planet’s history as a crust factory. Scientists knew that Earth produces crust a few inches a year. But has it done so consistently throughout its history?
His analysis, Humayun said, indicates that “The crust formation rates could not have been radically different from what they are today, which is not what anyone expected.”
Magazine reference:
- Elementary restrictions on the amount of crust recycled in the generation of basalts of the middle oceanic crest (MORB) ”Science Advances (2020). DOI: /10.1126/sciadv.aba2923
