Scientists examined hundreds of samples taken along global ridges that contain recycled ancient oceanic crust in varying amounts. The “depleted” segments of the ridge received lower amounts of recycled bark than the “normal” ones, while the “enriched” segments contain a higher proportion of recycled bark. Credit: Caroline McNiel / National MagLab
Thank God for the earth’s crust: it is, after all, that solid, outermost layer of our planet that supports everything above it.
But much of what happens underneath that layer remains a mystery, including the fate of the fading sections of crust on Earth. Now, a geochemical team based on the Florida State UniversityThe headquartered National High Magnetic Field Laboratory has uncovered key clues to where those rocks have been hidden.
The researchers provided new evidence that while most of the Earth’s crust is relatively new, a small percentage is actually made up of ancient chunks that had long since sunk into the mantle and then re-emerged. They also found, based on the amount of that “recycled” crust, that the planet has been consistently producing the crust since its formation 4.5 billion years ago, an image that contradicts prevailing theories.
Munir Humayun, geochemist at the National High Magnetic Field Laboratory and professor of geology at the Department of Earth, Ocean and Atmospheric Sciences at Florida State University. Credit: Courtesy of the National High Magnetic Field Laboratory.
Their research is published in the journal. Scientific advances.
“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,” said co-author Munir Humayun, a geochemist at MagLab and a professor at the Earth Department. , Ocean and Atmospheric of the State of Florida. Science (EOAS). “We use a new technique to show that this process is essentially a closed loop, and that the recycled crust is unevenly distributed along the ridges.”
In addition to Humayun, the research team included MagLab postdoctoral researcher Shuying Yang, lead author of the paper, and the Director of the MagLab Geochemistry Group and EOAS President Vincent Salters.
Earth’s oceanic crust is formed when the mantle rock melts near fissures between tectonic plates along underwater volcanic ridges, producing basalt. As a new crust forms, it pushes the older crust away from the ridge toward the continents, like a super slow conveyor belt. Eventually, it reaches areas called subduction zones, where it is forced under another plate and returned to Earth.
Scientists have long theorized about what happens to the subducted crust after it is reabsorbed in the hot, high-pressure environment of the planet’s mantle. It can sink deeper into the mantle and settle there, or rise back to the surface in plumes, or spin through the mantle, like chocolate threads through a yellow marble cake. Some of that “chocolate” could eventually rise, melt back into the ridges of the mid-ocean, and form a new rock for another service journey of millions of years at the bottom of the sea.
This new evidence supports the “marble cake” theory.
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 shed more light on the mystery of the disappearing crust, the team chemically analyzed 500 basalt samples collected from 30 oceanic ridge regions. Some got rich, others ran out, and others stayed in the middle.
Initially, the team discovered that the relative proportions of germanium and silicon were lower in the melt of the recycled crust than in the “virgin” basalt that emerges from the melted mantle rock. They then developed a new technique that used that ratio to identify a different chemical footprint for the subducted cortex.
They devised an accurate method to measure that relationship using a mass spectrometer at 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.
Shuying Yang, postdoctoral scholar in geochemistry at the National High Magnetic Field Laboratory. Credit: Courtesy of Florida State University.
At first, the analysis revealed nothing remarkable. Concerned, Yang, a doctoral candidate at the time, consulted with his adviser. Humayun suggested looking at the problem from a broader angle: instead of comparing basalts from different regions, they could compare rich and depleted basalts.
After quickly re-analyzing the data, Yang was excited to see clear differences between those groups of basalts.
“I was very happy,” recalled Yang, lead author of the article. “I thought, ‘I’ll be able to graduate!'”
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.
It was a lesson in losing the forest to the trees, Humayun said.
“Sometimes you’re looking too closely, with your nose in the data, and you can’t see the patterns,” he said. “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 in the global ridges of the mid-ocean, the team was able to calculate that around 5 to 6 percent of Earth’s mantle is made from recycled crust, a figure that sheds new light on the planet’s history. like a factory crust. 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, “Scab formation rates cannot have been radically different from what they are today, which is not what anyone expected.”
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Reference: June 26, 2020, Scientific advances.
DOI: 10.1126 / sciadv.aba2923
MagLab is funded by the National Science Foundation and the State of Florida. It is based at Florida State University with additional locations in University of Florida and Los Alamos National Laboratory.
