Submarine reliefs produced by a survey off the coast of Mayotte in 2019, showing the new underwater volcano Fani Maoré
Campagne MAYOBS2
An emerging underwater volcano near Madagascar is releasing chemical remnants from a primordial magma ocean dating back to Earth’s first 100 million years.
Scientists have long believed that Earth’s mantle, the vast layer of heated rock below the crust, has been in motion for over 4 billion years, gradually erasing almost all chemical evidence of the planet’s earliest phases.
“This is going to change a lot of things [in earth science], because now we have proof that materials dating back 4.5 billion years – from the very beginning of Earth’s history – still exist in sufficient quantities to be sampled in a volcano,” says Catherine Chauvel at the French National Centre for Scientific Research (CNRS) in Paris.
During the Hadean eon, a Mars-sized object collided with Earth, ejecting debris believed to have formed the moon. The impact was so intense that it enveloped the young planet in a global magma ocean. Over several million years, this molten rock cooled and solidified, forming the early crust above the mantle.
Some scientists theorized that remnants of this initial crystallization lingered in the mantle, but lacked the tools to confirm it, says Chauvel.
In May 2018, a series of unusual earthquakes near Mayotte, a French island situated between Madagascar and Mozambique, led to the discovery of a new volcano about 50 kilometers east, named Fani Maoré. Over the ensuing three years, eruptions drained significant magma from beneath Mayotte, causing it to subside by about 20 centimeters.
Chauvel and her team collected volcanic rock samples from Fani Maoré and nearby Mayotte to compare the chemical composition of the new volcano with that of the older volcanic system. Collaborating with Claudine Israel at the University of Cambridge, they used a newly developed ultra-precise technique to measure slight differences in neodymium isotopes. According to Israel, these isotopes maintain a chemical record of how Earth’s ancient magma ocean crystallized as the planet cooled.
The team discovered that the Fani Maoré lava exhibited a marginally higher ratio of neodymium-142 to neodymium-144 compared to Mayotte’s lava. This elevated ratio likely indicates a pocket of ancient mantle material that has remained relatively unmixed and is still rich in bridgmanite, a mineral thought to have been among the first to crystallize from the primordial magma ocean.
“It’s always exciting to find something you’ve been looking for – and that nobody else has found yet,” says Chauvel.
The findings suggest that Earth’s mantle may not have mixed as thoroughly as previously believed by many geologists. This could aid scientists in reconstructing the solidification process of Earth’s early magma ocean, says Israel.
“For the first time, we’ve shown experimentally how the mantle crystallised from the magma ocean, and how that crystallisation created chemical heterogeneity from the very beginning,” she says.
The new findings provide plausible evidence that Earth’s mantle still contains ancient material, says Tim Johnson at Curtin University in Perth, Australia. “This seems to be an exciting advance,” he says.
“It takes an enormous amount of work to get a technique like that working properly, and it looks like they’ve succeeded,” says Bernard Bourdon at the CNRS in Lyon.
The study provides an unprecedented perspective on a period of Earth’s history for which almost no direct evidence exists, he adds. “It’s a bit like discovering a sample of Earth’s core that somehow made it all the way to the surface,” says Bourdon.
For Richard Carlson at Carnegie Science in Washington, D.C., the precision of the study is particularly remarkable. “Anyone who has experience with these measurements would recognise it as a major achievement,” he says.
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