A rising underwater volcano off the coast of Madagascar has been spewing up chemical traces of material from a primordial magma ocean in the first 100 million years of Earth’s history.

Scientists generally suspect that Earth’s mantle – the thick layer of hot rock beneath the crust – has been steadily churning for more than 4 billion years, gradually erasing most chemical traces of the planet’s earliest history.

“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, breaking off debris that scientists believe later formed the moon. The impact heated the young planet so intensely that it became covered by a global magma ocean. Over the next few million years, the molten rock cooled and crystallised, and the earliest crust began to form above the mantle.

Some scientists suspected that traces of this primordial crystallisation survived in Earth’s mantle, but they lacked the analytical precision to prove it, says Chauvel.

In May 2018, an unusual swarm of earthquakes off the French island of Mayotte, between Madagascar and Mozambique, led scientists to discover a new volcano about 50 kilometres further east, named Fani Maoré. Eruptions over the following three years drained so much magma from underneath Mayotte that it sank about 20 centimetres.

Chauvel and her colleagues recovered volcanic rock samples from Fani Maoré and nearby Mayotte to compare the chemistry of the new volcano with that of the older volcanic system. They teamed up with Claudine Israel at the University of Cambridge to investigate further, using a newly developed ultra-precise technique to measure tiny differences in neodymium isotopes. Such isotopes preserve a chemical record of how Earth’s primordial magma ocean crystallised as the young planet cooled, says Israel.

The team found that, compared to Mayotte’s lava, the Fani Maoré lava had a slightly higher ratio of neodymium-142 to neodymium-144. That higher ratio probably reflects a pocket of ancient mantle that escaped billions of years of mixing and is still relatively rich in bridgmanite – a mineral thought to have been among the first to crystallise from Earth’s 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 Earth’s mantle might never have become as thoroughly mixed as many geologists had assumed. That could help scientists reconstruct how Earth’s primordial magma ocean solidified, 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 preserves extremely 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 offers an unprecedented glimpse into a period of Earth’s history for which almost no direct evidence survives, 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 alone is noteworthy. “Anyone who has experience with these measurements would recognise it as a major achievement,” he says.