Earth was once blanketed by a worldwide ocean of molten rock that cooled and hardened over time. Chemical signs of this early phase have now appeared in lava from a recently active submarine volcano in the Indian Ocean.
An undersea volcano near Madagascar has released material from a magma ocean that existed in the planet’s first 100 million years. Researchers had thought steady churning in the mantle over 4 billion years would have erased most such ancient chemical signatures.
Catherine Chauvel of the French National Centre for Scientific Research noted that the discovery shows materials from 4.5 billion years ago remain in large enough amounts to be sampled by a volcano, which could alter many ideas in earth science.
In the Hadean period a Mars-sized body struck Earth, ejecting debris that formed the Moon and leaving the planet covered in molten rock. Within a few million years this ocean cooled, crystals formed, and the first crust appeared.
Earlier studies suspected remnants of that crystallisation lingered in the mantle but lacked the precision to confirm it. In 2018 a swarm of earthquakes off Mayotte led to the discovery of the new volcano Fani Maoré. Eruptions over three years caused the island to subside about 20 centimetres.
Samples from Fani Maoré and older volcanoes around Mayotte were analysed with a new high-precision method for neodymium isotopes. The new lava showed a higher ratio of neodymium-142 to neodymium-144, pointing to a pocket of mantle that escaped mixing and still contains bridgmanite, one of the first minerals to crystallise from the primordial ocean.
The results indicate the mantle was never mixed as completely as many had believed. They also give the first direct experimental evidence of how crystallisation from the magma ocean created lasting chemical differences, according to the researchers.
