Ancient crystal graveyards carry clues of Earth's magma ocean phase
It may be a cool, comfortable paradise nowadays, but Earth had a pretty hotheaded youth – some scientists speculate that for a while the surface was one big ocean of magma. Now, a team of researchers at Cambridge has found elusive evidence from that time, dredged up from an ancient crystal graveyard at the fringe of the mantle and core.
Around 4.5 billion years ago, the solar system was a much more volatile place. Proto-planets regularly crashed into each other, growing and merging and throwing off debris that became moons. That’s how our own Moon was born when the Earth was a mere toddler of 50 million years old.
These regular cataclysms are believed to have melted the rocky surface of the planet, creating a worldwide ocean of bubbling magma several hundred kilometers deep. Over time, as things calmed down that magma would have cooled and crystallized into rock, locking in the structure and chemistry of Earth and even contributing to its early atmosphere.
Unfortunately it’s hard to study this period in Earth’s past as tectonic activity tends to recycle older rocks, pushing them into the mantle and melting them down.
But evidence from that period may survive in other ways. In a new study, researchers from the University of Cambridge have found the chemical fingerprints of this ancient magma ocean in rocks in Greenland that date back some 3.7 billion years.
The find was made in Greenland’s Isua supracrustal belt, a region known to be home to some of the oldest exposed rocks on Earth and what may be the earliest evidence for microbial life.
The team conducted chemical analyses of these rocks and found certain iron isotopes with tungsten anomalies, which indicates they were leftovers from the crystals formed as the ancient magma ocean cooled. But they haven’t just been sitting at the surface for 3.7 billion years – they’d journeyed almost to the center of the Earth and back.
It’s thought that over time, these crystals would have sunk more than 700 km (435 miles) below the surface, to the boundary between the mantle and the core. There, they would have sat undisturbed for billions of years, in what they describe as an “ancient crystal graveyard.”
Later, they made the return journey through a series of melting and recrystallization stages, first migrating to the upper mantle. There they would have mixed with rocks from other depths, creating a layered “marble cake” structure. This mixture was then melted again, and floated upwards to the surface. This whole journey would have been like a “distilling” process, the team says, but the rocks still retained the chemical signatures of the magma ocean phase of Earth’s history.
“There are few opportunities to get geological constraints on the events in the first billion years of Earth’s history,” says Helen Williams, lead author of the study. “It’s astonishing that we can even hold these rocks in our hands – let alone get so much detail about the early history of our planet.”
The team plans to investigate other areas of ancient rock deposits, such as Hawaii, to determine if these processes happened across the world or if it was a rarer anomaly.
The research was published in the journal Science Advances.