Traces of Earth’s early magma ocean have been identified in the rocks of Greenland

New research led by the University of Cambridge has found rare evidence – preserved in the chemistry of ancient Greenland rocks – that tells of a time when the Earth was almost completely melted.

The study, published in the journal Science Advances, provides information about an important period in our planet’s formation, when a deep sea of ​​glowing magma stretched across the Earth’s surface and stretched hundreds of miles into its interior.

It is the gradual cooling and crystallization of this “magma ocean” that determines the chemistry of the Earth’s interior – a defining phase in the assembly of our planet’s structure and the formation of our early atmosphere.

Scientists know that catastrophic consequences during the formation of the Earth and Moon would have generated enough energy to melt the interior of our planet. But we don’t know much about this distant and fiery phase of Earth’s history, as tectonic processes have recycled nearly all rocks older than 4 billion years.

Now researchers have found the chemical remains of the magma ocean in 3.6 billion-year-old rocks from southwest Greenland.

The findings support the long-standing theory that Earth was once almost completely melted and provide a window into a time when the planet began to solidify and evolve the chemistry that now controls its internal structure. The research suggests that other rocks on Earth’s surface may also preserve evidence of ancient magma oceans.

“There are very few possibilities to impose geological constraints on the events of the first billion years of Earth’s history. It’s amazing that we can even hold these rocks in our hands – let alone get so much detail about our planet’s early history, ”said lead author Dr. Helen Williams, Cambridge Earth Science Department.

The study combines forensic chemical analysis with thermodynamic modeling to find the original origin of the Greenland rocks and how they came to the surface.

At first glance, the rocks that make up Greenland’s Isua supracrustal belt look like any modern basalt you’d find on the sea floor. But this outcrop, first described in the 1960s, is the oldest rock exposure on Earth. It is known to contain the earliest evidence of microbial life and plate tectonics.

The new research shows that the Isua rocks also contain rare evidence even older than plate tectonics – the residues of some of the crystals left behind when that magma ocean cooled.

“It was a combination of some of the new chemical analyzes we did and the previously published data that gave us the impression that the Isua rocks may contain traces of ancient material. The hafnium and neodymium isotopes were really tempting because they isotopic systems are very difficult to modify – so we had to look at their chemistry in more detail, ”said study co-author Dr. Hanika Rizo of Carleton University.

Iron isotopic systematics confirmed to Williams and the team that the Isua rocks are derived from parts of the Earth’s interior formed as a result of magma ocean crystallization.

Most of this primordial rock has been mixed into the mantle by convection, but scientists believe some isolated zones deep at the mantle-core boundary – ancient crystal burial sites – may have remained undisturbed for billions of years.

It is the remains of these crystal burial sites that Williams and her colleagues observed in the Isua rock chemistry. “Those iron fingerprint samples also have a tungsten aberration – a hallmark of Earth’s formation – that makes us think their origin can be traced back to these primal crystals,” said Williams.

But how did these signals from the deep mantle make their way to the surface? Their isotopic composition shows that they were not led to the core-mantle boundary by melting alone. Their journey was more lengthy and involved several stages of crystallization and remelting – a kind of distillation process. The mix of ancient crystals and magma is said to have first migrated to the upper mantle, where it was churned to create a ‘marble cake’ of rocks from different depths. Later melting of that hybrid of rocks is what produced the magma that fed this part of Greenland.

The team’s findings suggest that modern hot spot volcanoes, believed to have formed relatively recently, may in fact be affected by ancient processes.

“The geochemical signals we report in the Greenland rocks show similarities to rocks erupted by volcanoes like Hawaii – something we’re interested in is whether they can also tap into the depths and reach areas of the interior that are mostly outside of us. range, ”said Dr. Oliver Shorttle, who works jointly at the Department of Earth Sciences and the Cambridge Institute of Astronomy.

The team’s findings stemmed from a project funded by Deep Volatiles, a five-year research program funded by the NERC. They now plan to continue their quest to understand the magma ocean by expanding their search for clues in ancient rocks and experimentally modeling isotopic fractionation in the lower mantle.

“Billions of years ago we were able to unravel what part of the interior of our planet was doing, but to further fill in the picture we need to keep looking for more chemical clues in ancient rocks,” said study co-author Dr. Simon Matthews. from the University of Iceland.

Scientists were often reluctant to look for chemical evidence of these ancient events. “The evidence is often changed over time. But the fact that we found out what we did suggests that the chemistry of other ancient rocks may provide more insights into the formation and evolution of the Earth – which is hugely exciting,” Williams said.