Traces of Earth’s first magma ocean identified in Greenland rocks

New research from 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 Scientific progress, provides information about an important period in the formation of our planet, when a deep sea of ​​glowing magma spread across the Earth’s surface and stretched hundreds of kilometers inland.

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

Scientists know that catastrophic impacts during the formation of the Earth and the 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 almost all rocks over 4 billion years old.

Today, researchers have discovered chemical remnants of the magmatic ocean in 3.6 billion-year-old rocks in southwest Greenland.

The results support the long-held theory that the Earth was once almost completely melted, and provide a window into a time when the planet began to solidify and develop the chemistry that now governs its internal structure. Research suggests that other rocks on the Earth’s surface may also preserve evidence of ancient magmatic oceans.

“There is little possibility of obtaining geological constraints on the events of the first billion years of Earth’s history. It is amazing that we can even hold these rocks in our hands – let alone obtain so many details about the early history of our planet, ”said lead author Dr. Helen Williams, Cambridge Department of Earth Sciences.

The study combines forensic chemical analysis with thermodynamic modeling to research the primitive origins of Greenland rocks and how they came to the surface.

At first glance, the rocks that make up the Isua supracrustal belt in Greenland look like any modern basalt you will find on the seabed. But this outcrop, which was first described in the 1960s, is the oldest exhibit of rocks on Earth. It is known to contain the first evidence of microbial life and plate tectonics.

The new research shows that Isua rocks also retain rare evidence that even predates plate tectonics – the residue of some of the crystals left behind by the cooling ocean of magma.

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

Isotopic systematics of iron confirmed to Williams and the team that the Isua rocks were derived from parts of the Earth’s interior that formed as a result of the crystallization of oceanic magma.

Most of this early rock was mixed by convection in the mantle, but scientists believe that some isolated areas deep at the mantle core boundary – ancient crystal cemeteries – may have remained untouched for billions of years. .

These are the relics of those crystal cemeteries that Williams and his colleagues observed in Isua rock chemistry. “These samples with the iron imprint also have a tungsten anomaly – a signature of the formation of the Earth – which makes us think their origin can be traced back to these early crystals,” said Williams.

But how did these deep mantle signals come to the surface? Their isotopic composition shows that they were not simply driven by fusion at the core-mantle boundary. Their journey was more roundabout, involving several stages of crystallization and reflow – a sort of distillation process. The mixture of ancient crystals and magma would have first migrated to the upper mantle, where it was stirred to create a “ marble cake ” of rocks of varying depths. The subsequent fusion of this rock hybrid is what produced the magma that nourished this part of Greenland.

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

“The geochemical signals we report in the rocks of Greenland have similarities to rocks that have erupted from hot volcanoes like Hawaii – something of interest to us is whether they can also tap into the depths and access regions. from within generally beyond our reach, “said Dr Oliver Shorttle, co-based at Cambridge’s Department of Earth Sciences and the Institute for Astronomy.

The team’s findings come from a project funded by Deep Volatiles, a 5-year NERC-funded research program. They now plan to continue their quest to understand the magmatic ocean by expanding their search for clues in ancient rocks and experimentally modeling isotope fractionation in the lower mantle.

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

Scientists have often been reluctant to seek chemical evidence for these ancient events. “The evidence is often altered over time. But the fact that we found what we did suggests that the chemistry of other ancient rocks may provide additional information about the formation and evolution of Earth – and it’s extremely exciting, ”said Williams.