Traces of the Earth’s early magma ocean identified in Greenlandic rocks

New research led by the University of Cambridge has found scarce evidence – preserved in the chemistry of ancient rocks from Greenland – 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 ​​light bulb stretched across the earth’s surface and stretched hundreds of miles inland.

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 composition of the structure of our planet and the formation of our early atmosphere.

Scientists know that catastrophic consequences during the formation of the Earth and Moon would generate enough energy to melt the inside of our planet. But we do not know much about this distant and fiery phase of the Earth’s history, because tectonic processes have recovered almost 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 southwestern Greenland.

The findings 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 controls its internal structure. The research suggests that other rocks on Earth may also preserve evidence of ancient magma oceans.

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

The study brings together forensic chemical analysis along with thermodynamic modeling in search of the primordial origin of the Greenlandic rocks and how it came to the surface.

At first glance, the rocks that make up the supracrustic belt of Greenland look like any modern basalt you will find on the sea floor. But this outcome, first described in the 1960s, is the oldest exposure to rocks 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 preserve rare evidence that even preceded plate tectonics – the residues of some of the crystals left behind when the magma ocean cooled.

“It was a combination of some new chemical analyzes we did and the previously published data that was revealed to us that the Isua rocks may contain traces of ancient material. The hafnium and neodymium isotopes were really disturbing. “Because the isotope systems are very difficult to adapt.” That’s why we had to look at their chemistry in more detail, “said co-author Dr. Hanika Rizo of Carleton University.

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

Most of these primeval rocks were confused by convection in the mantle, but scientists think that some isolated zones deep on the mantle core boundary – ancient crystal cemeteries – may have remained undisturbed for billions of years.

These are the remains of these crystal churches that Williams and her colleagues observed in the Isua rock chemistry. “The iron-fingerprint samples also have a tungsten aberration – a signature of the Earth’s formation – that makes us think that their origin can be traced back to these primordial crystals,” Williams said.

But how did these signals from the deep mantle find their way to the surface? Their isotopic makeup shows that they are not just melted at the core mantle boundary. Their journey was more surrounding and involved different phases of crystallization and melting – a kind of distillation process. The mixture of ancient crystals and magma would first migrate to the upper mantle, where it was hacked to create a ‘marble cake’ of rocks of different depths. Later, the hybrid of rocks melts the magma that fed this part of Greenland.

The team’s findings suggest that modern hotspot volcanoes, which presumably formed relatively recently, may actually be affected by ancient processes.

“The geochemical signals we report in the Greenland rocks have similarities with rocks that erupted from hotspot volcanoes like Hawaii – something we are interested in is whether it can also tap into the depths and gain access to the interior regions that is usually beyond our reach, “said Dr. Oliver Shorttle, who is jointly based at Cambridge’s Department of Earth Sciences and Institute of Astronomy.

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

“We were able to pick out one part of the inside of our planet billions of years ago, but to fill in the picture further, we need to keep looking for more chemical clues in ancient rocks,” said Dr. Simon Matthews, co-author, said. of the University of Iceland.

Scientists were often reluctant to search for chemical evidence of these ancient events. “The evidence is often changed over time. But the fact that we have found what we have done suggests that the chemistry of other ancient rocks can provide further insights into the formation and evolution of the earth – and it is very exciting, “Williams said.