Traces of Earth’s Early Magma Ocean Identified in Greenland Rocks

(CN) — Long before Earth was blue, it was covered in a deep sea of incandescent magma.

Now scientists have found evidence in Greenland of a time when Earth was mostly molten.

Researchers at the University of Cambridge in the U.K. say their discovery sheds light on an early period in our planet’s formation when the cooling and crystallization of this molten ocean helped assemble our planet’s structure and created our early atmosphere.

Preserved in ancient rocks, remnants of the magma sea that extended hundreds of miles below the surface increase scientists’ limited knowledge of this crucial period in Earth’s evolution, according to a study published in the journal Science Advances.

While scientists know that catastrophic impacts during Earth’s formation would have generated enough energy to melt our planet’s interior, little evidence remains of this fiery phase of Earth’s history because tectonic processes have recycled most rock layers older than 4 billion years.

Based on chemical analysis of 3.6 billion year old rocks discovered in southwestern Greenland, the study supports the theory that Earth was once almost entirely molten, providing a glimpse of a time when the planet started to solidify and develop the chemistry that now governs its internal structure.

The research suggests that other rocks may also hold clues to this ancient molten period.

“There are few opportunities to get geological constraints on the events in the first billion years of Earth's history. 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,” said Helen Williams of Cambridge's Department of Earth Sciences.

Utilizing forensic chemical analysis and thermodynamic modelling, researchers puzzled together the origins of the Greenland rocks and how they reached the surface of the planet.

While the rocks of Greenland's Isua supracrustal belt look like modern basalt found at the bottom of the ocean, this outcrop is the oldest exposure of rocks on Earth. First described in the 1960s, the formation contains the earliest evidence of microbial life and plate tectonics.

Scientists say the Isua rocks also preserve crystal residues left behind as the magma cooled — rare evidence that predates plate tectonics.

“It was a combination of some new chemical analyses we did and the previously published data that flagged to us that the Isua rocks might contain traces of ancient material,” said co-author Hanika Rizo of Carleton University.

After finding iron isotopic systematics in the samples, researchers realized that the Isua rocks came from parts of the Earth's interior when the magma crystalized.

While convection has mixed most of this primeval rock in the mantle, scientists think ancient crystal graveyards deep at the mantle-core boundary may have remained undisturbed for billions of years. It’s the relics of these crystal graveyards that researchers observed in the Isua rocks.

“Those samples with the iron fingerprint also have a tungsten anomaly - a signature of Earth's formation — which makes us think that their origin can be traced back to these primeval crystals,” said Williams.

But how did these crystal remnants from the deep mantle make their way to the surface? Based on their isotopic makeup, researchers say the migration relied on a distillation process involving stages of crystallization and remelting that initially pushed the mix of ancient crystals and magma to the upper mantle where it was churned in a “marble cake” of various rocks. Later melting of the rock hybrid produced the magma which fed this part of Greenland.

The findings suggest that modern volcanoes thought to have formed relatively recently may actually be influenced by ancient processes.

“The geochemical signals we report in the Greenland rocks bear similarities to rocks erupted from hotspot volcanoes like Hawaii — something we are interested in is whether they might also be tapping into the depths and accessing regions of the interior usually beyond our reach,” said Oliver Shorttle, who is jointly based at Cambridge's Department of Earth Sciences and Institute of Astronomy.

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