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Rocks collected in Greenland may contain traces of an ancient ocean of magma that bubbled over much of the Earth’s surface shortly after the planet was born, according to a new study.
Scientists collected rocks from the supracrustal belt of Isua, a region in the southwest Greenland where the exposed rocks are between 3.7 billion and 3.8 billion years old; the belt contains the oldest known rocks on Earth, which remain relatively undisturbed by tectonic plates, thermal and chemical alterations, according to Science Magazine.
The chemical traces of the first magmatic oceans are even older than the rocks themselves, dating to around 4.5 billion years ago, when an object the size of Mars smashed into Earth, knocking down a huge chunk of rock that later became the moon, according to the new study.
When celestial objects the size of Earth and Mars collide, “the near total fusion of the entire planet is an inevitable consequence,” said lead author Helen Williams, professor of geochemistry at the University of Cambridge, at Live Science. And as this molten rock cooled and crystallized, the Earth gradually came to resemble the blue marble we know today, she said.
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But although most scientists accept the Molten Earth theory, “a big challenge is that it is very difficult to find … geological evidence for something that has happened so early in our history,” Williams said. The new study, published on March 12 in the journal Scientific progress, shows that the rocks of the Isua belt still bear chemical “imprints” left by this primordial cooling process.
Williams began researching these fingerprints after she and her co-author Hanika Rizo, associate professor at Carleton University in Canada, met at a fall meeting of the American Geophysical Union (AGU). , an annual event which in the pre-pandemic era attracted tens of thousands of scientists from around the world.
Rizo had previously extracted rock samples from Isua’s supracrustal belt and wrote about them in a 2011 study, published in the journal Earth and Planetary Science Letters. In the article, she noted that rocks carry certain chemical signatures, namely single isotopes or chemical elements with varying numbers of neutrons. Williams later read the report, and these chemical signatures piqued his interest.
“Her article doesn’t contain direct geological evidence of the magma ocean. But so many of the chemical tracers she talks about… really point in that general direction,” Williams said. If they studied the samples any further, Williams thought, they might discover a snapshot of Earthis a melted past. So when she spotted Rizo across the vast AGU conference room, “I started running towards her, because I really wanted to talk … about the possibility of collaborating,” said Williams said.
To launch their collaboration, the scientists went to the laboratory. They selected a subset of volcanic rocks from the Isua samples, choosing only the purest, in terms of the wear and tear they experienced after they erupted to the surface and exposed to the elements. They then sawed off the exposed surfaces of the rocks, sanded them down, crushed them to a fine powder, and dissolved the powder in strong acids.
“By the time you’re done, it’s pretty amazing, that something that was a really hard, dense rock in your hand is now actually a little vial of liquid in your lab,” Williams said. Treating rocks in this way allowed the team to examine isotopes, or chemical elements with varying numbers of neutrons, in the samples.
More specifically, the team was looking for isotopes that would have formed during the crystallization of magmatic oceans. Models to suggest that some remains of these crystals would have been trapped in the lower mantle, near the Earth’s core, and preserved for billions of years. Over time, they would migrate through the lower mantle to the upper mantle, taking with them the “isotopic footprints” of the magma ocean, Williams said.
These fingerprints include hafnium and neodymium isotopes, which are formed when their parent isotopes break down; this degradation occurs in a specific pattern when the parent isotopes are placed under extremely high pressures, such as those found in the depths of the lower mantle, Williams said. The team found these unique isotopes in Isua’s samples, along with a rare form of the element tungsten; Known as a “tungsten anomaly,” these unusual isotopes of tungsten come from an ancient parent isotope that only existed in the first 45 million years of Earth’s history, Williams said.
As these crystal residues moved up the lower mantle to the upper mantle, they melted and mixed with other molten rocks, creating a mottling effect. Thus, once this mixed rock broke through the crust, it carried isotopic imprints with the rock of the upper and lower mantle; this was the case with the Isua samples. There are several theories as to how and why the crystals migrated upward through the layers of the Earth, one being that the crystals melted and recrystallized over and over, becoming more and more concentrated as they went. they were riding, Williams said.
After discovering the chemical traces of the magmatic oceans, “the question I ask myself is whether other ancient rocks on Earth have retained the same signatures,” Williams said. She and her team begin to search for these signatures at sites around the world, looking for sites with extremely ancient rocks and in modern hotspots for volcanic activity, such as Hawaii and Iceland.
“Numerous sources of evidence … suggest that these modern hotspots are derived from the fusion of materials really deep within the Earth, possibly even coming from near the border between the Earth’s mantle and its core,” Williams said. . This means that chemical traces of the magma ocean may also be lurking in these hot spots, although we don’t yet know for sure, she said.
Originally posted on Live Science.
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