New clues about the emergence of terrestrial continents

May 5. (EUROPE PRESS) – –

New research deepens understanding of the Earth’s crust by testing and ultimately eliminate a hypothesis popular question about why continental crust has less iron and is more oxidized compared to oceanic crust.

The iron-poor composition of the continental crust is one of the main reasons why vast portions of the earth’s surface rise above sea level as dry land, making terrestrial life possible today.

The study, published in the journal ‘Science’, uses laboratory experiments to show that the oxidized, iron-poor chemistry typical of Earth’s continental crust probably did not come from crystallization of the mineral garnet, as a popular explanation proposed in 2018.

The continents are part of what makes Earth a unique place for life among the planets of the solar system, but surprisingly little is known about what gave rise to these huge pieces of the planet’s crust and their special properties.

The new research by Elizabeth Cottrell, a research geologist and rock conservator at the Smithsonian’s National Museum of Natural History, and Megan Holycross, the study’s lead author, formerly a Peter Buck and National Science Foundation fellow at the museum and now Adjunct Professor at Cornell University, it eliminates a popular hypothesis about why continental crust has less iron and is more oxidized than oceanic crust.

The building blocks of the new continental crust arise from deep within the Earth in what are known as continental arc volcanoes, which are found in subduction zones where an oceanic plate dips under a continental plate.

In the garnet explanation for the iron-free, oxidized state of the continental crust, crystallization of garnet in magmas under these continental arc volcanoes removes non-oxidized (reduced or ferrous, as it is known among scientists) iron from the plates. terrestrial, simultaneously depleting iron from the molten magma and leaving it more oxidized.

One of the main consequences of the low iron content of the Earth’s continental crust relative to the oceanic crust is that it makes the continents less dense and more buoyant, causing the continental plates to sit higher above the sea. mantle of the planet than the oceanic plates.

This discrepancy in density and buoyancy is one of the main reasons why the continents present land while the oceanic crusts are underwater, as well as the reason why continental plates always meet oceanic plates at subduction zones.

The garnet-based explanation for iron depletion and oxidation in continental arc magmas was convincing, but Cottrell said there was one aspect that didn’t add up. “High pressures are needed for garnet to be stable, And this low-iron magma is found in places where the crust is not as thick and therefore the pressure is not as high.”Explain.

In 2018, Cottrell and colleagues set out to find a way to test whether garnet crystallization deep within these arc volcanoes is indeed essential to the process of creating continental crust as understood.

To do that, they had to find ways to replicate the intense heat and pressure of the Earth’s crust in the lab, and then develop techniques sensitive enough to measure not just how much iron was present, but to differentiate whether that iron was oxidized.

To recreate the massive pressure and heat found under continental arc volcanoes, the team used what are called piston-cylinder presses at the museum’s High Pressure Laboratory and at Cornell. A piston-cylinder hydraulic press is about the size of a mini-fridge and is made mostly of incredibly thick and strong steel and tungsten carbide.

The force applied by a large hydraulic ram produces very high pressures in tiny rock samples, about one cubic millimeter. The set consists of electrical and thermal insulators that surround the rock sample, as well as a cylindrical furnace. The combination of the piston-cylinder press and heating assembly allows for experiments that can reach the very high pressures and temperatures found under volcanoes.

In 13 different experiments, Cottrell and Holycross grew garnet samples from molten rock inside the piston-cylinder press at designed pressures and temperatures. to simulate the existing conditions inside the magmatic chambers of the depths of the earth’s crust.

The pressures used in the experiments ranged from 1.5 to 3 gigapascals, that is, between 15,000 and 30,000 Earth atmospheres of pressure, or 8,000 times more pressure than inside a soda can. Temperatures ranged from 950 to 1,230 degrees Celsius, hot enough to melt rock.

Next, the team collected garnets from the Smithsonian’s National Rock Collection and from other researchers around the world. This group of garnets had already been analyzed, so their concentrations of oxidized and non-oxidized iron were known.

Finally, they brought the materials from their experiments and those from collections to the Advanced Photon Source at the US Department of Energy’s Argonne National Laboratory where they used high-energy X-ray beams to carry out ray absorption spectroscopy. X, a technique that can tell scientists about the structure and composition of materials based on how they absorb X-rays.

Samples with known proportions of oxidized and non-oxidized iron were used to check and calibrate the team’s X-ray absorption spectroscopy measurements. and facilitated comparison with the materials of their experiments.

The results of these tests revealed that the garnets had not incorporated enough unoxidized iron from the rock samples to account for the levels of iron depletion and oxidation present in the magmas that form the Earth’s continental crust.

“These results make the garnet crystallization model an extremely unlikely explanation for why magmas from continental arc volcanoes are oxidized and iron-depleted,” Cottrell said. It is more likely that conditions in the Earth’s mantle below the continental crust set up these rusty conditions.”

Like so many other scientific results, the findings raise more questions like “what is oxidizing or depleting the iron?” Cottrell points out. If it’s not crystallization of the garnet in the crust and it’s something to do with how magmas are coming up from the mantle, then what’s going on in the mantle?”

Cottrell acknowledges that these questions are difficult to answer, but that now the leading theory is that oxidized sulfur could be oxidizing iron, something that a current Peter Buck fellow is investigating under his mentorship at the museum.