Surprisingly little is known about the origins of these enormous chunks of the Earth’s crust and their unique characteristics, despite the fact that continents play a key role in what makes Earth the only planet in the solar system that is habitable for life.
Megan Holycross, a lead study author and former Peter Buck Fellow and National Science Foundation Fellow at the museum who is now an assistant professor at Cornell University, and Elizabeth Cottrell, research geologist and curator of rocks at the Smithsonian’s National Museum of Natural History, have tested and ultimately disproved a popular theory about why the continental crust is lower in iron and more oxidized.
Large areas of the Earth’s surface are above sea level as dry land because of the iron-poor composition of the continental crust, which made terrestrial life conceivable on Earth today.
The research, which was published on May 4 in Science, uses laboratory tests to demonstrate that the iron-depleted, oxidized chemistry typical of Earth’s continental crust probably did not arise from the crystallization of the mineral garnet, as a common explanation provided in 2018.
At continental arc volcanoes, which are found at subduction zones where an oceanic plate descends beneath a continental plate, the building blocks of new continental crust emerge from the depths of the Earth.
The crystallization of garnet in the magmas beneath these continental arc volcanoes removes non-oxidized (reduced or ferrous, as it is known among scientists) iron from the terrestrial plates, simultaneously depleting the molten magma of iron and leaving it more oxidized, according to the garnet explanation for the continental crust’s iron-depleted and oxidized state.
The low iron content of Earth’s continental crust in comparison to oceanic crust has the effect of making the continents less thick and more buoyant, causing continental plates to rest higher atop the planet’s mantle than oceanic plates. This disparity in density and buoyancy is a fundamental reason why continents have dry land while oceanic crusts are underwater, and why continental plates always win when they collide with oceanic plates at subduction zones.
A research geologist and curator of rocks at the Smithsonian’s National Museum of Natural History, Elizabeth Cottrell said that a certain aspect of the garnet explanation did not sit right with her.
She further said, “You need high pressures to make garnet stable, and you find this low-iron magma at places where the crust isn’t that thick and so the pressure isn’t super high”.
Cottrell and her colleagues decided to put the garnet explanation to the test in 2018. A combination of piston-cylinder presses and a heating system encircling the rock sample enabled their studies to achieve the extremely high pressures and temperatures found beneath volcanoes.
Cottrell and his colleagues developed garnet samples from the molten rock inside the piston-cylinder press in 13 different trials. The pressures utilized in the studies varied from 1.5 to 3 gigapascals, which is approximately 8,000 times higher than the pressure inside a can of soda. Temperatures were between 950 and 1,230 degrees Celsius, which is hot enough to dissolve rock.
The team then acquired garnets from the Smithsonian’s National Rock Collection and from other researchers across the world that had already been tested for oxidized and unoxidized iron contents. These samples will be used for calibration.
Finally, the researchers used X-ray absorption spectroscopy to determine the concentrations of oxidized and unoxidized iron in the produced garnet samples, which showed the structure and composition of materials based on how they absorbed X-rays. This was performed at the Argonne National Laboratory of the US Department of Energy in Illinois.
These studies demonstrated that the garnets had not assimilated enough unoxidized iron from the rock samples to account for the levels of iron depletion and oxidation present in the magmas that serve as the foundation of Earth’s continental crust.
Cottrell stated, “These results make the garnet crystallization model an extremely unlikely explanation for why magmas from continental arc volcanoes are oxidized and iron-depleted”.
“It’s more likely that conditions in Earth’s mantle below continental crust are setting these oxidized conditions”, said added.
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