The discrepancy in density and buoyancy has been found to be a major reason why Earth’s continental plates always lie up when they meet oceanic plates in subduction zones | Representative image of tawatchai07 on Freepik
New research has shown that the low-iron, oxidized chemistry typical of Earth’s continental crust likely does not originate from crystallization of the mineral garnet, a popular explanation proposed in 2018.
The low-iron content of Earth’s continental crust compared to oceanic crust made the continent less dense and more buoyant, causing the continental plates to sit higher on the planet’s mantle than the oceanic plates, making life on Earth possible today.
Continental and oceanic plates
The discrepancy in density and buoyancy has been found to be a major reason why the continents feature dry land while oceanic crust is submerged, and why continental plates always surface up when meeting oceanic plates in subduction zones where an edge of a Crust lies plate is pushed sideways and down into the mantle under another plate.
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This study from the Smithsonian National Museum of Natural History, USA, was published in the journal Sciencesaid the results deepened the understanding of the Earth’s crust, testing a popular hypothesis and eventually eliminating why the continental crust contains less iron and is more oxidized compared to oceanic crust.
Elizabeth Cottrell, research geologist and curator of rocks at the Smithsonian National Museum of Natural History, said she disagreed with one aspect of the garnet explanation.
“You need high pressure to make garnet stable, and you find this low-iron magma in places where the crust isn’t that thick and so the pressure isn’t very high,” she said.
Test Garnet Declaration
In 2018, Cottrell and her colleagues set out to test the garnet explanation. A combination of piston-cylinder presses and a heating arrangement surrounding the rock sample allowed their experiments to reach very high pressures and temperatures found beneath volcanoes.
In 13 different experiments, Cottrell and his team grew garnet samples from molten rock in the piston-cylinder press. The pressure used in the experiments was between 1.5 and 3 gigapascals – about 8,000 times more pressure than in a can of cola. Temperatures ranged from 950 to 1,230 degrees Celsius, hot enough to melt rock.
The team then collected shells from the Smithsonian National Rock Collection and from other researchers around the world, which had already been analyzed for their levels of oxidized and unoxidized iron. These samples would be used for calibration purposes.
The concentrations of oxidized and unoxidized iron in the grown garnet samples were measured using X-ray absorption spectroscopy, which revealed the structure and composition of materials based on the way they absorbed X-rays. This was done at the US Department of Energy’s Argonne National Laboratory in Illinois.
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The results of these tests showed that the shells had not contained enough unoxidized iron from the rock samples to account for iron degradation and oxidation in the magmas that are the building blocks of 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’s more likely that conditions in the mantle below the continental crust are driving these oxidized conditions,” Cottrell said.
(With agency entries)