New study debunks garnet theory for Earth's low-iron continental crust

A new research has shown that the iron-depleted, oxidised chemistry typical of Earth’s continental crust likely did not come from crystallisation of the mineral garnet, a popular explanation proposed in 2018.

Earth’s continental crust’s low-iron content, relative to oceanic crust, made the continent less dense and more buoyant, causing the continental plates to sit higher atop the planet’s mantle than oceanic plates, making terrestrial life possible today.

Continental and Oceanic plates

The discrepancy in density and buoyancy was found to be a major reason that the continents feature dry land while oceanic crusts are underwater, as well as why continental plates always come out on top upon meeting oceanic plates at subduction zones, where one edge of a crustal plate is forced sideways and downward into the mantle below another plate.

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This study from the Smithsonian National Museum of Natural History, US, published in the journal Science, said the findings deepened the understanding of Earth’s crust by testing and ultimately eliminating one popular hypothesis about why the continental crust is lower in iron and more oxidised compared to the oceanic crust.

Elizabeth Cottrell, research geologist and curator of rocks at the Smithsonian National Museum of Natural History, said a certain aspect of the garnet explanation did not sit right with her.

“You need high pressure to make garnet stable, and you find this low-iron magma at places where crust isn’t that thick and so the pressure isn’t super high,” she said.

Testing garnet explanation

In 2018, Cottrell and her colleagues set out to test the garnet explanation. A combination of piston-cylinder presses and heating assembly surrounding the rock sample, allowed for their experiments to attain very high pressure and temperature found under volcanoes.

In 13 different experiments, Cottrell and team grew samples of garnet from molten rock inside the piston-cylinder press. The pressure used in the experiments ranged from 1.5 to 3 gigapascals — roughly 8,000 times more pressure than inside a can of soda. Temperatures ranged from 950 to 1,230 degrees Celsius, hot enough to melt rock.

The team then collected garnets from Smithsonian National Rock Collection and from other researchers around the world, already analysed for their concentrations of oxidised and unoxidised iron. These samples would be used for calibration purposes.

The concentrations of oxidised and unoxidised iron in the grown garnet samples were measured by using X-ray absorption spectroscopy, which revealed the structure and composition of materials based on how they absorbed X-rays. This was done at the US Department of Energy’s Argonne National Laboratory in Illinois.

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A mismatch

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

“These results make the garnet crystallization model an extremely unlikely explanation for why magmas from continental arc volcanoes are oxidised and iron-depleted,” Cottrell said.

“It’s more likely that conditions in Earth’s mantle below continental crust are setting these oxidised conditions,” said Cottrell.

(With Agency inputs)