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.
This study from the Smithsonian’s National Museum of Natural History, US, and published in the journal Science, said that 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’s National Museum of Natural History, said that a certain aspect of the garnet explanation did not sit right with her.
“You need high pressures 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.
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 the very high pressures and temperatures found under volcanoes.
In 13 different experiments, Cottrell and team grew samples of garnet from molten rock inside the piston-cylinder press. The pressures 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. Next, the team collected garnets from Smithsonian’s 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.
Finally, the study authors measured the concentrations of oxidised and unoxidised iron in the grown garnet samples by using X-ray absorption spectroscopy, which revealed the structure and composition of materials based on how they absorbed X-rays. This was accomplished at the US Department of Energy’s Argonne National Laboratory in Illinois.
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.