New research suggests that the iron-depleted, oxidized chemistry typical of the Earth’s continental crust did not result from the crystallization of the mineral garnet, as was widely believed in 2018.
The low iron content of Earth’s continental crust in comparison to oceanic crust caused the continents to be less compact and more buoyant, causing continental plates to reside higher atop the planet’s mantle than oceanic plates and making terrestrial life possible.
The disparity in density and buoyancy was discovered to be a major factor in why continents have dry land and oceanic crusts are submerged, as well as why continental plates always come out on top when colliding with oceanic plates at subduction zones, where one edge of a crustal plate is forced sideways and downward into the mantle below another plate.
Earth’s continental crust is less solid and more buoyant than oceanic crust, allowing terrestrial life to exist.
This study from the Smithsonian’s National Museum of Natural History, US, and published in the journal Science, stated that the findings deepened understanding of the Earth’s crust by testing and ultimately refuting a popular theory regarding why the continental crust is lower in iron and more oxidized than the oceanic crust.
Elizabeth Cottrell, a research geologist and curator of rocks at the Smithsonian National Museum of Natural History, stated that a particular facet of the garnet explanation did not set well with her.
“You need high pressures to make garnet stable, and you find this low-iron magma in places where the crust isn’t that thick, so the pressure isn’t super high,” she explained.
Cottrell and her collaborators set out to verify the garnet explanation in 2018. Utilizing a combination of piston-cylinder presses and a heating assembly encircling the rock sample, the researchers were able to replicate the extremely high pressures and temperatures found beneath volcanoes.
Cottrell and his colleagues produced garnet samples from molten rock inside the piston-cylinder press in 13 separate experiments. The experimental pressures ranged from 1.5 to 3 gigapascals, which is roughly 8,000 times greater than the pressure inside a beverage can. Rock-melting temperatures ranged between 950 and 1,230 degrees Celsius.
Terrestrial life exists because continental crust is less solid and buoyant than oceanic crust.
Next, the team gathered garnets from the Smithsonian’s National Rock Collection and from other researchers around the globe that had already been analyzed for their oxidized and unoxidized iron concentrations. These samples would be utilized for the objectives of calibration.
Using X-ray absorption spectroscopy, which disclosed the structure and composition of materials based on how they absorbed X-rays, the authors of the study determined the concentrations of oxidized and unoxidized iron in the grown garnet samples. This was accomplished at the Argonne National Laboratory in Illinois, which is part of the US Department of Energy.
These studies revealed that the garnets did not contain enough unoxidized iron from the rock samples to account for the levels of iron-depletion and oxidation present in the magmas that comprise the Earth’s continental crust.
“These results render the garnet crystallization model a highly improbable explanation for why magmas from continental arc volcanoes are oxidized and iron-depleted,” Cottrell stated.
“It is more likely that conditions in the Earth’s mantle beneath the continental crust are causing these oxidized conditions,” Cottrell explained.