The mystery behind the unique chemistry of Earth’s continents

Oxidized sulfur, and not garnet, may be behind it.
A microscope image from an experiment conducted for a study on continental crust. The image contains glass (brown), large garnets (pink), and other small mineral crystals. The field of view is 410 microns wide, about size of a sugar crystal

A microscope image from an experiment conducted for a study on continental crust. The image contains glass (brown), large garnets (pink), and other small mineral crystals. The field of view is 410 microns wide, about size of a sugar crystal G. Macpherson and E. Cottrell, Smithsonian.

Alongside our planet’s oxygen rich atmosphere and plentiful water, the Earth’s continents are part of what makes the planet uniquely habitable for sustaining life. Despite this, little is known about what gave rise to these massive pieces of the Earth’s crust and the properties that make them special. One prevailing hypothesis is that since continental crust is lower in iron compared to oceanic crust, the iron-poor composition in continental crust is part of why large parts of Earth’s surface stand above sea level as dryland for terrestrial life. 

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However, scientists found that the iron-depleted, oxidized chemistry in Earth’s continental crust likely didn’t come from crystallization of the mineral garnet, as proposed in 2018. Instead, the team believes that oxidized sulfur may be behind it.  They published their findings on May 4 in the journal Science

New continental crust comes from continental arc volcanoes found at subduction zones like the Cascadia subduction zone off the coast of the Pacific Northwest. These zones are where an oceanic plate dives beneath a continental plate. 

In 2018, the explanation of why the continental crust is iron-poor and oxidized came down to the crystallization of garnet, a group of silicate minerals. According to this theory, the garnet found in the magmas beneath continental arc volcanoes is removing non-oxidized iron from the Earth’s terrestrial plates. At the same time, the molten magma is depleted of iron and is more oxidized. 

This garnet explanation for iron depletion and oxidation in continental arc magmas was pretty compelling to study co-author Elizabeth Cottrell, a research geologist and curator of rocks at the Smithsonian’s National Museum of Natural History. However, one aspect of it 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,” Cottrell said in a statement.

Five years ago, Cottrell and Megan Holycross from Cornell University, along with their colleagues began to search for ways to test whether crystallization of garnet beneath arc volcanoes is actually essential for creating continental crust.

To test this, the team used piston-cylinder presses to recreate the massive pressure and heat that is found beneath continental arc volcanoes. In 13 different experiments, the team grew samples of garnet from molten rock inside the piston-cylinder press under pressures and temperatures similar to conditions inside of the Earth’s crust deep magma chambers– roughly 8,000 times more pressure than what’s inside a can of soda. The temperature in the experiment ranged from 1,742 degrees to 2,246 degrees Fahrenheit, hot enough to melt rock.

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The team then used garnets from the Smithsonian’s National Rock Collection and other collections from around the world where the concentrations of oxidized and unoxidized iron were already known.

The materials were then taken to the Advanced Photon Source at the US Department of Energy’s Argonne National Laboratory in Illinois. Using high-energy X-ray beams, the team conducted X-ray absorption spectroscopy. This technique can tell scientists about the structure and composition of materials based on how they absorb X-rays. The team looked at the concentrations of oxidized and unoxidized iron to check and calibrate X-ray absorption spectroscopy measurements and compare it with the materials from earlier experiments.

They found that the garnets had not incorporated enough unoxidized iron from the rock samples to account for the levels of iron-depletion and oxidation in the magmas that build up 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 Earth’s mantle below continental crust are setting these oxidized conditions.”

The findings led to more questions, such as what oxidizing and depleting the iron actually does  in the crust, as well as what is modifying the compositions. 

The leading theory is that oxidized sulfur could be oxidizing the iron and it is currently being investigated at the Smithsonian. This research was supported by funding from the Smithsonian (part of the Our Unique Planet initiative), the National Science Foundation, the Department of Energy, and the Lyda Hill Foundation.