Continents are part of what makes Earth uniquely habitable for life among the planets of the solar system, yet surprisingly little is understood about what gave rise to these huge chunks of planetary crust and their special properties. Advances understanding of the Earth’s crust by testing and finally by dispelling one popular hypothesis as to why continental crust has less iron and is more oxidized compared to oceanic crust.
The iron-poor composition of the continental crust is the main reason why huge parts of the Earth’s surface stand above sea level as dry land, making terrestrial life possible today.
The building blocks of the new continental crust emerge from deep within the Earth in places known as continental arc volcanoes, which are located in subduction zones where an oceanic plate sinks beneath a continental plate.
In the garnet explanation for the iron-depleted and oxidized state of the continental crust, the crystallization of garnet in the magmas beneath these continental arc volcanoes removes unoxidized (reduced or ferric, as it is known among scientists) iron from the Earth’s plates while depleting it. molten magma of iron and leaves it more oxidized.
One of the key consequences of the low iron content of Earth’s continental crust compared to oceanic crust is that continents are less dense and buoyant, causing continental plates to sit higher on the planet’s mantle than oceanic plates. This discrepancy in density and buoyancy is the main reason why continents have dry land while oceanic crust is underwater, and also why continental plates always rise on top when they meet oceanic plates at subduction zones.
Garnet’s explanation for iron depletion and oxidation in continental arc magmas was compelling, but Cottrell said one aspect of it didn’t sit well with her.
To recreate the enormous pressure and heat found beneath continental arc volcanoes, the team used so-called piston-cylinder presses at the museum’s High-Pressure Laboratory and at Cornell. A hydraulic piston cylinder is about the size of a mini fridge and is mostly made of incredibly strong and strong steel and tungsten carbide. The force applied by a large hydraulic piston results in very high pressures on small rock samples about a cubic millimeter in size.
In 13 different experiments, Cottrell and Holycross grew garnet samples from molten rock inside a piston-cylinder press under pressures and temperatures designed to simulate conditions inside magma chambers deep in the Earth’s crust.
The pressures used in the experiments ranged from 1.5 to 3 gigapascals—that’s roughly 15,000 to 30,000 Earth atmospheres of pressure, or 8,000 times the pressure inside a can of soda. Temperatures ranged from 950 to 1230 degrees Celsius, which is hot enough to melt rock.
Additionally, the team collected garnets from the Smithsonian’s National Rock Collection and from other researchers around the world. Significantly, this group of garnets had already been analyzed so that their oxidized and unoxidized iron concentrations were known.
Technique that can tell scientists about the structure and composition of materials
Finally, the study authors took materials from their experiments and materials gathered from collections at the Advanced Photon Source at the US Department of Energy’s Argonne National Laboratory in Illinois. There, the team used high-energy X-rays to perform X-ray absorption spectroscopy, a technique that can tell scientists about the structure and composition of materials based on how they absorb X-rays. In this case, the researchers looked at the concentrations of oxidized and unoxidized iron.
Samples with known ratios of oxidized to unoxidized iron provided a way to check and calibrate the team’s X-ray absorption spectroscopy measurements and facilitate comparisons with materials from their experiments.
The results of these tests revealed that the garnets did not contain enough unoxidized iron from the rock samples to match the levels of iron depletion and oxidation present 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 is more likely that conditions in the Earth’s mantle beneath the continental crust create these oxidized conditions.”
Like many scientific results, these findings lead to further questions: “What causes iron oxidation or depletion?” Cottrell asked. “If it’s not the crystallization of garnet in the crust and it’s something about magmas coming up from the mantle, then what’s going on in the mantle? How were their songs edited?
Cottrell said these questions are difficult to answer, but that the leading theory now is that oxidized sulfur could oxidize iron, which the current Peter Buck Fellow is researching under her guidance at the museum.