Continents play a vital role in supporting life on Earth by providing a platform for the growth of diverse ecosystems. Yet, surprisingly little is understood about the formation of these huge pieces of the planet’s crust and their unique properties. A new study by Elizabeth Cottrell, research geologist and curator of rocks at the Smithsonian’s National Museum of Natural History, and lead study author Megan Holycross, deepens our understanding of Earth’s crust by testing and ultimately eliminating one popular hypothesis about why continental crust is lower in iron and more oxidized compared to oceanic crust.
The study, published in Science, uses laboratory experiments to show that the iron-depleted, oxidized chemistry typical of Earth’s continental crust likely did not come from the crystallization of the mineral garnet, as a popular explanation proposed in 2018.
The iron-poor composition of continental crust is a significant reason why vast portions of the Earth’s surface stand above sea level as dry land, making terrestrial life possible today.
The building blocks of new continental crust issue forth from the depths of the Earth at what are known as continental arc volcanoes, which are found at subduction zones where an oceanic plate dives beneath a continental plate. In the garnet explanation for continental crust’s iron-depleted and oxidized state, the crystallization of garnet in the magmas beneath these continental arc volcanoes removes non-oxidized iron from the terrestrial plates, simultaneously depleting the molten magma of iron and leaving it more oxidized.
The study authors set about finding a way to test whether the crystallization of garnet deep beneath these arc volcanoes is indeed essential to the process of creating continental crust as is understood. The team used what are called piston-cylinder presses to recreate the intense heat and pressure found beneath continental arc volcanoes in the lab. In 13 different experiments, they grew samples of garnet from molten rock inside the piston-cylinder press under pressures and temperatures designed to simulate conditions inside magma chambers deep in Earth’s crust.
Next, the team collected garnets from Smithsonian’s National Rock Collection and from other researchers around the world. This group of garnets had already been analyzed, so their concentrations of oxidized and unoxidized iron were known. The study authors then took the materials from their experiments and those gathered from collections to the Advanced Photon Source at the U.S. Department of Energy’s Argonne National Laboratory in Illinois.
There the team used high-energy X-ray beams to conduct 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 were looking into the concentrations of oxidized and unoxidized iron.
The results of these tests revealed that the garnets had not incorporated enough unoxidized 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. Therefore, the garnet crystallization model is an unlikely explanation for why magmas from continental arc volcanoes are oxidized and iron-depleted.
“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 study is an example of the kind of research that museum scientists will tackle under the museum’s new Our Unique Planet initiative, a public-private partnership that supports research into some of the most enduring and significant questions about what makes Earth special. Other research will investigate the source of Earth’s liquid oceans and how minerals may have served as templates for life.
This research was supported by funding from the Smithsonian, the National Science Foundation, the Department of Energy, and the Lyda Hill Foundation.
The findings of this study open new avenues of research for scientists who are trying to understand the origin of Earth’s continental crust. It suggests that the process of creating continental crust is much more complex than previously thought and that there might be other factors at play in the formation of these critical parts of our planet’s crust.
According to Cottrell, the study also raises new questions about what is doing the oxidizing or iron depleting. If it’s not garnet crystallization in the crust, then what is happening in the mantle? How did their compositions get modified?
“These questions are hard to answer, but now the leading theory is that oxidized sulfur could be oxidizing the iron,” she said.
The study is an essential contribution to our understanding of Earth’s geological processes, which are critical to supporting life on our planet. It highlights the importance of continued research into the workings of our planet and the need for new techniques and technologies to explore its mysteries.
The Smithsonian’s National Museum of Natural History, where the study was conducted, is a leading institution in the field of natural history, with collections of over 145 million specimens and artifacts. It is home to some of the world’s most renowned scientists and researchers, who are committed to advancing our knowledge of the natural world.
The Our Unique Planet initiative, which supported this study, is an exciting development that will help to fund groundbreaking research into some of the most fundamental questions about our planet’s history and evolution.
Overall, the study is a significant step forward in our understanding of Earth’s geological processes and will provide scientists with new avenues for research and exploration. It is a testament to the power of collaboration and the dedication of scientists and researchers worldwide who are committed to advancing our understanding of the natural world.