Researchers have created a new global map showing where unusual volcanic rocks linked to rare earth elements are found, uncovering a striking connection to the oldest and thickest parts of Earth’s continents.
The international team, led by scientists from the University of Cambridge’s Department of Earth Sciences, found that these rare earth-rich igneous rocks are strongly associated with variations in the lithosphere, the rigid outer shell of the planet. Their findings suggest that thick lithosphere plays a major role in forming the kinds of rocks capable of concentrating valuable metals over time.
The study, published in Nature Geoscience, could help scientists identify new rare earth deposits around the world.
“Our research is beginning to provide a kind of predictive power for where we can expect these rocks and, by extension, their associated rare earth element deposits, to form,” said Dr. Emilie Bowman, lead author of the study from Cambridge Earth Sciences.
Rare earth elements are critical components in many modern technologies, including smartphones, electric vehicles, and wind turbines. As demand for clean energy technologies rises, countries are increasingly searching for secure domestic supplies instead of relying heavily on imports from China.
Ancient Continental Crust Holds the Key
Scientists have long tried to understand why rare earth deposits appear in some regions but not others.
“There is significant scientific interest in why rare earth deposits form where they do,” said Professor Sally Gibson, senior author of the study from Cambridge Earth Sciences, who currently leads a £1-million research project focused on the topic.
Most previous studies examined individual deposits or specific regions. This research instead looked at the problem on a global scale while also exploring processes happening deep beneath Earth’s surface.
To carry out the study, Bowman compiled chemical information from about 9,000 igneous rock samples collected worldwide. All of the rocks were enriched in dissolved CO2, an important ingredient that increases the likelihood of rare earth element concentration.
“Until relatively recently, this subset of igneous rocks were mere curiosities,” said Gibson. “Geologists collected them avidly; undergraduates were baffled by them in practical classes. But in recent years they have become very relevant.”
Many of these rocks are highly unusual and were originally identified in the 19th and early 20th centuries. Their names often came from the places where they were discovered or from the strange minerals they contained.
“The terminology is so sprawling that you could almost make a new language from these rock names,” said Gibson. “This, and their scientific complexity, has added confusion, and people have tended to steer away from them.”
Earthquake Waves Reveal Hidden Rare Earth Clues
The researchers combined the rock database with detailed seismic imaging of Earth’s interior. Using earthquake waves, the team was able to map the thickness and structure of the lithosphere beneath different continents.
“Using seismic waves from earthquakes, we can create a slice-through image of the lithosphere, much like a sonar can pick out features on the seabed,” said Professor Sergei Lebedev, a geophysicist involved in the study. “From this mapping we can see that lithospheric thickness plays a guiding role in where we find these deposits.”
The scientists discovered that rocks with the right chemistry for rare earth enrichment are mainly found along the steep edges of Earth’s thickest and oldest lithosphere.
“We needed to put together these two pieces of the puzzle, the rock chemistry and seismic data, in order to make the connection,” said Gibson. “Rocks with the right chemistry for enrichment occur only in very specific places, mainly along the steep edges of Earth’s thickest and oldest lithosphere.”
How Rare Earth Deposits Slowly Form Deep Underground
According to the researchers, thick lithosphere keeps mantle rocks under high pressure and relatively cool conditions, limiting how much melting can occur. Under these conditions, only small amounts of magma form deep underground.
These pockets of magma often become trapped beneath the lithosphere, where they slowly cool and solidify into CO2-rich igneous rocks. Later geological events can partially melt those rocks again, allowing rare earth elements to become even more concentrated over time until economically valuable deposits eventually form.
The team now plans to expand its research to include rocks older than 200 million years, which contain many of the world’s major rare earth mines and deposits.
“For this work we focused initially on deposits that were formed after the main phases of breakup of Earth’s big continents,” said Gibson. She explained that geological activity such as mountain building and continental rifting has disturbed many older rocks, making them more difficult to analyze. “Now we have established this systematic behavior exists, we can go back further in time. It’s going to be more challenging, but I’m hopeful that this will be a key step in predicting mineral occurrences.”