A new atlas charts the global distribution of unusual, critical‑metal‑bearing igneous rocks, finding that they often form near the thick and ancient cores of the world's major continents.
Researchers from Cambridge's Department of Earth Sciences mapped occurrences of CO2-rich igneous rocks – the world's primary source of rare earth elements – finding that their distribution is strongly tied to variations in Earth's rigid outer layer, the lithosphere.
Thicker lithosphere is key to creating the right rocks for enrichment, say the researchers, allowing pockets of molten rock to become trapped at depth where they slowly steep to concentrate metals.
The findings, published in the journal Nature Geoscience , could be used to guide the search for new rare earth deposits, said Dr Emilie Bowman, lead author of the study from Cambridge Earth Sciences. "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."
Rare earth elements are used in the production of many everyday and advanced technologies, including smartphones and clean energy solutions such as wind turbines and electric vehicles.
Much of the world is dependent on imports of rare earth elements from China, but countries are seeing a growing need to move toward domestic sources which have greater security and sustainability of supply.
"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 holds a £1-million project to investigate this.
Previous investigations have tended to focus on rare earth formation at a specific site, or within a given region, said Gibson, "but we're scaling up and exploring the question at a global scale, whilst looking for deeper clues that might explain the surface geology."
Bowman assembled chemical data on 9,000 igneous rock samples from around the world, all enriched in dissolved CO₂ – a key ingredient that enhances the potential for 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."
Coming in a range of weird and wonderful forms, many of the rocks were first classified in the 19th and early 20th centuries – taking their names from the places where they were first collected or according to the unusual 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."
The team, including project co-lead Professor Sergei Lebedev and Dr Siyuan Sui, both geophysicists at Cambridge Earth Sciences, plotted the rock data onto a map alongside detailed information about Earth's interior.
"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 Lebedev. "From this mapping we can see that lithospheric thickness plays a guiding role in where we find these deposits."
"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," she explained.
Thicker parts of the lithosphere keep the underlying mantle rocks at high pressures and relatively cool, suppressing melting, Gibson explained. Only tiny amounts of the mantle can melt under these conditions, producing small pockets of magma that often get stuck at the base of the lithosphere where they solidify into CO₂‑rich igneous rocks. But it's only when those rocks are re-melted later that the metals get a second stewing – becoming concentrated enough to form a useful ore deposit.
Now the team plan to extend their map to include rocks older than 200 million years old, which host most of the economic rare earth element deposits and mines globally.
"For this work we focussed initially on deposits that were formed after the main phases of breakup of Earth's big continents," said Gibson. She explained that tectonic processes such as mountain building and rifting had churned up the older rocks, making them harder to study. "Now we have established this systematic behaviour 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."
