Here’s what you’ll learn when you read this story:

• Rare earth elements are actually everywhere, but deposits of them large enough to mine are not easily found in the lithosphere (the crust and mantle).
• Researchers used seismic data from earthquakes to create tomographic images that revealed where these elements are most likely to be found.
• The researchers plan to study much older rocks, possibly with larger deposits, in the future.

Deep in Earth, there could be troves of a type of undiscovered treasure more valuable than any diamond or ruby or sapphire: rare earth elements. REEs, as they’re also called, could be nestled in deposits far below Earth’s surface—and there’s now a treasure map that could lead us right to them.

Led by igneous petrologist Emilie Bowman from Cambridge University, an international team of researchers created a map that highlights bizarre volcanic rocks that might contain rare earth elements. These elements are actually scattered everywhere in Earth’s crust, but hardly ever occur in deposits large enough to be mined. Not only are they used in modern tech gadgets (like the neodymium in your smartphones), they’re also already fueling next-gen green technologies.

So far, most of the planet has relied on rare earth deposits found in China. These deposits tend to form in magma rich in carbon dioxide, which eventually hardens into igneous rocks such as silicates and carbonatites. Some species of these igneous rocks were previously thought to occur in shallower parts of the lithosphere, which comprises the crust and mantle. Others were associated with the edges of cratons, which make up the most ancient and stable parts of the continental crust. The question of why larger accumulations of rare earth elements are found in certain regions of the lithosphere eventually led Bowman and her team to investigate.

“An understanding of where these magmas are emplaced on the surface of the Earth and the processes that control their spatial distribution is thus crucial to advancing our knowledge of global CO₂ cycling as well as the locations of economic mineral deposits,” she said in a study recently published in Nature Geoscience.

Lithospheric depth can give away the locations of some of these deposits. The problem, however, is that estimates often rely on which types of magmas and igneous rocks have already been found at certain depths. Basanites are closest to the surface (if 62 miles or 100 km can be considered close). Carbonatites—the extremely strange rocks known for being made up of more carbonates than the usual silicate minerals in igneous rocks—are somewhere in the middle. And kimberlites are buried in the deepest reaches.

Bowman was searching for possible evidence of deposits beyond what had already been suggested. To do so, she mined a database of around 9,000 samples of igneous rocks unearthed around the world, and used seismic wave data from past earthquakes to find out more about what is really going on inside our Earth.

The researchers turned earthquake findings into images that sliced through the lithosphere, revealing where experts would be most likely to find igneous rocks with the right chemistry for the formation of rare earth elements. They saw that igneous rocks high in CO₂ occur under high pressure in the oldest and thickest regions of the lithosphere. Less intense heat in these regions means that rocks are not as susceptible to melting, so there are few bubbles of magma among what is mostly solid rock. If those rocks melt and re-solidify with the shifting of tectonic plates or other geological phenomena, rare earth elements can become more and more concentrated until they aggregate into deposits substantial enough to mine.

Rocks analyzed in this study were geologically young, at under 200 million years old. In future research, Bowman plans to go back much further in time and target rocks from before the breakup of Pangea, which she thinks may hold the largest undiscovered deposits of rare earth elements (some of the most important REE mines exist in rocks that may have come into being before a dinosaur ever hatched). While rare earth deposits in many of these rock layers may be more difficult to make out because of disturbances over the eons, they also might reveal even more treasure.

“Future studies [will determine] whether this lithospheric thickness–magma type relationship has changed through geologic time,” Bowman said, “a question of particular importance given some of the largest rare earth element deposits.”