Magma Magic: How Volcanoes Stash Rare Earth Treasures

Rare Earths: Crucial Ingredients for Green Tech (But Hard to Find)

Rare earth elements (REEs) are the unsung heroes powering our modern green technology revolution. These metals – with tongue-twisting names like neodymium, dysprosium, and europium – are essential in everything from the powerful magnets in electric vehicle motors and wind turbines to energy-efficient lights, screens, and advanced batteries. As the world races to build more EVs and wind farms to combat climate change, the demand for REEs has been skyrocketing. In fact, demand is projected to soar dramatically in the coming years as nations invest in renewable energy and electric transport.

For all their importance, however, rare earths come with a big challenge: they’re not actually rare in the ground, but they’re rarely found in rich, convenient chunks. Instead of large veins or nuggets, REEs are usually sprinkled in tiny quantities within other minerals. Extracting them can be like trying to pull needles out of a haystack – difficult, costly, and often messy for the environment. Currently, a handful of countries (notably China) dominate the production and refining of rare earths, meaning the rest of the world is highly dependent on those sources. This supply squeeze, combined with skyrocketing demand, has geologists and companies scrambling to develop better exploration models and find new places where nature might have concentrated REEs enough to mine. We need new clues and clever strategies to hunt down Mother Nature’s hidden stashes of these critical elements.

Iron Oxide-Apatite Deposits – Volcanoes with Hidden Rare Earth Riches

One promising clue has emerged from an unlikely source: old iron ore deposits formed by ancient volcanoes. In recent years, geologists have discovered that certain iron-rich mineral deposits – known as iron oxide-apatite (IOA) deposits – contain surprisingly high concentrations of rare earth elements. IOA deposits are basically what they sound like: enormous accumulations of iron oxides (minerals like magnetite or hematite, which are mined for iron) mixed with a lot of the mineral apatite. Apatite is a calcium phosphate mineral (famous as a source of phosphorus for fertilizer), and it turns out apatite can also host rare earth elements in its crystal structure. These IOA deposits are found in volcanic regions around the world and range from deposits over a billion years old to geologically young ones only a couple million years old. Famous examples include the giant Kiruna mine in Sweden (the largest underground iron mine in the world) and the El Laco deposit in Chile, an extinct volcano that erupted iron-rich lava. For decades, these places were tapped for iron (and sometimes phosphorus), with little thought given to rare earths lurking inside.

That changed when unexpected discoveries put IOA deposits on the rare earth map. Kiruna, Sweden – long known for its high-grade iron ore – was recently found to hold over one million tonnes of rare earth oxides mixed in with its iron ores. This makes it the largest known rare earth deposit in Europe, hiding in plain sight within an active iron mine! In the United States, the historic Pea Ridge iron mine (Missouri) turned out to contain rich rare earth minerals in its leftover rock, and in Chile, the iron ores from the El Laco volcano have yielded rare earth-rich minerals like monazite. It’s as if these iron-heavy volcanic systems had quietly been hoarding rare earth elements all along, and we’re only now realizing it.

Naturally, this raises the question: how did those volcanoes manage to stockpile so many rare earth elements in their iron-rich rocks? Was it just a fluke of geology, or a systematic process that we can understand (and use to guide exploration)? Geologists have been debating how IOA deposits form for years. Some theories argue that iron and phosphorus were concentrated by weird salty fluids or by magmas reacting with rocks, while others suspect a more direct magmatic process. One leading idea is that the answer involves a clever bit of volcanic alchemy called melt immiscibility – essentially, the magma equivalent of oil and water separation. If true, this would be the “trick” nature used to stash rare earths in these deposits. Recent research set out to test this idea, and the story that emerged is pretty amazing.

When Magma Separates Like Oil and Water

We usually imagine magma as a uniform, bubbling pot of melted rock, all mixed together. But under the right conditions, magma can split into two distinct liquids that refuse to mix – just like oil and water in a shaken salad dressing that gradually separate into two layers. Geologists call this phenomenon immiscibility, meaning the liquids won’t dissolve into each other. It turns out some volcanic magmas can essentially unmix themselves: one part of the magma becomes a silica-rich liquid (think of this as “normal” magma, the kind that forms common rocks like granite or basalt), and the other part becomes an unusual iron phosphate-rich liquid. This second liquid is packed with iron oxide and phosphorus (and often other components like fluorine), and it has a very different density and composition than the silica-rich magma. Once they split, these two liquids coexist like oil blobs suspended in water – two magmas in one chamber, each minding its own business.

If you’re having trouble visualizing it, picture a classic lava lamp. Inside a lava lamp, you have two fluids of different composition; when heated, the colored globs rise and fall but never mix with the surrounding liquid – they remain separate, floating around each other. Now replace the lamp’s colored wax with an iron-rich molten blob inside a silicate magma; that’s a decent analogy for what might happen in an immiscible magma. Another kitchen analogy is oil and vinegar: shake them together and you get droplets of one in the other, but given time they segregate into two layers. Mother Nature can do a similar trick in a magma chamber deep underground.

This immiscibility between a silicate melt and an iron-phosphate melt is believed to be a key to IOA deposits. The idea is that if a magma becomes saturated with iron, phosphorus, and perhaps volatiles like water or fluorine, it can spontaneously split. The iron-phosphate melt will be extremely rich in certain elements (phosphorus, iron, and others), while the silicate melt will be depleted in those. Crucially, many elements prefer one liquid over the other. It’s like a geochemical sorting hat: some ingredients in the magma “decide” they’d rather live in the dense iron-rich liquid than in the silica liquid. Rare earth elements, it turns out, are one such ingredient – and they have a strong preference for the iron-rich side. Geologists had long suspected this, because minerals that form from iron-phosphate liquids (like apatite or monazite) often contain lots of rare earths. But how to prove this process in action? That required creating the scenario in a laboratory and watching where the rare earths go.

Cooking Up a Mini-Volcano in the Lab

To test the magma immiscibility trick, a team of researchers led by Shengchao Yan and colleagues set up a clever experiment – essentially cooking up a mini-volcano in the lab. They took powders with the right chemical ingredients to mimic a natural volcanic system: one powder was a silica-rich “magma” mix (imagine ground-up bits of volcanic rock), and another was an iron-and-phosphate-rich mix (imagine a recipe enriched in iron oxide and phosphorus pentoxide, similar to what an IOA-forming magma might contain). They also added a pinch of water and fluorine-bearing minerals to the mix, because in real volcanoes those ingredients help encourage immiscibility. Instead of stirring all the ingredients together, the researchers layered them in a capsule – picture layering a dessert parfait, with a layer of iron-phosphate material and a layer of silicate material on top. This layering was done to help produce larger blobs of immiscible melt (past experiments found that if everything is too homogeneous, you get tiny droplets that are hard to analyze).

These tiny capsules (made of gold or platinum to withstand heat) were then placed into a piston-cylinder apparatus, a machine that can generate the immense pressures found deep in the Earth. The scientists squeezed the capsules to pressures of about 0.4–0.8 GPa (that’s roughly 4,000 to 8,000 times atmospheric pressure – equivalent to the pressure 15–30 kilometers underground) and heated them up to 800–1150 °C. In essence, they were simulating conditions in a magma chamber beneath a volcano. The capsules “cooked” at these hellish conditions for several days, giving the mixtures time to melt, interact, and – if the hypothesis was correct – separate into two liquids.

After the experiments, the capsules were rapidly cooled (quenched) to freeze in place whatever had formed. When the researchers opened them, it was like peeking inside a miniature volcano. The once-layered materials had melted and then split into two coexisting melts. There were clear signs of separation: blobs of one type of melt embedded in the other, and distinct compositions in different regions of the capsule. One melt was silica-rich and one was iron-phosphate rich, just as predicted. Tiny crystals of magnetite (iron oxide) had formed too, often in delicate branching shapes (like little metallic snowflakes), indicating that the iron-rich melt was saturated with iron oxide. There were even bubbles trapped alongside iron oxide globules, reminiscent of volcanic gases exsolving – a mini eruption frozen in time.

The experiment was a success: the team had recreated magmatic immiscibility in the lab. They effectively witnessed a magma dividing itself into two liquids, very much akin to the natural process theorized for IOA deposits. This meant they could now answer the golden question – or rather, the rare earth question: where did the rare earth elements end up in this great magma split?

Rare Earths Choose the Iron-Rich Side

With the experimental mini-magma solidified into tiny rock samples, the researchers then zoomed in with high-powered microscopes and analytical instruments to measure the chemical makeup of each type of melt. They especially looked at the distribution of rare earth elements between the two frozen magmas. The results were striking, even more dramatic than many might have expected. The rare earth elements overwhelmingly went into the iron-phosphate melt rather than the silicate melt. In other words, when faced with two liquid homes, the rare metals “chose” the iron-rich, phosphorus-rich liquid as their preferred residence by a huge margin.

How big a margin? In some cases, the iron-rich melt had on the order of 100 times more of a given rare earth element than the coexisting silicate melt. For the lighter rare earths (like lanthanum, cerium, neodymium – which are at the lighter end of the lanthanide series), the enrichment was even higher. The lightest rare earths were measured at nearly 150–200 times greater concentration in the iron-phosphate liquid compared to the silica liquid! The heavier rare earths (like ytterbium or lutetium at the heavy end) also concentrated in the iron melt, though their preference was a bit less extreme – roughly on the order of tens of times more concentrated than in the silicate melt (still a strong preference, just not a two-hundred-fold jump). This split behavior means the iron-rich melt became a veritable rare-earth soup, especially rich in the light rare earths, while the poor silicate magma was left relatively depleted in those elements.

To put it in perspective, imagine a vat of magma that somehow had, say, 100 parts per million of neodymium (a rare earth) when it was all one liquid. Once that magma split into two immiscible liquids, almost all of the neodymium would end up in the iron-phosphate liquid and practically none would remain in the silicate liquid. If you were to sample the iron-rich liquid, it would be loaded with neodymium (it might now have concentrations in the tens of thousands of ppm, i.e. a few weight percent, which is ore-grade level), whereas the silicate liquid might have only a scant amount left. Nature effectively separates the rare earths and concentrates them into one of the magma’s two “buckets.”

This experimental proof supports the long-suspected idea that immiscible magmas can generate rare earth-enriched zones. It explains a lot about IOA deposits: since those iron oxide-apatite bodies are thought to form from iron-phosphate-rich melts separating from a larger magma, it makes sense that they often contain rare earth minerals. The light rare earths in particular tag along with that iron-phosphate melt and later crystallize into minerals like monazite (a rare earth phosphate) or get incorporated into apatite. Essentially, the magma’s oil-and-water trick is also a rare-earth concentrating trick. Nature has a clever way to sort valuable metals using basic chemistry, long before humans ever came along looking for them.

New Clues for Exploration – A Win-Win for Finding Rare Earths

Now that we know how nature hides rare earth treasures inside iron-rich volcanoes, what do we do with this insight? For one, it gives geologists a powerful set of clues in the hunt for new REE sources. Rather than only looking in the usual places (like carbonatite intrusions or clay deposits in subtropical soils), explorers can now add IOA deposits and related iron-rich systems to their checklist of promising targets. If you’re exploring a region and find signs that an ancient magma might have separated into two liquids – for example, you encounter rocks that are unusually rich in magnetite and apatite, or you find droplets of former melts preserved in volcanic glass – that’s a hint you might be standing on a rare earth jackpot. Any Fe- and P-rich igneous rock (sometimes called a nelsonite when it’s mostly oxides and apatite) could be more than just an iron prospect; it could be a hidden rare earth deposit waiting to be recognized.

This knowledge also encourages a fresh look at existing mines. Around the world, there are operating or abandoned iron mines associated with extinct volcanic systems that have never been analyzed for rare earth elements. The new research suggests these could be low-hanging fruit. Why drill a brand-new mine in pristine wilderness if an old iron mine might have a mother lode of rare earths right in its waste piles or deeper extensions? In many cases, mining companies could potentially extract rare earth byproducts from the same ore they’re already processing for iron or phosphate. It’s a scenario that could reduce the need for separate rare earth mining operations, which are often environmentally challenging. As Dr. Michael Anenburg – an experimental petrologist and co-author of the study – noted about this approach:

“It’s a win-win,” he said, “because the company gets more value out of the stuff they’re mining anyway, and the environment wins since we don’t need to put a new hole in the ground.”

From a broader perspective, the discovery that iron-rich volcanic rocks can be REE-enriched helps us refine our exploration models at the planetary scale. We can begin to map out ancient volcanic provinces and ask: did any of those volcanoes produce immiscible iron-phosphate magmas? If yes, those are places to focus on for potential rare earth resources. It also aids scientific understanding – it resolves a piece of the puzzle of how IOA deposits form and why they so often feature rare earth minerals. Of course, there are still mysteries (for example, what exactly triggers the immiscibility in each natural case, and how the two melts segregate into large bodies that we can mine), but we’ve taken a big step toward answering those questions.

In the face of the green energy transition, having diverse and abundant sources of rare earth elements is critical. The story of silicate and iron phosphate melt immiscibility is a perfect example of nature’s clever chemistry at work: a process that separated and concentrated precious metals long before humans needed them. Now, with a bit of scientific sleuthing, we’ve uncovered that trick. Extinct volcanoes, once just seen as big iron ore providers, might turn out to be the sleeping giants of rare earth supply. All we have to do is recognize the signs of that ancient lava lamp in the geology, and we might reveal a cache of critical metals ready to power the technologies of tomorrow. It’s Mother Nature’s way of saying, “The treasure was here all along – you just needed to know how to look.”

Journal Reference:

1. S.C. Yan, B. Wan, M. Anenburg, J.A. Mavrogenes. Silicate and iron phosphate melt immiscibility promotes REE enrichment. Geochemical Perspectives Letters, 2024; 32: 14 DOI: 10.7185/geochemlet.2436