The weirdest volcano in the world may be Tanzania’s towering Ol Doinyo Lengai, an active peak that squeezes out a strange, low-temperature lava called carbonatite. Carbonatites are composed of more than 50% carbonate minerals, the same substances that form the ocean’s reefs. At Ol Doinyo Lengai, they are key components of the coldest lava on the planet.

Carbonatites are found on every continent and range in age from today-ish years old (in Tanzania) to about 3 billion years old (in Greenland). What’s more, they’re a major source of critical minerals.

In a new study published in Science Advances, a team of scientists led by Carl Spandler from Adelaide University in Australia identified a compelling correlation between carbonatites and specific sections of Earth’s continents—those proximal to past subduction zones.

Carbonatites and Critical Minerals

In the United States, the federal government defines critical minerals as those essential to the nation’s economic or national security. These minerals must also have supply chains that are vulnerable to distortions such as demand surges and foreign conflict. For example, most of the world’s terbium, used for everything from naval sonar systems to indoor lighting, comes from China. The United States considers terbium a critical mineral because the possibility of political or economic conflict within China or between China and another polity could directly or indirectly threaten the world’s supply of the element.

Critical minerals are either chemical elements (like terbium) or minerals. Important elements range from the familiar, like the lithium we need for batteries, to the sesquipedalian, like praseodymium, used for high-strength magnets. (Sesquipedalian means “having to do with a very long word.”)

Praseodymium is one of the 17 rare earth elements (terbium is another), all of which are considered critical minerals. Rare earth elements are not actually rare and are often (but not always) found in carbonatites. If you wanted to identify a rock that likely hosts rare earth elements, “carbonatite would be a good place to start,” said Kathryn Goodenough of the British Geological Survey, who was not involved in this study.

Fertilizing the Mantle

Much of Earth’s mantle is rock that remains after magma has been extracted—this mantle has been depleted. But carbonatites must come from mantle that’s quite the opposite—from parts that had to have been fertilized with volatiles containing trace metals, often critical minerals of interest. The question of how the mantle source for carbonatites came to be fertilized has no definitive answer.

Just as a garden can be fertilized in many ways ranging from synthetic sprays to coplanted cover crops, Earth’s mantle can be fertilized via myriad methods. “You must have volatiles or melts rising up from deeper in the mantle that are carrying metals with them,” Goodenough said.

For example, as a slab subducts beneath another tectonic plate, a volcanic arc typically arises above the zone at which the subducting slab reaches about 100 kilometers below Earth’s surface. This is the approximate depth at which the slab releases water, triggering melting in the overlying plate.

But fluids and melts can continue to exit the subducting slab far beyond the trace of the volcanic arc. That far out, the overriding plate almost always comprises a complete section of lithosphere—crustal lithosphere on top and mantle lithosphere on the bottom. The fluids and melts from the underlying slab, rich in halogens, carbon dioxide, phosphorus, and the like, rise into the overriding plate’s mantle lithosphere, changing the rocks via a process called metasomatism, Goodenough explained.

On the other hand, mantle plumes ascending from the core-mantle boundary are thought to be fertilized from a graveyard of subducted slabs that pond in the very deepest part of the mantle.

Spandler and his colleagues focused on testing whether that first method of fertilization, subduction-driven metasomatism, spatially correlates with carbonatites and rare earth element deposits. TL;DR—it does.

Fertilized Mantle Lithosphere

GPlates is a piece of software that allows users to rewind the movements of tectonic plates, exploring how continents have shifted their locations over the past 2 billion years. Using GPlates, Spandler’s coauthors Andrew Merdith and Amber Griffin, also of Adelaide University, mapped 43 polygons that denote regions of subduction lasting 100 million years or longer. These polygons, the authors infer, mark the locations of fertilized mantle lithosphere, which they call FML. These zones are thought to contain the good stuff—the critical minerals of interest.

Spandler and his colleagues compared the locations of carbonatites and rare earth elements with the polygons. They found that 67% of carbonatites and 72% of rare earth element ore deposits lie within these polygons. This correlation, though not perfect, suggests that mantle lithosphere fertilized by subduction could provide the source for many of these curious and critical deposits.

“If [the correlation were] 100%, I wouldn’t believe it myself because geology doesn’t work that way,” Spandler said.

Two Stepping

Spandler and his colleagues argue that carbonatites form in a two-step process. He emphasized that the new paper focuses on the first step—the process that led to fertilization of the eventual sources for carbonatites and rare earth element deposits.

The second step—the trigger—generates the carbonate-rich magma itself. It’s this event that provides the heat that causes melting of the mantle, said Richard Ernst, a scientist in residence at Carleton University in Canada who was not involved in this study.

“The trigger can be almost anything,” said Spandler, because the lithosphere needs only a nudge to melt. A plume can disrupt the structure of the lithosphere, triggering carbonatite magmatism, but so can continental rifting, he said. Indeed, Ol Doinyo is one of the mountains presiding over the East African Rift (which some scientists think also sits atop a plume).

Previous work by Ernst considered whether plumes could provide at least part of the source for some carbonatites by looking at the age of the deposits and those of nearby large igneous provinces—dramatic, long-lived outpourings of hot basalt thought to result from mantle plumes. In that work, Ernst and his colleague, the late Keith Bell, found the ages of large igneous provinces correlate with the ages of nearby carbonatite deposits; in short, the examples in that paper are potentially linked in both space and time.

Where carbonatite ages match those of nearby flood basalts from large igneous provinces, Spandler said, “I suspect that may just be the trigger mechanism.”

Plume Problems

For some carbonatites, there’s a time difference between when the mantle was fertilized and when the magmas were emplaced, explained Goodenough. “We can track that in several different localities,” she said. This observation would support something like the two-step process outlined above, as opposed to plumes driving the entire sequence.

Another problem with associating carbonatite formation exclusively with plumes, Goodenough said, is that carbonatites require cool conditions that result in relatively minor mantle melting. Plumes, and the large igneous provinces they appear to produce, are hot, and a lot. Plume proponents counter this critique by arguing that carbonatites are often found near the edges of large igneous provinces, away from the hottest zones.

Ernst noted, however, that though Spandler and his colleagues have made the spatial argument for subduction, “they haven’t made the isotopic argument that requires a subduction zone mechanism [for the source].” That sets up a testable hypothesis for future studies that could make use of existing data-rich geochemical studies of deposits within FMLs.

Moreover, even newer research may link the two camps, at least in some cases, with geochemical indicators pointing to both mantle plumes and mantle lithosphere being involved in forming some carbonatites. The latter component, said Ernst, may result from subduction-based fertilization as proposed by Spandler and his colleagues.

The Future of FMLs

“This is just an example of what we could do [with GPlates],” said Spandler. “In the next decade, we’ll see these models getting much more sophisticated and applied to all sorts of things.”

Computing power has improved to allow these models to run in a reasonable time frame. Plus, there’s lots of data. “We have a much better understanding about the history of each little bit of the continental crust around the planet,” he said.

And although people rightly point out that details become fuzzy in plate models that reach into the Proterozoic and beyond, “you’ve just got to pick one model and use it,” said Goodenough. “They’ve…taken the most widely available, repeatable model out there and used that.”

And on the basis of that model, Spandler and colleagues have shown a correlation between subduction—via FMLs—and carbonatites and rare earth element deposits. If someone comes up with another explanation, Spandler said, “that’s fine as well.”

—Alka Tripathy-Lang (@dralkatrip.bsky.social ), Science Writer

Citation: Tripathy-Lang, A. (2026), Ancient subduction may have seeded today’s critical mineral deposits, Eos, 107, https://doi.org/10.1029/2026EO260173. Published on 29 May 2026.

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Editors’ Vox is a blog from AGU’s Publications Department.

The supply of magma from the Earth’s mantle is a primary source of heat to volcanic arc crust, where the heat is then dissipated through various processes. Much of this magmatic heat is dissipated as heated water, or aqueous fluid.

A new article in Reviews of Geophysics compares 11 different volcanic-arc segments where heat discharge via aqueous fluid has been well-inventoried to better understand the factors that influence this process. Here, we asked the authors to give an overview of heat discharge from volcanic arcs, how scientists measure it, and what questions remain.

Why is it important to study how heat is dissipated from volcanic arcs?

Volcanic arcs are the chains of volcanoes on top of subduction zones. They can produce some of Earth’s most explosive and hazardous eruptions. But much of the magma beneath the surface never erupts. Nevertheless, the heat from these magmas—and the simple fact of their existence and abundance—still matters for geothermal energy, patterns of groundwater flow, and the patterns of volcanic activity at the surface.

What are the main modes in which heat is discharged from volcanic arcs?

Heat at volcanic arcs can be carried by magmas, transmitted through the crust conductively, and carried by water seeping slowly through the crust. At the base of the crust, magmas are probably most important, with conduction coming in second. But as magmas move upwards through the crust, some of them solidify and impart their heat to their surroundings where it is transferred by conduction. Within a few kilometers of the surface, fluids seeping through the crust begin to take up all that heat, and so if we can quantify the heat carried by those fluids, we can retrace it to its origins in magmas.

How do scientists measure these different forms of heat loss?

Scientists measure the heat carried by erupting magmas using satellites, or by adding up the erupted masses and making an estimate of how much energy was released by cooling from eruption temperatures to solid igneous rocks at Earth’s surface. Conductive heat flow is measured by drilling holes in the Earth’s crust to see how quickly it gets hotter with depth.

Measuring the heat carried by aqueous fluids in the crust is in some ways the trickiest. One approach is to find all the springs where hot or even slightly warm water is trickling out and measure the temperature and discharge to estimate how much extra heat that water is carrying.

What are the challenges and uncertainties in measuring hydrothermal heat discharge?

One challenge is that many springs are only slightly warmer than you’d expect. There is good data for many hot springs, but there are data tracking these ‘slightly warm’ springs for only a subset of arcs. Another challenge is that warm underground fluids can flow laterally, so you have to try to account for that. This is not an uncertainty in hydrothermal discharge, but one additional big uncertainty for our study, where we were trying to quantify the proportion of magmas that freeze underground versus erupting, is in the estimates of how much magma has actually erupted through time.

What are some of the factors that influence hydrothermal heat loss?

A main factor that influences hydrothermal heat loss is the magmas that solidify underground. This link is the key motivation for our study. A major goal of our paper is to try to quantify these hidden magmas—how much magma needs to intrude the crust beneath the surface to supply the hydrothermal heat fluxes that we observe? The composition of magmas influences how much heat they can release. The depth at which magmas are emplaced also matters, because magmas that intrude the shallow crust eventually cool to lower temperatures than magmas emplaced in the lower crust and therefore release more heat.

What are the remaining questions or knowledge gaps where additional research efforts are needed?

A big outstanding challenge is combining estimates from hydrothermal data of how much magma is coming into the crust – like ours – with other approaches to estimating the same thing. The magmatic systems beneath volcanoes span the crust. At the base of the crust, you have magma supply, sort of like the water main feeding your plumbing system. Despite how fundamental magma supply is, we know remarkably little about it. It’s exciting to think about how the rates of magma supply could vary through time and space and why. Applying a range of techniques—including geophysical imaging, hydrothermal budgets, gas and igneous geochemistry, and petrology—to understand these questions could really be a game changer.

—Benjamin A. Black (bblack@eps.rutgers.edu; 0000-0003-4585-6438), Rutgers University, United States; S. E. Ingebritsen (steve.ingebritsen@gmail.com; 0000-0001-6917-9369), Kyoto University, Japan; and Kazuki Sawayama (sawayama@bep.vgs.kyoto-u.ac.jp; 0000-0001-7988-3739), Kyoto University, Japan

Editor’s Note: It is the policy of AGU Publications to invite the authors of articles published in Reviews of Geophysics to write a summary for Eos Editors’ Vox.

Citation: Black, B. A., S. E. Ingebritsen, and K. Sawayama (2026), Hydrothermal heat flow as a window into subsurface arc magmas, Eos, 107, https://doi.org/10.1029/2026EO265017. Published on 28 April 2026.

This article does not represent the opinion of AGU, Eos, or any of its affiliates. It is solely the opinion of the author(s).

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Editors’ Highlights are summaries of recent papers by AGU’s journal editors.

Source: Tectonics

At subduction zones, one tectonic plate dives beneath another, dragging rocks tens of kilometers into Earth’s interior where they are transformed by extreme pressures and temperatures. Some of these deeply buried rocks make it back to the surface, carrying a record of conditions along the plate boundary at depth. Geologists have long debated how these high-pressure rocks are exhumed and how they end up mixed into younger, lower-grade surrounding material.

Wang et al. [2026] address this question with detailed geologic mapping, Ar-Ar analyses, and U-Pb geochronology from subduction complex rocks on Cedros Island, offshore Baja California, Mexico. Their data show that high-pressure blocks yield cooling ages between 172 and 144 million years old, yet they are hosted in sedimentary rocks no older than about 92 million years. This age mismatch, combined with field evidence that the blocks are enveloped in sedimentary matrix rather than tectonically sheared into place, leads the authors to propose that the high-pressure rocks were exhumed to the surface, eroded, and recycled back into the subduction trench as sedimentary debris, potentially multiple times. The authors suggest that rapid exhumation was driven by extension within the forearc wedge. When plate convergence rates dropped abruptly, the wedge became gravitationally unstable and stretched along brittle-ductile shear zones, bringing deeply buried rocks to shallow crustal levels.

This polycyclic model is incompatible with alternative interpretations in which exotic blocks were mixed into their host matrix by viscous return flow within the subduction channel, because such models predict that blocks and their surrounding matrix should share similar thermal histories. Instead, the data require that blocks completed their journey to depth and back long before the surrounding sediments even entered the trench. The new understanding of subduction dynamics on Cedros Islands illuminates connections with the broader Franciscan Complex of California, where the origin of similar high-pressure blocks in younger matrix has been debated for decades. Together, these findings offer new perspectives on how subduction zones operate over long timescales and how their fragmentary rock record preserves fundamental evidence of the tectonic history of the continental margin. 

Citation: Wang, J. W., Kapp, P., Holder, R., He, J., Hernández-Uribe, D., & Worthington, J. (2026). Polycyclic metamorphism, exhumation, and recycling of subduction complex rocks, Cedros Island, Baja California. Tectonics, 45, e2025TC009340. https://doi.org/10.1029/2025TC009340

­­—Alexis Ault, Associate Editor, Tectonics

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Source: Geophysical Research Letters

As seismic waves travel through Earth, they gradually lose energy, a process called attenuation. That energy loss doesn’t happen uniformly—some features in the crust sap far more energy from seismic waves than others. Researchers can map underground features by watching where seismic waves lose more or less energy. The Southern Array for the Lithosphere and Uplift of Taiwan Experiment (SALUTE) is doing just that, providing information that could lead to improved seismic hazard planning in the country.

Lin et al. report attenuation results from SALUTE focused on the convergence between the Eurasian plate and the Luzon Arc, an understudied, geologically dynamic area where Earth’s crust is deforming. Using the overall attenuation rate and relative attenuation rates of P and S seismic waves, the authors imaged active faults, identified distinct lithologies, and better resolved the Luzon forearc block that sits just offshore of Taiwan.

The authors used data from the SALUTE high-density seismographic network, spanning December 2020 to December 2023, to construct both 2D and 3D attenuation models. They found clear changes in attenuation associated with major faults, as well as areas of high attenuation associated with fluid-rich, ductile zones in the lower crust that cause tectonic tremors. Their attenuation imaging additionally revealed that the Luzon forearc block, which had been poorly imaged in the past, dips northward and narrows as it nears the convergence zone.

The authors say their results agree well with previous velocity-based seismic imaging studies and show that attenuation can image features, such as transition zones, that were previously difficult to capture. Their data could also be useful for better understanding seismic hazard throughout the region, they note. (Geophysical Research Letters, https://doi.org/10.1029/2025GL121583, 2026)

—Nathaniel Scharping (@nathanielscharp), Science Writer

Citation: Scharping, N. (2026), Seismic attenuation techniques reveal what lies beneath Taiwan, Eos, 107, https://doi.org/10.1029/2026EO260150. Published on 11 May 2026.

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