Hidden Crystal Archives Reveal La Palma’s Deep Mush Reservoir Erupts Again and Again

Beneath the Canary Island of La Palma, the magma that eventually poured from the Cumbre Vieja ridge in 2021 did not spend its final years in a shallow crustal chamber, as many volcanologists once assumed. Instead, a new study of three historical eruptions shows that the island’s volcanic plumbing is anchored deep in the upper mantle, where a cold, crystal-rich reservoir of evolved phonolitic magma has been repeatedly broken apart and flushed out by hot basaltic recharge before every eruption for at least three centuries. The finding, published in Nature Communications, offers a strikingly consistent picture of how low-flux ocean island volcanoes store and evacuate magma, and it carries sobering implications for volcano monitoring worldwide.

A team led by Alberto Caracciolo of the University of Perugia examined clinopyroxene crystals erupted during the El Charco eruption of 1712, the Teneguía eruption of 1971, and the Tajogaite eruption of 2021, the destructive nine-week event that devastated parts of the island and displaced thousands of residents. Clinopyroxene is an ideal mineral archive: it crystallizes over a wide range of depths, and its slow rates of chemical diffusion preserve trace element zoning that records the temperature, pressure, and composition of the magmas through which the crystal grew. By reading these zoned crystals like tree rings, the researchers reconstructed the pre-eruptive history of each eruption in remarkable detail.

The team combined quantitative trace element mapping by laser ablation inductively coupled plasma mass spectrometry, electron microprobe analyses, clinopyroxene-based thermobarometry, and unsupervised machine learning cluster analysis. The clustering, applied to more than 316 single-spot analyses and nearly 400,000 mapped pixels, resolved six geochemically distinct populations of clinopyroxene. Two of these populations, found exclusively in resorbed crystal cores, turned out to be the smoking gun for the deep evolved reservoir: they crystallized from melts so differentiated that up to 80 to 90 percent fractional crystallization of a primitive basanite would be required to generate them, matching the degree of differentiation needed to produce phonolite from basanite beneath La Palma.

Thermobarometry told the researchers where and how hot these processes occurred. Primitive clinopyroxene cores record crystallization pressures of roughly 6 to 9 kilobars, equivalent to depths of about 22 to 32 kilometers, well below the local Moho at 10 to 14 kilometers. Evolved cores, the green, sodium-rich crystals that grew in the phonolitic mush, record the coolest temperatures in the entire dataset, between 1000 and 1025 degrees Celsius, at depths of roughly 17 to 21 kilometers. In contrast, the primitive inner rims that overgrow these cores record much hotter conditions of 1100 to 1150 degrees Celsius, the signature of fresh, mafic basanitic magma invading the reservoir from below.

The geometry of the zoning is what makes the story so compelling. In nearly every crystal examined, a chromium-rich, magnesium-rich inner rim abruptly overgrows an evolved, resorbed core, marking the moment when hot basanitic recharge disrupted the resident mush and swept its antecrysts into the ascending magma. Using measured rim thicknesses of roughly 40 to 50 micrometers and growth rates borrowed from comparable alkaline systems such as Mount Etna, the team estimated that these recharge events occurred at least four to seven days before eruption. In other words, the crystals recorded a countdown: the mush was unlocked days to weeks before magma reached the surface.

Yet the recharge was not the immediate trigger. In the early-erupted tephrites, the recharge signature is followed by a slightly more fractionated rim composition, indicating that after the mafic input the system cooled and fractionated rather than erupting immediately. The authors argue that eruption onset was more likely driven by progressive pressurization of the reservoir as volatile exsolution during continued crystallization raised the internal overpressure until the surrounding rock failed. This interpretation is consistent with independent evidence from the 2021 Tajogaite eruption, where phase equilibrium experiments and olivine zoning indicate pre-eruptive cooling of the tephritic magma.

Perhaps the most remarkable result is the temporal consistency of the system. Across eruptions separated by as much as 259 years, the same crystal populations appear, the same storage depths recur, and the same recharge-to-fractionation trends hold. The microcrystalline matrix compositions, which serve as crystal-free proxies for the carrier melts, cluster around 4.7 to 4.9 weight percent MgO in all three eruptions, and thermodynamic fractional crystallization modeling shows that the transition from basanite to tephrite within each eruption required only about 10 to 20 percent crystallization. The plumbing system beneath Cumbre Vieja, in short, has been operating in a stable configuration for centuries, with magma storage dominantly in the upper mantle and only transient stalling in the crust at depths of roughly 5 to 10 kilometers, a zone also suggested by fluid inclusion data and shallow seismicity during the 2021 eruption.

To test whether this deep-mush model applies beyond La Palma, the team compiled clinopyroxene major element data from low-flux ocean island basalt volcanoes around the world, including the Canary Islands, Cape Verde, the Azores, and the Galápagos, and projected the data into principal component space. The analysis revealed that volcanoes in the early, high-flux stage of island evolution, such as La Palma, El Hierro, Fogo, and Pico, commonly preserve clinopyroxene compositions consistent with recycling of evolved mush in their mafic lavas. Mature islands with waning magma flux, such as Tenerife and Gran Canaria, lack this signature in their mafic rocks, suggesting that as a volcanic system ages and its magma supply declines, the deep evolved mush solidifies between recharge events and its record is erased.

The implications for hazard assessment are significant. Because a large share of magma evolution at low-flux ocean island volcanoes occurs at depths of 18 to 25 kilometers or deeper, much of the pre-eruptive process may be poorly resolved by conventional geophysical monitoring, which is most sensitive to shallower crustal signals. The 2021 Tajogaite eruption did produce deep seismicity at 20 to 25 kilometers in the days before magma broke the surface, and the new petrological evidence suggests that such deep signals may mark the critical moment when basanitic recharge begins to evacuate the mantle mush. Recognizing that evolved, potentially more explosive melts can be generated and stored near the Moho without prolonged shallow accumulation, and that their mobilization may precede eruptions by only days to weeks, gives monitoring networks a clearer target: the deep, hidden reservoirs where La Palma’s eruptions, and perhaps those of many ocean island volcanoes, truly begin.

Subject of Research: Magma storage and mush evacuation in the ocean island volcano La Palma revealed by clinopyroxene chemistry

Article Title: Recurrent evacuation of mantle mush in ocean islands revealed by clinopyroxene from La Palma

Article References: Caracciolo, A., Ubide, T., Ágreda-López, M., Herrera, R., Marquez, A., González-García, D., Huertas, M. J., Ancochea, E., Chicharro, N., Coello-Bravo, J. J., & Petrelli, M. (2026). Recurrent evacuation of mantle mush in ocean islands revealed by clinopyroxene from La Palma. Nature Communications, 17(1), Article 9362. https://doi.org/10.1038/s41467-026-77213-9

Image Credits: AI Generated

DOI: 10.1038/s41467-026-77213-9

Keywords: La Palma, Cumbre Vieja, clinopyroxene, crystal mush, magma recharge, thermobarometry, ocean island basalt, phonolite, Canary Islands, volcanology, Tajogaite eruption, magma plumbing system