Volcanic crystals (especially clinopyroxene) act as “black boxes” because they preserve layered chemical records of magma conditions deep underground, much like flight data recorders or tree rings, allowing scientists to reconstruct the hidden pre- eruptive history of magma storage, rejuvenation, and ascent.

Magma beneath volcanoes is typically not pure molten liquid but a crystal- rich “magma mush” (often compared to a semi- frozen granita: solid crystals suspended in melt). In a recent study published in Nature Communications, researchers examined clinopyroxene crystals from lava samples of La Palma’s 1712, 1971, and 2021 (Tajogaite) eruptions in the Canary Islands. High- resolution chemical imaging of these crystals revealed consistent patterns spanning more than 300 years.

Clinopyroxene grows slowly in concentric layers (zones) as magma cools, incorporating chemical signatures of the surrounding temperature, pressure and depth, and composition at each stage:

• Crystal cores record an ancient, cooler, crystal- packed mush reservoir at roughly 18- 25 km depth in the upper mantle. These older crystals sat largely undisturbed for long periods.
• Intermediate zones capture a sudden change day to weeks before eruption: injection of hotter, more primitive magma from deeper in the mantle. This heats and partially melts the mush, alters outer crystal chemistry, and stirs the mixture, remobilizing older crystals.
• Outermost rims record the final, relatively rapid ascent through the crust to the surface, where the

This is analogous to reading tree rings for climate history: the chemistry of successive growth zones preserves the sequence of events that prepared the volcano to erupt.

Standard volcano monitoring (seismicity, ground deformation, gas emissions) mainly detects shallower magma movement and is less sensitive to deep processes. The crystal records show that significant magma accumulation and eruption triggering at La Palma occur at great depth in the upper mantle, where unrest can be hard to detect.

Knowing this helps interpret monitoring signals more accurately, e.g., assessing how quickly an eruption might develop once signs appear, even if it does not provide exact forecasts of timing.

Similar chemical zonation patterns appear in clinopyroxene from other oceanic volcanic islands (Azores, Cape Verde, Galápagos, and other Canary Islands), suggesting that deep, long- lived crystal mushes that are periodically reactivated by fresh mantle magma are common, especially during the earlier, higher-magma- supply stages of island growth. As supply later declines, these mushes may cool and become less easily remobilized.

In short, these black crystals survive the journey from deep storage through mixing and ascent, carrying a durable archive of conditions that would otherwise be inaccessible. Analyzing them improves understanding of how volcanic systems prepare for eruption and supports better hazard assessment for places like La Palma and analogous island volcanoes.

Clinopyroxene chemical zoning is one of the most powerful archives of magma history available in volcanic rocks. These black or dark- green crystals grow by adding concentric (or sometimes sector- or patchy) layers, each of which chemically “records” the temperature, pressure, and melt composition present at the moment of growth. Because diffusion of many elements in clinopyroxene is relatively slow, the original zoning is often preserved even after the crystal is carried to the surface.

How zoning forms and what it records

Clinopyroxene (most commonly diopside- augite in alkaline basalts) crystallizes over a wide depth range, from the upper mantle into the crust. As it grows:

• Compatible elements (Cr, Ni, Sc, Mg) are preferentially incorporated from more primitive melts.
• Incompatible elements (Zr, REE, Ti, Mn, Na) become enriched in more evolved melts.
• Major- element ratios such as Mg#
• (= 100 × Mg, (Mg and Fe)) track the degree of melt evolution or mixing.

Common zoning styles include:

• Normal zoning: progressive decrease in Mg# and Cr outward (simple cooling and fractionation).
• Reverse zoning: abrupt increase in Mg# and Cr (injection of hotter, more mafic magma).
• Oscillatory or multiple reverse zones: repeated recharge events.
• Patchy or sector zoning: complex growth or partial resorption in a crystal- rich mush.
• Thin outer rims: final growth during ascent or eruption.

Because the crystal is a solid archive, the sequence of zones from core → inner rim → outer rim reconstructs the chronological order of events before eruption.

Zoning patterns revealed at La Palma (1712, 1971, 2021 eruptions)

In the Nature Communications study, high -resolution major- and trace- element mapping of clinopyroxene from the three historical Cumbre Vieja eruptions shows a remarkably consistent architecture:

Textural zone	Typical chemistry	Interpretation	Depth / Temperature
Cores	Broad range: primitive (high Cr, Ni, Sc, Mg#) to highly evolved (low Cr, high Zr, Mn, enriched REE; often green in thin section). Patchy or sector- zoned.	Antecrysts from a long-lived, cool (~1000- 1025 °C) phonolitic–tephriphonolitic crystal mush	Upper mantle (~18- 25 km)
Inner rims	Strong reverse zoning: sharp rise in Cr (up to ~3500 ppm), Ni, Sc, Mg#; lower Zr. Often euhedral.	Mafic (basanitic) recharge that remobilized and partially resorbed the mush. Minimum growth timescales ~4- 7 days	Mostly upper mantle; possible brief crustal stalling (~5- 10 km)
Outer rims and microlites	Lower Cr/Mg# relative to inner rims; compositions closer to the carrier melt	Final differentiation and or ascent	Shallower levels during final rise

Cluster analysis of the trace- element data isolates six compositional groups. The most evolved clusters occur almost exclusively in cores and represent crystallization from residual melts after 80- 90 % fractionation of a basanitic parent (producing phonolitic compositions). The most primitive clusters dominate the high- Cr inner rims and record the recharge magma itself.

Why the zoning is so informative

• Cr is an especially sensitive tracer of mafic recharge because it is strongly compatible in clinopyroxene and its diffusion is slow; high- Cr zones therefore mark the arrival of primitive melt.
• REE patterns help distinguish phonolitic mush- derived cores (often showing HREE enrichment characteristic of high- Na clinopyroxene) from the less- fractionated carrier melts.
• Thermobarometry applied zone- by- zone (both clinopyroxene, liquid exchange and machine-learning clinopyroxene- only calibrations) confirms that the evolved mush resided in the upper mantle, consistent with deep seismicity observed before the 2021 eruption.
• Matrix (groundmass) compositions remain relatively constant (~5 wt% MgO), showing that the carrier melts themselves did not change dramatically over 300 years; the main variation is in the recycled crystal cargo.

Broader significance

The same style of clinopyroxene zoning, cool, evolved upper- mantle mush cores overgrown by high- Cr recharge rims, appears in other ocean-island and alkaline volcanoes. It indicates that deep crystal mushes can persist for long periods and are periodically “unlocked” by hotter, primitive magma injections. Because these processes occur at depths that are difficult to image geophysically, the crystal record supplies information that monitoring networks alone cannot easily capture.

In short, clinopyroxene chemical zoning turns each crystal into a miniature stratigraphic log of the magma plumbing system, revealing storage conditions, recharge timing, mush recycling, and ascent pathways that would otherwise remain hidden.

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

Temporal variations in magma plumbing systems influence eruption priming and how we interpret volcanic unrest, but these are poorly constrained in low- flux ocean island volcanoes. The study examines clinopyroxene zoning from three historical eruptions at Cumbre Vieja on La Palma (Canary Islands): El Charco (1712), Teneguía (1971), and Tajogaite (2021).

Using quantitative trace- element mapping, thermobarometry, and cluster analysis, the authors show:

• Preceding all three eruptions, an evolved phonolitic (to tephri- phonolitic) crystal mush resided in the upper mantle at ~18- 25 km depth under relatively cool conditions (~1000- 1025 °C).
• At least one week before eruption, hotter basanitic recharge (~1100- 1150 °C) recycled/remobilized this mush and promoted magma ascent, potentially with brief stalling in the crust (~5- 10 km).
• Mafic recharge was critical for unlocking the mush but may not have been the immediate eruption trigger (subsequent cooling, fractionation and pressurization likely played a role).
• Upper-mantle evolved mushes appear common in ocean- island basalt (OIB) volcanoes; their presence is likely controlled by the island’s evolutionary stage and magma flux.

Methods and Data

• Samples: Lavas from the three eruptions (early tephrites followed by later basanites).
• Techniques: Petrography, major- element electron microprobe analysis (EMPA), laser- ablation ICP-MS for trace elements and quantitative mapping, clinopyroxene, liquid and machine- learning (ML) clinopyroxene- only thermobarometry, unsupervised cluster analysis of trace- element compositions, and comparison with matrix (carrier- melt) compositions.
• Clinopyroxene textures show complex cores (often patchy and or sector- zoned), reverse-zoned inner rims (higher Mg#), and thinner outer rims.

Main Results

Carrier melts: Microcrystalline matrices are relatively homogeneous (~5 wt% MgO) across eruptions and follow a fractional- crystallization trend; only modest differences exist (e.g., slightly more evolved El Charco 1712 melts).

Clinopyroxene chemistry: Cores span primitive to highly evolved compositions. High-Cr, high- Mg# inner rims record mafic recharge. Trace- element maps highlight patchy evolved cores and consistent high- Cr inner rims (Cr up to ~3500 ppm).

Thermobarometry: Exchange- based methods place most crystallization in the upper mantle (~18- 25 km, consistent with deep pre- 2021 seismicity). Evolved cores are cooler (~1000- 1025 °C). ML-based barometry additionally suggests possible transient crustal crystallization of rims/microlites (~5-10 km or shallower).

Cluster analysis: Six geochemical clusters link to processes, highly evolved clusters (0-1) form antecrystic cores from the phonolitic mush; primitive clusters (especially 5, high- Cr) form recharge- related inner rims and phenocrysts.

Interpretation and Broader Implications

An upper- mantle crystal mush of evolved (phonolitic) composition persists and is repeatedly evacuated and or recycled by basanitic recharge before historical eruptions. Inner- rim thicknesses imply minimum timescales of ~4- 7 days for the final recharge-related growth.

Recharge unlocks the mush but is followed by some cooling/fractionation, so it is not necessarily the direct trigger; progressive pressurization (e.g., from volatile exsolution) likely leads to eruption.

The paper is open access. A PDF is available on the Nature page. This is the scientific study underlying the recent Conversation/UQ article on volcanic crystals as “black boxes.”

Journal information: Nature Communications 17, Article number: 9362 (2026) 

DOI: 10.1038/s41467-026-77213-9

Provided: The Conversation

Authors: Alberto Caracciolo,
Teresa Ubide,
Mónica Ágreda-López,
Raquel Herrera,
Alvaro Marquez,
Diego González-García,
María José Huertas,
Eumenio Ancochea,
Nicolás Chicharro,
Juan Jesús Coello-Bravo &
Maurizio Petrelli 

Abstract

Temporal variations in magma plumbing influence eruption priming and the interpretation of unrest signals, yet remain poorly constrained in low-flux ocean island volcanoes. Here, we examine temporal changes in the active volcanic system beneath La Palma, Canary Islands, using clinopyroxene zoning from the 1712, 1971, and 2021 eruptions. Combining quantitative trace element mapping, thermobarometry, and cluster analysis, we show that preceding all eruptions, an evolved phonolitic crystal mush resided in the upper mantle (∼18–25 km depth) under relatively cool (1000–1025 °C) conditions. At least one week before eruption, basanitic recharge (1100–1150 °C) recycled this mush and promoted magma ascent, potentially with brief stalling in the crust (∼5–10 km). Mafic recharge was critical in unlocking the mush, yet may not represent the immediate eruption trigger. Globally, upper mantle evolved mushes may be common in ocean island basalt volcanoes, and their occurrence is likely controlled by the island evolutionary stage and magma flux.