Clinopyroxene thermobaromatry uncovers trigger mechanisms and reservoir configuration preceding eruptions at Campi Flegrei, Italy

Since 2005, Campi Flegrei (Italy) has been experiencing volcanic unrest characterized by earthquake swarms with magnitudes up to 4.6 and ground uplift rates of 10–30 mm/month, amounting to approximately 160 cm since the onset of unrest. With ~500,000 residents living within the red zone, it is critical to assess how the ongoing crisis may evolve and to identify potential eruption-triggering scenarios. Here, we apply clinopyroxene-only thermobarometry based on supervised machine learning [1] to pyroclastic samples collected from the opening and upper units of emblematic eruptions spanning a wide range of ages, eruption styles and intensities, and locations within the caldera.

Clinopyroxenes from the opening units display Mg# values (0.7–0.95) comparable to those of the upper units, except for Triflisco, but show stronger bimodality. In contrast, Mg# distributions in the upper units are generally more homogeneous. Thermobarometric estimates indicate that all eruptions were preceded by magma storage at shallow depths of 1–2 kbar (~4 km). Except for Monte Nuovo and Agnano Monte Spina, clinopyroxenes from the opening units also record the extraction of hot magma (~1100 °C) from depths exceeding 2.5 kbar (>8 km). In general, high-intensity eruptions (i.e., Neapolitan Yellow Tuff and Triflisco) show bimodality in the crystallisation temperature estimates separated by a clear gap at 900–1000 °C. Among the studied eruptions, only Triflisco shows clear petrological evidence for magma recharge as a triggering mechanism, whereas Agnano Monte Spina and Monte Nuovo were likely triggered by external processes, such as elastic crustal weakening [2]. Strikingly, the Campanian Ignimbrite displays continuous temperature estimates similar to those observed in low-intensity eruptions (e.g., Solfatara, Santa Maria delle Grazie, Averno 1), suggesting that reservoir configuration alone does not control eruption magnitude or intensity, and that surface deformation characteristics, mostly controlled by dynamics in the shallow portion of the plumbing system, might not be directly linked to the magnitude of a future eruption.

The shallowest depths of magma emplacement correspond to the present-day source of seismicity and ground deformation, while deeper storage levels match the deep reservoir imaged by seismic and recent magnetotelluric surveys [3], highlighting strong links between petrological and geophysical observations. We infer that the volume of eruptible magma present within the mid-lower crustal reservoir is a key parameter in estimating the intensity of a future eruption. This study provides new constraints on preferred magma pathways that may precede eruptive activity, as well as on plausible eruptive scenarios, both of which are essential for volcanic hazard assessment and risk mitigation.

[1] Ágreda-López et al. (2024) Computers & Geosciences, 193
[2] Kilburn et al. (2023) Commun Earth Environ 4, 190
[3] Isaia et al. (2025) Commun Earth Environ 6, 213