Insights into the processes and timescales of magma storage and ascent at Campi Flegrei caldera: From natural samples to experiments

The Campi Flegrei caldera is an outstanding example of large silicic calderas, offering critical insights into magma storage, transfer dynamics, and eruptive precursors in such systems. Since the last eruption in 1538 CE, the caldera experienced subsidence, interrupted by unrest episodes in 1950–52, 1970–72, 1982–84, and ongoing unrest since 2005. Here we integrated the results from recent 2D/3D microstructural and chemical characterizations of representative natural eruptive products with advanced high-temperature, high-pressure experiments conducted under controlled conditions to comprehensively investigate the magma storage and ascent conditions of this volcanic system. Petrological evidence reveals a long-lived, multi-level magmatic system, with a deep mafic reservoir (~300–400 MPa, 12–16 km depth) and a shallower, zoned, sill-shaped chamber (~150-200 MPa, 6–8 km depth), still detected today by recent geophysical surveys in this area. Small-volume intrusions occasionally reach upper crustal levels, near lithological discontinuities, to rapidly cool or erupt. These shallower intrusions are typically linked to small-scale eruptions, marked by slow magma ascent, open-system degassing and potential stasis at shallower depths, leading to prolonged unrests or failed eruptions. In contrast, large explosive eruptions involve rapid, sustained magma ascent and closed-system degassing, likely associated with fast conduit propagation and brief, deeper precursory signals. In this context, the combination of these petrological results with geochemical data highlights that the recent dynamics started during the 1982–84 unrest, reflects significant magma transfer (~1-3 km³) in the deeper part of the system (≥200 MPa), which released hot gases into the overlying hydrothermal system, deforming and fracturing the upper crust.