Long-Term magmatic evolution reveals the beginning of a new caldera cycle at Campi Flegrei

Understanding the mechanisms that control the accumulation of large silicic magma bodies in the upper crust is key to determining the potential of volcanoes to form caldera-forming eruptions. Located in one of the most populated regions on Earth, Camp Flegrei is an active and restless volcano that has produced two cataclysmic calderaforming eruptions and numerous smaller eruptive events over the past 60,000 years. Here, we combine the results of an extensive petrological survey with a thermomechanical model to investigate how the magmatic system shifts from frequent, small eruptions to large caldera-forming events. Our data reveal that themost recent eruption ofMonte Nuovo is characterized by highly differentiated magmas akin to those that fed the pre-caldera activity and the initial phases of the caldera-forming eruptions. We suggest that this eruption is an expression of a state shift in magma storage conditions, whereby substantial amounts of volatiles start to exsolve in the shallow reservoir. The presence of an exsolved gas phase has fundamental consequences for the physical properties of the reservoir andmay indicate that a largemagma body is currently accumulating underneath Campi Flegrei.