Paleomagnetic constraints on emplacement temperatures in phreatomagmatic deposits from the astroni volcano (Campi Flegrei, Italy)

Explosive volcanic eruptions encompass a broad spectrum of eruptive styles and transportdepositional processes, generating diverse deposits and volcanic hazards. Among these, phreatomagmatic eruptions are particularly hazardous due to their ability to produce highly mobile pyroclastic density currents (PDCs) capable of causing severe destruction. Magma-water interaction processes have played a major role in the evolution of Campi Flegrei (Italy), one of the largest and most densely populated active calderas worldwide. Constraining the physical parameters that control PDC generation, mobility, and emplacement under these conditions is therefore critical for reconstructing eruption dynamics and improving hazard assessment and numerical modelling of eruptive scenarios. In this study, we use paleomagnetic methods to constrain the emplacement temperatures of the ~4 ka pyroclastic deposits from the Astroni volcano. The Astroni pyroclastic succession represents a unique and well-preserved eruptive sequence characterised by phreatomagmatic and magmatic events closely spaced in time, providing an excellent opportunity to examine fragmentation processes, transport-depositional mechanisms, and thermal conditions during PDC emplacement. We collected and oriented in situ two hundred one lithic clasts (0.3 - 4 cm in size) from ten intraand extra-crater sites representative of six different volcanic units. Paleomagnetic analyses indicate that the clasts experienced different thermal histories, displaying either a single (CH) or two (HT-LT) magnetic components. Clasts with a CH component were subdivided based on magnetic directions: cold (≤ 160 °C to ≤ 470 °C) clasts display randomly oriented directions, whereas hot (≥ 550 °C) ones show directions subparallel to the expected geomagnetic field. Clasts with HT-LT magnetic components record temperatures between 190 °C and 470 °C. Lithic clasts recording lower temperatures are interpreted as fragments incorporated cold and subsequently heated by the deposit; these clasts were therefore used to constrain the emplacement temperature of the PDC. In contrast, clasts recording higher temperatures are interpreted as hot lithic fragments, derived from the upper part of the plumbing system. Based on the lowest values identified at each locality, inferred emplacement temperatures range between 220-250 °C for Unit I, ≤160 °C for Unit II, 220-250 °C for Unit III, and 250-280 °C for Unit IV. Unit V, investigated in an ultra-proximal intra-crater area and at extra-crater localities, yields values ≤280 °C within the crater and ≤160-280 °C outside the crater. Unit VI, including an ultra-proximal intra-crater exposure and a proximal extra-crater exposure, shows temperatures of 220-250 °C and ~190 °C, respectively. Low temperatures both within the crater and on the outer slopes of the volcano consistently characterise the products of all Astroni eruptions, indicating relatively cool PDCs even in proximal depositional settings. A comprehensive explanation can be obtained by comparing the paleomagnetic results with data obtained from sedimentological observations and morphological and textural studies of the products. The occurrence of juvenile pumice clasts characterised by well-developed vesicles indicates that magma fragmentation was largely driven by magmatic volatile exsolution during eruption, resulting in rapid bubble growth and expansion typical of magmatic explosive activity. The coexistence of these pumiceous juvenile fragments with deposits containing accretionary lapilli and emplaced by relatively cool PDCs suggests that the Astroni eruptions involved a complex interplay between dominantly magmatic fragmentation and subsequent interaction with external water. While vesiculated pumice records primary magmatic degassing and fragmentation, the comparatively low emplacement temperatures inferred from paleomagnetic data indicate that cooling likely occurred during the final stages of magma ascent and/or at the surface, possibly through interaction with shallow water bodies and incorporation of external water and cold material. This scenario implies that fragmentation remained predominantly magmatic, whereas magmawater interaction occurred mainly after fragmentation, promoting cooling, ash aggregation and early deposition, and the emplacement of relatively dilute and low-temperature pyroclastic density currents.