The source-to-surface journey of volatile-flooded magmas: The archetypal case of the Campi Flegrei unrest Caldera

We attempt to reconstruct the architecture of the magmatic feeding system of the Campi Flegrei volcanic field, currently experiencing an unrest phase threatening several hundred thousand people, to shed light on the processes interplaying during magma evolution and transfer to the surface. To this aim, we provide, for the first time, a complete chemical dataset (major, trace and volatile elements, including hydrogen isotopes) of melt inclusions and their host pyroxenes. Case studies are the Campanian Ignimbrite eruption (∼40 ka) and explosive events that occurred in a short time-span (from ∼5 ka to ∼4 ka), in sectors of the Campi Flegrei caldera identified as having the highest probability of future eruptive activity.

Melt inclusions point to high dissolved volatiles content (H₂O ≈ 0.5–2.9 wt.%; CO₂ ≈ 120–2600 ppm; Cl ≈ 4000–10600 ppm; F ≈ 1400–3400 ppm; SO₃ ≈ 0.2–0.05 wt.%) which suggest that magma differentiation and degassing took place at pressures ranging approximately between 645 MPa and 74 MPa. In fact, one of the major outcomes of the paper is the temporary coexistence, at pressures higher than 200 MPa of felsic magma batches with mafic magmas. Modelling of gas release shows that trachytic magmas formed due to crystallization in a system featured by a mean oxidation state with Fe³⁺/Fe_tot = 0.15, under variable but high initial volatile contents: total volatiles exceed 5 wt.%, with not less than 2 wt.% CO₂. The high volatile elements content in the deep feeding system is possibly responsible for repeated fluxing events that may prompt the ascent of differentiated magmas to the shallowest reservoirs. Water-loss, due to the combination of repeated crystallization and CO₂ fluxing events in crustal reservoirs, satisfactorily explains the δD variability in melt inclusions, producing multiple paths approaching the δD of the local magmatic water discharged from present-day, low-temperature fumaroles. Our findings suggest that: (1) the studied eruptions were fed by trachytic magma inputs in equilibrium with a gas phase featured by XCO₂ ∼0.9 at pressures greater than 250 MPa; (2) XCO₂ decreases nearly continuously while pressure decreases; and (3) the obtained pressures cannot be simply converted into crystallization/storage depths. Conversely, local overpressure associated with gas fluxing must be considered. The overpressure can explain the ascent of differentiated, trachytic magmas that enter the uppermost plumbing system levels with an excess of volatiles, which may drive further magma crystallization and eruption. The final stage of magma ponding and degassing, between ∼200 MPa and ∼74 MPa, may represent the ultimate engine of the unrest phases at Campi Flegrei that precede volcanic eruptions.