Timing of magma extraction during the Campanian Ignimbrite eruption (Campi Flegrei Caldera)

A core drilled within the northern part of the city of Napoli has offered the unique opportunity to observe in one
single sequence the superposition of the four pyroclastic flow units emplaced during the Campanian Ignimbrite (CI)
eruption. Such a stratigraphic succession has never been encountered before in natural or in man made exposures.
Therefore the CI sequence was reconstructed only on the basis of stratigraphic correlations and compositional data
(in literature). The occurrence of four superposed CI flows, together with all the data available (in literature) allowed
us to better constrain the chemical stratigraphy of the deposit and the compositional structure of the CI magma
chamber. The CI magma chamber includes two cogenetic magma layers, separated by a compositional gap. The upper
magma layer was contaminated by interaction with radiogenic fluids. The two magma layers were extruded either
individually or simultaneously during the course of the eruption. In the latter case they produced a hybrid magma.
But no evidence of input of new geochemically and isotopically distinct magma batches just prior or during the
eruption has been found. Comparison with the exposed CI deposits has permitted reconstruction of variable eruption
phases and related magma withdrawal and caldera collapse episodes. The eruption was likely to have began with
phreatomagmatic explosions followed by the formation of a sustained plinian eruption column fed by the
simultaneous extraction from both magma layers. Towards the end of this phase the upward migration of the
fragmentation surface and the decrease in magma eruption rate and/or activation of fractures formed an unstable
pulsating column that was fed only by the most-evolved magma layer. This plinian phase was followed by the collapse
of the eruption column and the beginning of caldera formation. At this stage expanded pyroclastic flows fed by the
upper magma layer in the chamber generated. During the following major caldera collapse episode, the maximum
mass discharge rate was reached and both magma layers were tapped, generating expanded pyroclastic flows.
Towards the end of the eruption, only the deeper and less differentiated magma layer was tapped producing more
concentrated pyroclastic flows that traveled short distances.

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