Evidence of shallow storage and re-equilibration of magmas feeding the 39.8 ka Campanian Ignimbrite (Italy) eruption

Abstract The 39.8 ka Campanian Ignimbrite eruption from Campi Flegrei, Italy, emplaced voluminous Plinian fallout and deposits from dilute pyroclastic density currents. Elevated concentrations of magmatic volatiles have been proposed as a gas source for particle dilution of these pyroclastic density currents. Trace, major, and volatile element concentrations measured in 51 clinopyroxene-hosted melt inclusions from six Campanian Ignimbrite units were analyzed via Secondary Ion Mass Spectrometry and electron microprobe. These melt inclusions record H 2 O concentrations from near-zero to 3.5 wt%. The most quickly cooled fallout deposits have markedly consistent H 2 O concentrations averaging 2.02 ± 0.25 wt%. Ignimbrite flow units record universally low H 2 O concentrations < 1 wt%, which we interpret as having been degassed during slow cooling. The highly incompatible trace element Zr serves as a proxy for melt evolution in MI glasses and varies over a factor of six, despite stable H 2 O concentrations. We propose that shallow re-equilibration of H 2 O in the magma during brief storage at ~ 2.3 ± 0.36 km (460 ± 72 bar) just prior to eruption is responsible for consistent volatile concentrations within the melts that span a relatively large compositional range. Three MI have higher H 2 O concentrations at 3.38 ± 0.14 wt% and lower trace element concentrations corresponding to eruption of magmas from deeper portions of the magma system around 4.6 ± 0.35 km (920 ± 70 bar). The magma system is envisaged to be composed of discrete sills at varying depths, which supplied magma to different vents during successive phases of the eruption. Therefore, most melt inclusions do not record the full magmatic volatile history of the Campanian Ignimbrite, but reflect magmatic volatile conditions during a short-lived, shallow storage event during which re-equilibration allowed H + to escape from the melt inclusions while H 2 O exsolved from the magma, providing exsolved gas for the expanded pyroclastic density current.