Evidence of Shallow Storage and Re-equilibration of Magmas Feeding the 39.8 ka Campanian Ignimbrite (Italy) Eruption

The Campanian Ignimbrite (CI) eruption at 39.8 ka from Campi Flegrei in Naples, Italy, was the largest eruption recorded in Europe in the last 200 ka, emplacing a plinian fallout and dilute pyroclastic density current (PDC) deposits. Elevated concentrations of magmatic volatiles (i.e., H2O) in the phonolitic-trachytic melts may represent a mechanism for dilution of these PDCs. Trace- and major-element and volatile concentrations of clinopyroxene-hosted melt inclusions (MIs) from the CI units were analyzed via Secondary Ion Mass Spectrometry and Electron Microprobe to quantify the magmatic volatile contributions. The most voluminous units of the CI are slow cooling, containing MIs with low H2O concentrations (0.01-0.84 wt%) that record post-emplacement H loss. However, the phoenix cloud of the CI PDCs distributed a widespread fast-cooling co-ignimbrite fallout. MIs from this co-ignimbrite fallout were used as a proxy for the main PDC phase. MIs in the CI show increasing incompatible trace elements (e.g., Zr), while H2O concentrations are consistent at ~2 wt%. The uniformity of measured H2O concentrations with variable incompatible trace elements suggests that these MIs were volatile saturated prior to eruption. It is unlikely that the MIs were all trapped at the same time and record a similarly H2O-undersaturated magma, especially because the MIs have trace element concentrations that vary over a factor of ~6. We propose that magmas feeding the CI eruption underwent shallow storage (~350 bars), with H2O contents re-equilibrating to the saturation limit at this storage depth prior to eruption. Higher H2O concentrations (~3.4 wt%) and lower trace element concentrations found in some MIs likely correspond to eruption of magmas from deeper portions of the magma system. It is envisaged that the magmatic system is composed of discrete sills at differing depths that supplied magmas for different portions of the CI eruption. Therefore, MIs do not record the full magmatic volatile history of the CI, but, rather, magmatic volatile conditions during a short-lived, shallow storage event during which re-equilibration allowed H to escape from the MIs while H2O exsolved from the magma. This re-equilibration may have provided a large amount of free gas prior to eruption, with little time for it to escape the system before eruption.