Evidence of pre-eruptive volatile-saturated magmas before the eruption of Agnano-Monte Spina (Phlegrean Fields, Southern Italy)

Assessing the pre-eruptive dynamic and volatile-saturation conditions of magmas of past eruptions at active volcanic systems is of paramount importance to understand the future behavior of volcanoes capable of high explosive events. Among all active volcanoes on the Earth, Campi Flegrei caldera (CFc) in southern Italy is a well monitored volcano considering the past highly explosive eruptions such as that of the Neapolitan Yellow Tuff (40 km³) occurred 15 ka ago. At the present, CFc has been undergoing through an unrest crisis consisting of a ~120 cm uplift of the central part of the caldera since 2005 (INGV, November 2023 bulletin), rising a debate relative to the origin of this crisis among scientists. The high volcanic risk is derived by the high degree of urbanization at CFc and the proximity to the city of Naples. For assessing the volcanic risk of CFc, scientists have selected the Agnano-Monte Spina (A-MS) eruption as a reference. The A-MS, in fact, represents the highest explosive event for the last 4.5 ka of CFc activity.

We studied 100 clinopyroxenes from pumices of air fall deposit associated to the A-MS, and we studied the petrography of their hosted melt and fluid inclusions (MI and FI respectively). The major and minor element compositions of the selected clinopyroxene were characterized based on electron-microprobe analysis. In addition, the volatile contents of the bubble of MI and FI were studied by using Raman microspectroscopy.

Two groups of clinopyroxene were identified based on mineral chemistry and petrography. On one hand, clinopyroxenes of one group show (1) a light green colour as hand specimen, (2) diopsidic composition (MgO=16-18 wt.%), and (3) resorbed margins. This group of clinopyroxenes was named as MgO-rich cpx. On the other hand, clinopyroxenes of the other group show (1) a showing a dark green colour as hand specimen, (2) salitic composition (MgO=12-14 wt.%), and (3) larger size grain relative to MgO-rich cpx. This group of clinopyroxenes was named as MgO-poor cpx. The most striking difference of these two groups of cpx are the types of MI and FI hosted in the crystals. Glassy MI are found exclusively hosted in the MgO-poor cpx. These glassy MI hosted in MgO-poor cpx show shrinkage bubbles (bubble-bearing MI). In contrast, MgO-rich cpx host MI and FI which were trapped heterogeneously together. Shrinkage bubbles of MI hosted in MgO-poor cpx did not show any Raman signals of volatile species, while FI and shrinkage bubbles of MI hosted in Mg-rich clinopyroxene showed CO₂, carbonate, and H₂S Raman signals.

Based on our results and data of MI and cpx from the literature, we suggest that a magma mush of relatively less evolved composition associated to MgO-rich cpx was present at shallow depth (~3 km). A deeper and more evolved magma ascended from deeper the plumbing system and mixed with a volatile-saturated mush triggering the A-MS eruption. We further suggested that the shallow magma mush was saturated with a CO₂-H₂S-rich magmatic fluids, while the deeper and more evolved magma did not present evidence of volatile saturation.