Hydrothermal versus magmatic: Geochemical views and clues into the unrest dilemma at campi flegrei

The goal of fluid geochemistry of fumarolic systems is to constrain the thermodynamic response of a multiphase (at least two-phase) and multicomponent hydrothermal fluid system, which has enough degrees of freedom to respond to variations of external constraints, such as magmatic gas injections, by reequilibrating phase proportions and compositions of dissolved gas components. For active volcanic systems, such as Campi Flegrei caldera (CFc), this puts first-order constraints on the nature of the source that geophysical inversion of geodetic and gravimetric data can neither identify nor resolve. In this study, based on the geochemical record at CFc in the last 35 years, we review two main approaches appearing in the literature, yielding diametrically opposite conclusions for the 1982-84 and ongoing (post-2000) CFc unrest episodes. We show that inert gases help evaluate the geochemical signature of deep ascending gas, not compatible with magma migration at shallow depths during episodes of unrest. After exhaustion of the volatile content of the shallow magma emplaced in 1982-84, only the deep-sourced (8 km) magmatic gas (actually a supercritical fluid) feeds and warms up the present-day hydrothermal system. In addition, shallow steam condensation occurs certainly in the surroundings of fumarole emissions, but it was attained at fumaroles in few circumstances during the 1982-84 unrest. We establish that the nature of the 1982-84 unrest was magmatic due to the emplacement of a shallow (3-4 km deep) magma. This interfered with the “normal” degassing dynamics from the deep (8 km) magmatic reservoir of regional size. On the contrary, the post-2005 unrest is unlikely magmatic and likely hydrothermal. The pictured scenarios confirm in all cases, and independently of the type of unrest, the strong role played by the CO2-rich gas release of deep provenance.