A geochemical and geophysical reappraisal to the significance of the recent unrest at Campi Flegrei caldera (Southern Italy)

Abstract

Volcanic unrest at calderas involves complex interaction between magma, hydrothermal fluids, and crustal stress and strain. Campi Flegrei caldera (CFc), located in the Naples (Italy) area and characterized by the highest volcanic risk on Earth for the extreme urbanization, undergoes unrest phenomena involving several meters of uplift and intense shallow microseismicity since several decades. Despite unrest episodes display in the last decade only moderate ground deformation and seismicity, current interpretations of geochemical data point to a highly pressurized hydrothermal system. We show that at CFc, the usual assumption of vapor‐liquid coexistence in the fumarole plumes leads to largely overestimated hydrothermal pressures and, accordingly, interpretations of elevated unrest. By relaxing unconstrained geochemical assumptions, we infer an alternative model yielding better agreement between geophysical and geochemical observations. The model reconciles discrepancies between what observed (1) for two decades since the 1982–1984 large unrest, when shallow magma was supplying heat and fluids to the hydrothermal system, and (2) in the last decade. Compared to the 1980's unrest, the post‐2005 phenomena are characterized by much lower aquifers overpressure and magmatic involvement, as indicated by geophysical data and despite large changes in geochemical indicators. Our interpretation points out a model in which shallow sills, intruded during 1969–1984, have completely cooled, so that fumarole emissions are affected now by deeper, CO2‐richer, magmatic gases producing the modest heating and overpressure of the hydrothermal system. Our results have important implications on the short‐term eruption hazard assessment and on the best strategies for monitoring and interpreting geochemical data.

The Campi Flegrei deep drilling project (CFDDP): Caldera structure and hazard

“…In this case, deep drilling provides the best mean to obtain direct information on processes occurring at depth and represents a key tool for reconstructing the caldera structure and understanding the volcano dynamic and volcanic risk assessment [10]. The highly urbanised Campi Flegrei Caldera have been experiencing decades of unrest, with uplift rates up to 1 m/y, thousands of microearthquakes per year [11] and strong geochemical anomalies [12], leading to people evacuation of Pozzuoli town in [1982][1983]. At the present, the alert level for the caldera has been moved from 'base' (ordinary activity of volcano) to 'attention' status.…”

Increasing CO<sub>2</sub> flux at Pisciarelli, Campi Flegrei, Italy

“…The aforesaid could justify the discrepancy recently highlighted by Moretti et al (2017) between weak geophysical signals (moderate uplift and low seismicity) and drastic changes in geochemical indicators characterizing the present stage of the CF history.…”

“…As all calderas, CF represents a complicated makeup that includes a magmatic plumbing system with a depth of up to ∼ 8 km (Bodnar et al, 2007;Zollo et al, 2008;Vitale et al 2014;Moretti et al, 2017), feeding the overlying hydrothermal system (Chiodini et al, 2010;Troiano et al, 2011;De Siena et al, 2017a) through an intricate network of fractures (Zollo et al, 2008;De Siena et al, 2010;Byrdina et al, 2014). A clear picture of the feeding mechanisms and its dynamics is one of the central open questions regarding CF and is subject to ongoing debate.…”

“…An increase in the supply of fluids and associated thermal energy into the hydrothermal system for depressurization of the magmatic source (Allard et al, 1991;Chiodini et al, 2016). This increased supply is thought to stem from either the ∼ 8 km deep main magma reservoir (Bodnar et al, 2007;Zollo et al, 2008;Moretti et al, 2017) or from a contribution of a magma batch that intruded into the shallow subsurface (∼ 3-4 km depth) concomitantly with the 1982-1984 unrest episode (Chiodini et al, 2010;Caliro et al, 2014) and is periodically rejuvenated by arrivals of deep, more primitive magma (Bagagli et al, 2017).…”

Understanding volcanic hazard at the most populated caldera in the world: Campi Flegrei, Southern Italy

“…[]; (b) picture of fumarole emissions at Solfatara, analyzed by Moretti et al . []; the ratio between H 2 O and CO 2 , determined in the last 34 years by these authors, is shown; (c) picture of the Roman Market (Macellum) in Pozzuoli town, with the marble columns of the “Serapis” Temple which, with their bores (caused by molluscs) indicating the marine ingression levels during the time, allowed to reconstruct the secular variations of the ground level since Roman times (shown in this frame, from Bellucci et al , ); (d) picture of the tuff cliff of Rione Terra, at the center of Pozzuoli town and the Port; this is the site of maximum measured deformation at Campi Flegrei, whose trend since 1905 to present is shown below the picture, as measured by precision levellings until 2000 and by continuous GPS after this date; (e) picture of the dismissed steel factory of Bagnoli (ILVA), which hosted the drilling site CFDDP; on the right side, the stratigraphic sequence recovered in the well is shown with recovered ages and depths of main samplings (red stars). (Figures and composition by Claudio Serio, photos by Alessandro Fedele).…”