Long time-series of chemical and isotopic compositions of Vesuvius fumaroles: evidence for deep and shallow processes Stefano Caliro 1,* , Giovanni Chiodini 1 , Rosario Avino 1 , Carmine Minopoli 1 , Berardino Bocchino 1 1 Istituto Nazionale di Geofisica e Vulcanologia, sezione di Napoli, Osservatorio Vesuviano, Naples, Italy ANNALS OF GEOPHYSICS, 54, 2, 2011; doi: 10.4401/ag-5034 ABSTRACT Long time-series of chemical and isotopic compositions of Vesuvius fumaroles were acquired in the framework of the volcanic surveillance in the 1998-2010 period. These allow the identification of processes that occur at shallow levels in the hydrothermal system, and variations that are induced by deep changes in volcanic activity. Partial condensation processes of fumarolic water under near-discharge conditions can explain the annual 18 O and deuterium variabilities that are observed at Vesuvius fumaroles. Significant variations in the chemical compositions of fumaroles occurred over the 1999-2002 period, which accompanied the seismic crisis of autumn 1999, when Vesuvius was affected by the most energetic earthquakes of its last quiescence period. A continuous increase in the relative concentrations of CO 2 and He and a general decrease in the CH 4 concentrations are interpreted as the consequence of an increment in the relative amount of magmatic fluids in the hydrothermal system. Gas equilibria support this hypothesis, showing a P CO2 peak that culminated in 2002, increasing from values of ~40 bar in 1998 to ~55-60 bar in 2001- 2002. We propose that the seismic crisis of 1999 marked the arrival of the magmatic fluids into the hydrothermal system, which caused the observed geochemical variations that started in 1999 and culminated in 2002. 1. Introduction The present volcanic–hydrothermal activity at Vesuvius volcano is relatively low level. The main evidence of this activity is: (a) widespread fumarolic emissions that are accompanied by diffuse soil CO 2 degassing in the crater area [Chiodini et al. 2001, Frondini et al. 2004]; (b) CO 2 -rich groundwaters along the southern flank of Vesuvius and in the adjacent plain [Caliro et al. 1998, Federico et al. 2002, Caliro et al. 2005]; and (c) seismic activity with epicenters clustered inside the crater [Saccorotti et al. 2002, Del Pezzo et al. 2004]. The most notable seismic activity was registered on October 1999. This crisis included an earthquake of M L 3.6, which was the highest magnitude recorded for at least 25 years, and possibly since the last eruption of Vesuvius in 1944 [Zollo et al. 2002, Del Pezzo et al. 2004]. After the October 1999 seismic crisis, seismic activity decreased to low levels, with shallow hypocenters mainly clustered within the volcanic edifice [Del Pezzo et al. 2004]. The degassing area of the Vesuvius crater is characterized by the presence of fumarolic vents that are sited on the crater rim and at the bottom of the crater (Figure 1). Fumarolic fluids discharged by these fumaroles on the crater rim are of relatively low temperatures (<75 ˚C) and are mainly composed of atmospheric components. Fumaroles from the crater bottom have a composition that shows H 2 O and CO 2 as the major components, followed by H 2 , H 2 S, N 2 , CH 4 , CO and He (in order of decreasing content), and discharge temperature of about 95 ˚C, i.e. the condensation temperature of fumarolic fluids at the crater altitude (P atm = 0.91 bar). The presence of significant CH 4 and NH 3 contents provides evidence to the origin of these fluids being from a hydrothermal environment [Chiodini et al. 2001]. In particular, on the basis of a comprehensive geochemical study of crater fumarolic fluids collected in the period 1998-1999, Chiodini et al. [2001] suggested the presence of a high-temperature hydrothermal system that is located below the Vesuvius crater. The aim of this study is to investigate the eventual changes that have affected this hydrothermal system over a relatively long period (1998-2010), on the basis of the data acquired in the framework of volcanic surveillance. These data include chemical compositions of the rim fumaroles, chemical and isotopic compositions of the crater-bottom fumaroles, and soil CO 2 flux data relative to a continuous monitoring station that is located at the bottom of the Vesuvius crater (Figure 1). We show modifications of the fumarolic fluids due to the occurrence of seasonal and deep processes. In particular, we focus on the variations that accompanied and followed the seismic crisis of 1999, which caused detectable anomalies in the Vesuvius groundwaters [Federico et al. 2004, Madonia et al. 2008]. Article history Received December 20, 2010; accepted May 3, 2011. Subject classification: Vesuvius, Hydrothermal system, Geochemical monitoring, Stable isotopes. 137