Long time-series of chemical and isotopic compositionsof vesuvius fumaroles: Evidence for deep and shallow processes

Abstract

Long time-series of chemical and isotopic compositions of Vesuviusfumaroles were acquired in the framework of the volcanic surveillance inthe 1998-2010 period. These allow the identification of processes thatoccur at shallow levels in the hydrothermal system, and variations that areinduced by deep changes in volcanic activity. Partial condensationprocesses of fumarolic water under near-discharge conditions can explainthe annual 18O and deuterium variabilities that are observed at Vesuviusfumaroles. Significant variations in the chemical compositions offumaroles occurred over the 1999-2002 period, which accompanied theseismic crisis of autumn 1999, when Vesuvius was affected by the mostenergetic earthquakes of its last quiescence period. A continuous increasein the relative concentrations of CO2 and He and a general decrease in theCH4 concentrations are interpreted as the consequence of an increment inthe relative amount of magmatic fluids in the hydrothermal system. Gasequilibria support this hypothesis, showing a PCO2 peak that culminated in2002, 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 themagmatic fluids into the hydrothermal system, which caused the observedgeochemical variations that started in 1999 and culminated in 2002. © 2011 by the Istituto Nazionale di Geofisica e Vulcanologia. All rights reserved.