PII S0016-7037(01)00583-X Geochemical evidence for the existence of high-temperature hydrothermal brines at Vesuvio volcano, Italy GIOVANNI CHIODINI, 1, *LUIGI MARINI, 2 and MASSIMO RUSSO 1 1 Osservatorio Vesuviano, via Manzoni 249, 80122 Napoli, Italy 2 DIPTERIS, Universita ` di Genova, Corso Europa 26, 16132 Genova, Italy (Received December 20, 1999; accepted in revised form January 29, 2001) Abstract—A high-temperature hydrothermal system is present underneath the crater area of Vesuvio volcano. It is suggested that NaCl brines reside in the high-temperature reservoir and influence the chemical compo- sition of the gases discharged by the fumaroles of the crater bottom (vents FC1, FC2, and FC5). These have typical hydrothermal compositions, with H 2 O and CO 2 as major components, followed by H 2 ,H 2 S, N 2 , CH 4 , and CO (in order of decreasing contents) and undetectable SO 2 , HCl, and HF. Fumarolic H 2 O is either meteoric water enriched in 18 O through high-temperature water-rock oxygen isotope exchange or a mixture of meteoric and arc-type magmatic water. Fumarolic CO 2 is mainly generated by decarbonation reactions of marine carbonates, but the addition of small amounts of magmatic CO 2 is also possible. All investigated gas species (H 2 O, CO 2 , CO, CH 4 ,H 2 ,H 2 S, N 2 , and NH 3 ) equilibrate, probably in a saturated vapor phase, at temperatures of 360 to 370°C for vent FC1 and 430 to 445°C for vents FC2 and FC5. These temperatures are confirmed by the H 2 -Ar geoindicator. The minimum salt content of the liquid phase coexisting with the vapor phase is 14.9 wt.% NaCl, whereas its maximum salinity corresponds to halite saturation (49.2–52.5 wt.% NaCl). These poorly constrained salinities of NaCl brines reflect in large uncertainties in total fluid pressures, which are estimated to be 260 to 480 bar for vents FC2 and FC5 and 130 to 220 bar for vent FC1. Pressurization in some parts of the hydrothermal system, and its subsequent discharge through hydrofractur- ing, could explain the relatively frequent seismic crises recorded in the Vesuvio area after the last eruption. An important heat source responsible for hydrothermal circulation is represented by the hot rocks of the eruptive conduits, which have been active from 1631 to 1944. Geochemical evidence suggests that no input of fresh magma at shallow depths took place after the end of the last eruptive period. Copyright © 2001 Elsevier Science Ltd 1. INTRODUCTION At present, Vesuvio volcano is in a quiescent period char- acterized by widespread fumarolic activity in the inner slopes and the bottom of the crater (Fig. 1). The chronogram of the maximum temperatures measured in the crater area after the last eruption, which took place in 1944 (Fig. 2), clearly shows a hot period from 1944 to 1960 when temperatures of 600 to 800°C were recorded, and a cold period afterward. Tempera- tures close to the boiling point of water at the crater altitude (95°C) were attained during the 1990s. This temperature decline was accompanied by remarkable changes in the mineralogy of sublimates and alteration products deposited at the fumarolic vents (Parascandola, 1951; Russo and Langella, 1996, in prep.; Russo, 1997b). Chloride of Na (halite) and Pb (cotunnite) and oxide of Cu (tenorite) were the most important minerals at temperatures of 500 to 800°C, whereas the present mineral assemblage includes sulfur, gyp- sum, and compound sulfates, such as alunite, potassium alum, metavoltine, pickeringite, and halotrichite. Despite the absence of chemical data on fumarolic fluids, both outlet temperatures and minerals collected at the fumarolic vents suggest that magmatic gases were probably discharged during the hot period. The fumaroles of the crater rim were sampled in 1979 and 1982, when maximum temperatures ranged between 220 and 230°C, and showed chemical compo- sitions dominated by air components with 2 vol.% of CO 2 (Roberto Cioni, pers. comm.). The fumaroles of the crater bottom were sampled in 1987 (T = 210°C) and in 1988 (T = 101°C) by Patrick Allard (pers. comm.). Discharged fluids resulting were made up mainly of H 2 O (93.9 –95.4 vol.%) and CO 2 (4.4 –5.8 vol.%). Since 1996, we started to sample and analyze, both chemi- cally and isotopically, crater fumarolic effluents in the frame- work of the volcanic surveillance programme. These data are presented and discussed in this communication with the aim to elaborate a conceptual geochemical model of the Vesuvio magmatic-hydrothermal system. These initial results will be of help to interpret future vari- ations in the chemical and isotopic characteristics of fumarolic gases and, in particular, to detect the evolution of the Vesuvio magmatic system toward the next eruption. Therefore, this initial geochemical information is of great importance for the evaluation and mitigation of volcanic risk in this densely in- habited area. 2. GEOVOLCANOLOGIC BACKGROUND Somma-Vesuvio is a volcanic complex consisting of a rela- tively old strato volcano, Monte Somma, which underwent a series of caldera collapses (Cioni et al., 1999), and a younger intracaldera cone, Vesuvio. The deep geothermal borehole Trecase-1 (Fig. 1) drilled on the southern flanks of the volcanic complex, met 373  22 ka old lavas at a depth of 1125 m *Author to whom correspondence should be addressed (chiod@ischia. osve.unina.it). Pergamon Geochimica et Cosmochimica Acta, Vol. 65, No. 13, pp. 2129 –2147, 2001 Copyright © 2001 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/01 $20.00 + .00 2129