Geochemical evidence for the existence of high-temperature hydrothermal brines at Vesuvio volcano, Italy

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 composition of the gases discharged by the fumaroles of the crater bottom (vents FC1, FC2, and FC5). These have typical hydrothermal compositions, with H2O and CO2 as major components, followed by H-2, H2S, N-2, CH4, and CO (in order of decreasing contents) and undetectable SO2, HCl, and HF. Fumarolic H2O is either meteoric water enriched in O-18 through high-temperature water-rock oxygen isotope exchange or a mixture of meteoric and are-type magmatic water. Fumarolic CO(2)is mainly generated by decarbonation reactions of marine carbonates, but the addition of small amounts of magmatic CO2 is also possible. All investigated gas species (H2O, CO2, CO, CH4, H-2, H2S, N-2, and NH3) equilibrate, probably in a saturated vapor phase, at temperatures of 360 to 370 degreesC for vent FC1 and 430 to 445 degreesC 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 similar to 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 hydrofracturing, 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 (C) 2001 Elsevier Science Ltd.