Magma-derived gas influx and water-rock interactions in the volcanic aquifer of Mt. Vesuvius, Italy

Abstract

We report in this paper a systematic investigation of the chemical and isotopic composition of groundwaters flowing in the volcanic aquifer of Mt. Vesuvius during its current phase of dormancy, including the first data on dissolved helium isotope composition and tritium content. The relevant results on dissolved He and C presented in this paper reveal that an extensive interaction between rising magmatic volatiles and groundwaters currently takes place at Vesuvius. Vesuvius groundwaters are dilute (mean TDS ∼ 2800 mg/L) hypothermal fluids ( mean T = 17.7°C) with a prevalent alkaline-bicarbonate composition. Calcium-bicarbonate groundwaters normally occur on the surrounding Campanian Plain, likely recharged from the Apennines. δD and δ 18O data evidence an essentially meteoric origin of Vesuvius groundwaters, the contribution from either Tyrrhenian seawater or 18O-enriched thermal water appearing to be small or negligible. However, the dissolution of CO 2-rich gases at depth promotes acid alteration and isochemical leaching of the permeable volcanic rocks, which explains the generally low pH and high total carbon content of waters. Attainment of chemical equilibrium between the rock and the weathering solutions is prevented by commonly low temperature (10 to 28°C) and acid-reducing conditions. The chemical and isotope (C and He) composition of dissolved gases highlights the magmatic origin of the gas phase feeding the aquifer. We show that although the pristine magmatic composition may vary upon gas ascent because of either dilution by a soil-atmospheric component or fractionation processes during interaction with the aquifer, both 13C/ 12C and 3He/ 4He measurements indicate the contribution of a magmatic component with a δ 13C ∼ 0‰ and R/R a of ∼2.7, which is consistent with data from Vesuvius fumaroles and phenocryst melt inclusions in olivine phenocrysts. A main control of tectonics on gas ascent is revealed by data presented in this paper. For example, two areas of high CO 2 release and enhanced rock leaching are recognized on the western (Torre del Greco) and southwestern (Torre Annunziata-Pompeii) flanks of Vesuvius, where important NE-SW and NW-SE tectonic structures are recognized. In contrast, waters flowing through the northern sector of the volcano are generally colder, less saline, and CO 2 depleted, despite in some cases containing significant concentrations of magma-derived helium. The remarkable differences among the various sectors of the volcano are reconciled in a geochemical interpretative model, which is consistent with recent structural and geophysical evidences on the structure of Somma-Vesuvius volcanic complex.