The chemical and isotopic composition of fumarolic gases and spring discharges from Galeras Volcano, Colombia

Galeras fumarole discharges have been collected since its reactivation, in 1988, through December 1995. The gases are dominated by H2O, CO2, S (as SO2 and H2S) and HCl. The relative proportions of these gases classify them as ‘magmatic’. Thermodynamic equilibrium temperatures of the gases range from 260 to > 600 °C. The relative abundance of inert gases, N2, Ar and He, can be used as ‘tracers’ to identify the source of the fumarole discharges. At Galeras the majority of the samples have a composition characteristic of gases originating from arc-related magmas, with relatively high N2 contents and minor He and Ar. During 1993, the year of frequent eruptions, the gas composition changed to basaltic or ‘mantle-derived’ gases, with significantly higher He contents. This is interpreted to be the result of injection of volatiles from a basaltic magma body at depth prior to and during the increased eruptive activity of 1993. The δ13C values for CO2 in fumarole discharges are typical of andesitic volcanoes and may indicate addition of MORB-derived CO2. The δ15N values for N2 may indicate significant contribution of N2 from marine sediments and only minor contribution of MORB-derived N2. The δ D and δ18O values of the discharging steam lie on a mixing trend between the isotopic composition of ‘arc-related’ magmatic water and18O-shifted meteoric water. The most magmatic discharges have δ D values of −30 to −35‰; while the most meteoric discharges have values of −70 to −75‰, similar to Galeras thermal spring waters. s thermal water discharges consist of acid sulfate and bicarbonate waters.S/Cl ratios in the acid sulfate waters are similar to fumarole ratios, suggesting direct absorption of magmatic gases into shallow ground waters. This is supported by the essentially meteoric δD and δ18O values of the discharges and by elevated3He/4He ratios of thermal spring waters. The absorption of acid S- and Cl-rich gases yield acid waters which are capable of dissolving rocks. The thermal waters, however, are far from equilibrium with Galeras lavas and pyroclastic rocks, providing evidence of the immaturity of the Galeras hydrothermal system. The SO4 and Cl content, as well as the O and H isotopic composition of Galeras thermal springs vary with the activity of the volcano. year sampling program at Galeras revealed intriguing results concerning the activity of Galeras, its magmatic-hydrothermal system and the origin of the volatiles. Despite decreasing outlet temperatures since 1992, deep temperatures remain high, implying continued unrest in the Galeras magmatic system.