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 c lassify them as 'magmatic'. Thermodynamic equilibrium temperatures of the gases range from 260 to > 600 degrees C. The relative abundance of inert gases, N-2, Ar and He, can be used as 'tracers' to identify the source of the fumarole discharges. At Galeras the majority of the sam ples have a composition characteristic of gases originating from are-r elated magmas, with relatively high N-2 contents and minor He and Ar. During 1993, the year of frequent eruptions, the gas composition chang ed to basaltic or 'mantle-derived' gases, with significantly higher He contents. This is interpreted to be the result of injection of volati les from a basaltic magma body at depth prior to and during the increa sed eruptive activity of 1993. The delta(13)C values for CO2 in fumaro le discharges are typical of andesitic volcanoes and may indicate addi tion of MORE-derived CO2. The delta(15)N values for N-2 may indicate s ignificant, contribution of N-2 from marine sediments and only minor c ontribution of MORE-derived N-2. The delta D and delta(18)O values of the discharging steam lie on a mixing trend between the isotopic compo sition of 'arc-related' magmatic water and O-18-shifted meteoric water . The most magmatic discharges have SD values of -30 to -35 parts per thousand; while the most meteoric discharges have values of -70 to -75 parts per thousand, similar to Galeras thermal spring waters. Galeras thermal water discharges consist of acid sulfate and bicarbonate wate rs. S/Cl ratios in the acid sulfate waters are similar to fumarole rat ios, suggesting direct absorption of magmatic gases into shallow groun d waters. This is supported by the essentially meteoric delta D and de lta(18)O values of the discharges and by elevated He-3/He-4 ratios of thermal spring waters. The absorption of acid S- and Cl-rich gases yie ld acid waters which are capable of dissolving rocks. The thermal wate rs, however, are far from equilibrium with Galeras lavas and pyroclast ic rocks, providing evidence of the immaturity of the Galeras hydrothe rmal. system. The SO4 and CI content, as well as the O and H isotopic composition of Galeras thermal springs vary with the activity of the v olcano.