Chemical and isotopic analyses of fumarole gases from Galeras Volcano, Colombia provide significant insights into the
origin of volatile components in the magma.. Water comprises a mixture of magmatic (rock-equilibrated at high
temperature) and meteoric, with the relative proportions varying through time. Carbon isotopes are most consistent with
a mantle source, although other contributions to the system cannot be ruled out. Nitrogen most likely originated from
organic matter in subducted oceanic sediments. All of these "primary" components have been modified somewhat by
interaction or exchange with other reservoirs during ascent to the surface. In addition to information on the origin of the
gases, these data show significant temporal variations precursory to eruptive events. Chemical ratios change and water
becomes more meteoric in character prior to eruptions.