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.