THE CHEMISTRY OF FUMAROLIC VAPOR AND THERMAL-SPRING DISCHARGES FROM THE NEVADO-DEL-RUIZ VOLCANIC-MAGMATIC-HYDROTHERMAL SYSTEM, COLOMBIA

On the basis of three sets of gas samples, taken during the three months preceding the November 13, 1985 eruption of Nevado del Ruiz from fumaroles on the crater floor, the composition of the vapors discharged, following minor eruptive activity in September, corresponded to that of an essentially magmatic gas. During the subsequent quiet period in October, the proportion of a hydrothermal component in the vapor discharge increased steadily up until the major eruption. The gases were found to be exceptionally high in total sulfur and are super-saturated with respect to elemental sulfur; their thermodynamic equilibration temperatures range from 200 to 600°C. Gases and waters from fumaroles and thermal springs on the flanks of the volcano are likely to be derived from a two-phase vapor-brine envelope surrounding the magmatic system. The chemistry of the fumarole discharges is compatible with the assumption of an eruptive mechanism involving the build-up of pressure in an extensive vapor zone by gas released from an essentially stagnant, already extensively degassed body of magmatic material at depth. Production of such a residual gas over extended periods may account for the large amount of SO2 discharged from the volcano in relation to the small amount of ejected solids. © 1990.