CHARACTERIZATION AND MODELING OF THE COMPLETE VOLCANIC GAS-PHASE

During the survey on the eruption of the volcano ''La Soufriere de la Guadeloupe'' (French West Indies), the behaviour of sulfur in volcanic gases was examined. Matsuo (1962) studied chemical equilibrium in vol canic gases which led us to develop the ''in situ'' gas analysis. A fi eld gas chromatograph allowed direct injection of hot gases, before wa ter and sulfur condensation occurred. A silica tube equipped with ther mocouples was used for sublimates sampling and for measuring the conde nsation temperature. Conventional condensers and caustic soda bottles were used for sampling and later complementary analyses in the laborat ory. A free energy minimisation computational method modelled the phys ical and chemical changes that occurred during cooling of volcanic gas es. The high temperature composition of the gas mixture was recalculat ed from the concentrations of the gaseous and solid components obtaine d during sampling. The equilibrium composition was first calculated at the collection temperature for 22 elements. The model then calculated the equilibrium compositions at 50 degrees C intervals using the resi dual gas composition after condensation at the previous temperature. T he depositional sequence observed in the silica tube depend on the tem perature and the concentration of elements in the initial mixture. The computational method was applied to gases sampled from Mt, St Helens. The calculated results agree with observed sublimates. A new method f or volcano monitoring is proposed which allow to determine the magmati c origin of volcanic gases and their emission temperature from remote plume analysis. The model is also applicable to estimate the temperatu re and the composition of the gases entering hydrothermal systems or p articipating in ore deposits in the basement of the volcano. The model predicts the behaviour of the main and minor species emitted in the v olcanic gases. This approach is not only restricted to the volcanic ga s studies but can be applied to studies of high temperature reactive g as reactor to simulate cooling reactions.