ABSTRACT

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 volcanic gases which led us to develop the “in situ” gas analysis. A field gas chromatograph allowed direct injection of hot gases, before water and sulfur condensation occurred. A silica tube equipped with thermocouples was used for sublimates sampling and for measuring the condensation temperature. Conventional condensors and caustic soda bottles were used for sampling and later complementary analyses in the laboratory. A free energy minimisation computational method modelled the physical and chemical changes that occurred during cooling of volcanic gases. The high temperature composition of the .gas mixture was recalculated from the concentrations of the gaseous and solid components obtained during sampling. The equilibrium composition was first calculated at the collection temperature for 22 elements. The model then calculated the equilibrium compositions at 50°C intervals using the residual gas composition after condensation at the previous temperature. The depositional sequence observed in the silica tube depend on the temperature 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 for volcano monitoring is proposed which allow to determine the magmatic origin of volcanic gases and their emission temperature from remote plume analysis. The model is also applicable to estimate the temperature and the composition of the gases entering hydrothermal systems or participating in ore deposits in the basement of the volcano. The model predicts the behaviour of the main and minor species emitted in the volcanic gases. This approach is not only restricted to the volcanic gas studies but can be applied to studies of high temperature reactive gas reactor to simulate cooling reactions.