The collection and analysis of volcanic and hydrothermal gases

Abstract

A knowledge of the composition of the gas phase associated with the various surface emanations of volcanic and geothermal systems forms an essential part of the study of these systems. Gases undoubtedly play a major role in eruptive phenomena, such as lava fountains and volcanic explosions, and may be useful in predicting eruptions. The interpretation of the composition of gases emitted from fumaroles, hot springs and drillholes in a geothermal field gives a better understanding of the underlying hot water aquifer. A collecting technique for volcanic or hydrothermal gases, ideally, must meet two requirements if the sample is to be representative of the gases. First, it must take in the gases of interest while excluding all possible air, and secondly, once collected, the composition of the gas mixture must be preserved. The method chosen will also depend on the source of the gases (volcanic gases are usually hotter and more corrosive than hydrothermal gases), whether all of the gases or only a few specific gases are to be collected from a particular source, and the technique to be used for the analysis. A survey of the literature since the beginning of this century shows that five basic methods have been employed for taking gas samples; displacement of air from the sample container, displacement of water or mercury, vacuum tubes or flasks, condensation and absorption. Specific examples are given of the application of these methods either singly or in combination. The advantages and disadvantages of these techniques are discussed, particularly with respect to their success in meeting the above requirements for the collection of a gas sample. As with the collecting techniques, specific examples of methods used for the analysis of gas samples are discussed. The techniques include absorption methods, wet chemical methods both titrimetric and gravimetric, and instrumental methods (spectroscopy, mass spectrometry and gas chromatography). Possible ways to improve collecting techniques are discussed and the possible application of some more recent instrumental method of analysis are suggested.