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The term “fluid” is used in different ways in the geologic literature. Sometimes “fluid” is used to denote any kind of mobile phase, including silicate melts. In this chapter, we will, for purely pragmatic reasons, define a fluid as a mobile phase which is not a silicate or carbonate melt. Sometimes the term is defined even more narrowly as a mobile phase in a regime of pressure and temperature where no distinction between “vapour” and “liquid” is possible anymore. We will not follow this use, i.e. a “fluid” in the sense as it will be used in this chapter can have either “vapour-like” or “liquid-like” or transitional properties, unless otherwise stated.

Evidence for the composition of fluids in the Earth’s interior comes essentially from three sources of evidence: (i) the analysis of volcanic gases, (ii) the investigation of fluid inclusions and (iii) considerations of phase equilibria. Gases from volcanoes with a non-explosive eruption style can sometimes be directly sampled, while direct sampling is impossible during major explosions. Naturally, this introduces some bias in the data on volcanic gas compositions, since gas analyses can be much more easily acquired from basaltic magmas than from the often highly explosive andesitic and rhyolitic ones. However, in recent years remote sensing of gas compositions by infrared spectroscopy has become possible and it is to be expected that the further development of these methods will ultimately allow a more representative sampling of volcanic gases from a variety of magma sources and tectonic environments. In any case, although there are quite significant variations, virtually all available analyses show that the predominant constituents

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