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This chapter considers the phase relations of simple systems applicable to aqueous and petroleum fluid inclusions in sedimentary environments. One cannot appreciate the potential power of fluid inclusion observations, or the inherent limitations of the technique of microthermometry, without a sound understanding of the phase equilibria and their representation as phase diagrams. The equilibrium phase relations provide the link between the laboratory measurements of temperatures at which phase transitions occur within inclusions when heated and cooled and the interpretations of the measurements. Without knowledge of the phase diagrams, one cannot be a careful, critical inclusionist because one will not know the appropriate phase transitions to look for, and one will not know the assumptions upon which interpretations of phase transitions are made. Even the most basic petrographic approach to the fluid inclusion technique begins with a strong foundation in phase equilibria. These fundamental principles are elucidated in this chapter by first evaluating the phase relations of the unary systems pure H2O and pure CH4. Then, the effects of adding NaCl to water are evaluated, followed by a treatment of the effects of adding CH4 to water. Finally, a generalized treatment of the phase relations of petroleum fluids are considered. A more detailed presentation of phase behaviors that are observed within inclusions, as well as discussions of inherent observational and interpretative limitations are presented in Chapter 7.

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