Fluid immiscibility in natural processes: Use and misuse of fluid inclusion data: I. Phase equilibria analysis — A theoretical and geometrical approach

Abstract

Many occurrences of fluid immiscibility in natural geologic systems have been reported recently, most often from fluid inclusion studies. However, the interpretation of fluid inclusion data in terms of immiscibility sometimes suffers from ambiguity of the vocabulary, insufficient knowledge of the immiscibility constraints and insufficient knowledge of the topology (in the TPX space) of natural fluid systems. For such reasons some authors have been misled to erroneous interpretations. The term “chemical immiscibility” is unambiguously redefined as a multiphase multicomponent equilibrium. The consequences of this definition are directly derived from the phase rule and concern the possible equations that relate the various parameters (temperature, pressure, volumes, compositions) to each other. These equations already put constraints on the topology of the phase equilibria in fluid systems. A particular expression of the phase rule is proposed, which takes into account the multiphase-constant bulk volume-constant bulk composition constraints in fluid inclusions. The consequences of such an expression are of major importance in fluid inclusion studies. The phase relations of some simple systems that approximate quite efficiently the natural complex fluids are then detailed: H 2ONaCl, CO 2CH 4, H 2OCO 2, H 2OCO 2NaCl. The effects of these topologies and of the supplementary constraints (constant bulk composition and constant bulk volume) assumed for fluid inclusions (isopleth-isochoric systems) are discussed.