Fluid immiscibility in natural processes: Use and misuse of fluid inclusion data: II. Interpretation of fluid inclusion data in terms of immiscibility

Abstract

Phase equilibrium analysis of the relevant systems together with the application of the principles of chemical equilibrium put severe constraints on the interpretation of fluid inclusion data in terms of immiscibility (in Part I). Following from that point, the major limits on the accuracy, and even the validity, of fluid inclusion quantitative data and their interpretation in terms of fluid composition and density are briefly discussed. The practical implications of the general constraints (temperature, pressure, topology of the fluid systems) are envisaged. Emphasis is laid on some important consequences such as: the use of isochore intercepts (and the possible resulting interpretation of fluid mixing rather than unmixing), the case of highly saline inclusions, the identification and interpretation of heterogeneous trapping. The composition and density constraints on coexisting fluids are presented, and illustrated by natural examples. Taking into account all the measurable parameters in fluid inclusions (volume, temperature and nature of phase transitions, more or less complete individual spectroscopic analyses), all the available experimental data, and all the theoretical constraints, may be long and difficult. However, it is most generally very informative and productive although part of these data is often sufficient to deny unmixing. Nevertheless, a final example on metamorphic fluids demonstrates how such an approach can “prove”; and also characterize a fluid unmixing during a geologic process.