Fluids in metamorphic rocks

Abstract

Basic principles for the study of fluid inclusions in metamorphic rocks are reviewed and illustrated. A major problem relates to the number of inclusions, possibly formed on a wide range of P– T conditions, having also suffered, in most cases, extensive changes after initial trapping. The interpretation of fluid inclusion data can only be done by comparison with independent P– T estimates derived from coexisting minerals, but this requires a precise knowledge of the chronology of inclusion formation in respect to their mineral host. The three essential steps in any fluid inclusion investigation are described: observation, measurements, and interpretation. Observation, with a conventional petrographic microscope, leads to the identification and relative chronology of a limited number of fluid types (same overall composition, eventually changes in fluid density). For the chronology, the notion of GIS (Group of synchronous inclusions) is introduced. It should serve as a systematic basis for the rest of the study. Microthermometry measurements, completed by nondestructive analyses (mostly micro-Raman), specify the composition and density of the different fluid types. The major problem of density variability can be significantly reduced by simple considerations of the shape of density histograms, allowing elimination of a great number of inclusions having suffered late perturbations. Finally, the interpretation is based on the comparison between few isochores, representative of the whole inclusion population, and P– T mineral data. Essential is a clear perception of the relative chronology between the different isochores. When this is possible, as illustrated by the complicated case of the granulites from Central Kola Peninsula, a good interpretation of the fluid inclusion data can be done. If not, fluid inclusions will not tell much about the metamorphic evolution of the rocks in which they occur.