Fluids in metamorphic rocks

Basic principles for the study of fluid inclusions in metamorphic rocks are reviewed and illustrated. A major problem relates to the number of inclusi ons, possibly formed on a wide range of P-T conditions, having also suffere d, in most cases, extensive changes after initial trapping. The interpretat ion of fluid inclusion data can only be done by comparison with independent P-T estimates derived from coexisting minerals, but this requires a precis e knowledge of the chronology of inclusion formation in respect to their mi neral host. The three essential steps in any fluid inclusion investigation are describe d: observation, measurements, and interpretation. Observation, with a conve ntional petrographic microscope, leads to the identification and relative c hronology of a limited number of fluid types (same overall composition, eve ntually changes in fluid density). For the chronology, the notion of GIS (G roup of synchronous inclusions) is introduced. It should serve as a systema tic basis for the rest of the study. Microthermometry measurements, complet ed by nondestructive analyses (mostly micro-Raman), specify the composition and density of the different fluid types. The major problem of density var iability can be significantly reduced by simple considerations of the shape of density histograms, allowing elimination of a great number of inclusion s having suffered late perturbations. Finally, the interpretation is based on the comparison between few isochores, representative of the whole inclus ion population, and P-T mineral data. Essential is a clear perception of th e relative chronology between the different isochores. When this is possibl e, as illustrated by the complicated case of the granulites from Central Ko la Peninsula, a good interpretation of the fluid inclusion data can be done . If not, fluid inclusions will not tell much about the metamorphic evoluti on of the rocks in which they occur. (C) 2001 Elsevier Science B.V. All rig hts reserved.