Fluids of various nature are present at all levels of the lithosphere.It is mainly water ( with dissolved elements) and carbon dioxide.These fluids, which are released during metamorphism, play an important role in driving and controlling the mineralogical reactions.They move along fractures.They are then trapped in small cavities when these fractures are sealed. In this microphotograph, small inclusions of a few tenths of a millimetre in the cordierite contain CO2 in both liquid and gaseous form.The distribution of the inclusions mimics the old fractures. The small spherical crystal of zircon (Z : Zirconium silicate) The small spherical zircon crystal is about one tenth of a mm in size. Microphotography in plane polarised light.
