Fluid inclusions in metamorphic minerals

Garnet, kyanite and quartz are common mineral in the earth crust and has shown to preserve primary fluid inclusions. Fluid inclusions in metamorphic minerals provide direct evidence of the fluid phase present in most geological significant processes in the earth crust and mantle. Controversial theories exist about the applicability of metamorphic grown minerals and their fluid inclusions for petrological interpretations. This project attempts to clarify the important factors influencing the trapping and reequilibration of metamorphic fluid inclusions in garnet, kyanite and quartz. The physicochemical properties of the fluids (pressure, temperature and composition) before, at and after their entrapment and influences of other factors like deformation will be qualified and, as far as possible, quantified. The strategy includes field-related studies and an experimental approach. In the field related study selected areas of the Austroalpine Basement will be investigated, in which metamorphic minerals hosting fluid inclusions are known. Parageneses suitable for geothermobarometric and geochemical investigations will be selected. Reequilibration experiments will be performed on suitable natural fluid inclusions in garnet, kyanite and quartz by imposing them to pressure, temperature and fugacity gradients and by measuring their P-V-T-X properties before and after reequilibration. Attempts will also be made to synthesize fluid inclusions in garnet and kyanite.

If one wants to experience something about the geologic history of our planet, he is looking for garnet, kyanite and quartz. These common metamorphic minerals - i.e. formed under high pressure and temperature in the earth crust - preserve so called fluid inclusions - small cavities filled with liquids, solids and gases. Fluid inclusions provide direct evidence of the very important fluid phase, which strongly influences most geological processes. By combining the classical geologic field work, mapping of rocks and structures and sampling, with experiments in a high-pressure, high-temperature chamber, the project delivered new knowledge about the formation and changes of fluid inclusions in metamorphic minerals over tens of million years. In the field related study, in the geologic time scale relatively "young" rocks in the age between 70 and 120 million years within two selected areas of the Eastern Alps of Austria were investigated. Microscopy of ultra-thin rock sections in polarized light and chemical analyses of the minerals in the sections with a spatial resolution of one thousand of a millimetre were applied. Cooling and heating of small rock pieces between -170 and +600C caused the liquids and gases inside the inclusions to freeze or to homogenize, which provides information about their composition and density. The results of all these examinations are extensive knowledge about changes in depth, temperature, deformation of the rocks and the fluids within them during the millions of years lasting Alpine orogeny. In the experimental part of the project it was possible for the first time to produce artificial fluid inclusions in gemstone quality garnet. The mineral garnet is particularly useful for reconstructing geological processes because 1) it tends to retain a chemical record of events associated with its growth; 2) it is a common mineral in the crust and 3) it has been shown to preserve fluid inclusions. These artificial fluid inclusions were compared with natural ones in garnet of the Kaunertal in Tyrol. One interesting question was why garnet in some rocks contains millions of fluid inclusions whereas in others is almost totally free of them. The reasons for that are garnet - fluid reactions and different chemical composition of the fluid in the inclusions.