Multicomponent fluids show miscibility gaps at temperatures below 400 °C. Implications of this are expected to be found in diagenetic and low-grade metamorphic terraines in the Central Alps. A systematic search revealed fluid immiscibility in three distinct fluid environments:
1. CH4-H2O-salt in diagenetic and low-grade anchizonal, hydrocarbon bearing sedimentary rocks;
2. CO2-H2O-salt at high-grade anchizonal and greenschist facies conditions, in carbon and carbonate bearing metasedimentary and metaigneous rocks;
3. N2-H2O-salt at anchizonal and greenschist facies conditions in ammonium bearing metasedimentary and metaigneous rocks.
In addition to the major volatile component, fluids may contain smaller amounts of CH4, CO2, N2, H2S. The dissolved electrolyte species is mainly NaCl.
Observations of natural fluid systems permit to distinguish six different geological processes that lead to fluid unmixing:
1. Isobaric decrease in temperature;
2. isothermal decrease in pressure;
3. input of salt-enriched fluids (salting out);
4. volatile production during cracking of kerogen and higher hydrocarbon and by decarbonation and oxidation or reduction of carbonaceous matter;
5. upward and downward movement of the gas-water contact, mimicking unmixing phenomena;
6. channeled influx of unmixed allochtonous volatiles.
Depending on the tectonometamorphic and lithologic environment, fluid immiscibility in the Central Alps is controlled by one or several unmixing processes:
- In CH4 bearing diagenetic and low-grade anchimetamorphic rocks, unmixing processes 1,2,4,5 and 6 are dominant.
- In CO2 bearing rocks that suffered high-grade anchizonal and greenschist-facies conditions, unmixing processes 2 and 6 are dominant.
- In N2 bearing rocks exposed to anchizonal and greenschist facies conditions, process 2 is dominant.
- In rocks containing little or no volatiles, process 6 is connected to fractures and faults.