Points to Consider:
Rock masses contain a fluid phase, except in areas where high temperatures or impermeable rocks prevail. Usually the fluid phase is dominated by H2O, but CO2, CH4, N2, Cl, S, B, Na, K and other components may also be present.
Some evidence for the presence of a fluid phase in rocks during metamorphism are:
- Fluid inclusions in metamorphic minerals.
- The formation of metamorphic phases that include CO2, H2O etc. requires the presence of a fluid phase.
- Metamorphic reactions often involve dehydration and decarbonation reactions that yeild a fluid phase.
Within a rock, the fluid phase may be in equilibrium with the solid phase. However, if the composition of the fluid changes, disequilibrium results. The rock will adjust through mineralogical and/or textural changes in order to re-equilibrate. Fluids may also migrate from one rock mass to another as a result of temperature, pressure, or stress changes. Thus they may affect metamorphism in adjacent rocks.
[Modified from Raymond]
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