Equilibrium Thermodynamics in Petrology

Geologicalmaterials are not the result of well controlled chemicalexperi-ments. For example, during the metamorphism of a pelite, theoriginal clay or silt follows some temperature-pressure (P- T) paththrough time (r) as a result of burial and heating followed by upliftand cooling. Can we talk about equilibrium in the metamorphicprocess? This will depend almost completely on the rate at whichequilibrium is attained along the P-T-t path. If the rate ofequilibration is fast compared to the rate at which P and T arechanging with time, then the rock will maintain equilibriumthroughout its P-T path and would reach the Earth's surface as somelow temperature assemblage of clay minerals, etc. This does notusually happen, the rock reaches the Earth's surface with a high P-Tassemblage. A reasonable interpretation is that the rate ofattainment of equilibriumbecame much slower than the rate of change of P-T with time duringcooling, so that the mineralogy of the rock records some high P-Tequilibrium attained before substantial cooling and uplift tookplace.The equilibrium represented in the mineralogy has been frozenin.