This results in producing minerals that are less volatile rich with increasing grade of metamorphism. Ultimately, at the highest grade of metamorphism no hydrous or carbonate bearing phases exist. If retrograde metamorphism were a common process then upon uplift and unroofing metamorphic rocks would progressively return to mineral assemblages stable at lower pressures and temperatures. Yet, high grade metamorphic rocks are common at the surface of the Earth and usually show only minor retrograde minerals. Three factors inhibit retrograde metamorphism, two of which involve the fluid phase.
Chemical reactions run faster at higher temperatures.
During prograde metamorphism the reaction boundaries are continually being over-stepped to higher temperature. At higher temperature diffusion rates are higher and molecular vibration required to break chemical bonds is higher. Thus, during prograde metamorphism reaction rates are faster. As temperature is lowered on a rock, the reaction boundaries are over-stepped to low temperature, and as a result the reaction rates are much slower. Of course, this depends on the rate at which temperature is lowered.