8.1.7: Prograde and Retrograde Metamorphism

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8.11.png — Figure 8.11: Prograde and retrograde metamorphism

Prograde metamorphism occurs when low-grade or unmetamorphosed rocks change mineralogy or texture in response to a temperature increase. If the metamorphism is gradual and predictable, we call it progressive metamorphism. During progressive metamorphism, a series of reactions occur as the degree of metamorphism increases. Rock mineralogy changes multiple times before equilibrating at the highest temperature conditions. While this idea makes a convenient conceptual model, it is not correct for all metamorphic rocks. For example, many metamorphic rocks are deep in Earth where pressure and temperature are great. They were never unmetamorphosed rocks at low pressure and temperature. Other rocks go from low temperature to high temperature, perhaps because of rapid intrusion of a pluton, so rapidly that they skip intermediate stages. Still other rocks may only partially equilibrate during metamorphism.

Some metamorphic rocks form by retrograde reactions (metamorphism causing high-temperature rocks to change into low-temperature rocks). This is especially true for mafic rocks that were metamorphosed at high-grade conditions. Upon uplift and cooling, retrograde metamorphism may replace original high-temperature mineral assemblages with low-grade minerals. Figure 8.11 compares paths of prograde and retrograde metamorphism.

8.12.png — Figure 8.12: A large diamond crystal in kimberlite. The largest crystal is about 7 mm across.

One of the most intriguing questions about metamorphic rocks is: Why do we find high-grade metamorphic minerals at Earth’s surface where they are unstable? The Laws of Thermodynamics say that rocks will change mineralogy in response to increasing temperature (prograde metamorphism), so why don’t they undergo opposite (retrograde metamorphism) changes when temperature decreases as the rock reaches Earth’s surface? If rocks always went to equilibrium, we should have no samples of high-grade rocks or minerals to study. Yet, we do. For example, we have samples of diamond-bearing kimberlite, like the specimen seen in this photo (Figure 8.12), that are unstable and should break down at Earth’s surface.

Several considerations help answer these questions:

• Prograde metamorphic events are usually of much longer duration than retrograde events, giving minerals more time to achieve equilibrium.

• Prograde metamorphism liberates fluids not present when retrogression occurs. The fluids act as fluxes to promote prograde metamorphism; their absence may hinder retrogression. And, the absence of fluids means that some low-grade minerals cannot form.

• Prograde reactions are mostly endothermic, which means they consume heat. The heat that causes metamorphism naturally fuels the reactions. In contrast, retrograde reactions are mostly exothermic – they give off heat. There is no outside energy driving the reactions.

• At low temperature, reactions are very sluggish; they may not have time to reach equilibrium.

• More complex, low-grade minerals often have difficulty nucleating and growing.