Retrograde blueschist

Retrograde metamorphism: is the mineralogical adjustment of relatively high-grade metamorphic rocks to temperatures lower than those of their initial metamorphism, in which minerals characteristic of a lower metamorphic grade developed at the expense of minerals formed at a higher metamorphic grade. Retrograde metamorphism is also known as diaphthoresis (from the Greek "to degrade"); the term was first applied by Becke (1909) to a phyllonite containing relict high-grade metamorphic minerals.

In general, the changes in mineral assemblage and mineral composition that occur during burial and heating are referred to as prograde metamorphism, whereas those that occur during uplift and cooling of a rock represent retrograde metamorphism. If thermodynamic equilibrium were always maintained, one might expect all the reactions that occur during prograde metamorphism to be reversed during subsequent uplift of the rocks and re-exposure at Earth's surface; in this case, metamorphic rocks would never be seen in outcrop. Two factors mitigate against complete retrogression of metamorphic rocks during their return to Earth's surface:

(1) The efficient removal of the water and carbon dioxide released during prograde devolatilization reactions by upward migration of the fluid along grain boundaries and through fractures. Because almost all the water released during heating by reactions (such as when chlorite reacts with quartz to yield garnet and water) is removed from the site of reaction, the reaction cannot be reversed during cooling unless water is subsequently added to the rock. Thus, garnet can be preserved at Earth's surface even though it is thermodynamically unstable at such low temperatures and pressures.

(2) Metamorphic reactions do not typically operate in reverse during cooling is that reaction rates are increased by rising temperatures.

During cooling, reaction kinetics become sluggish, and metastable mineral assemblages and compositions can be preserved well outside their normal stability fields. Thus, prograde reactions are generally more efficient than retrograde reactions. It is common, however, to find at least some signs of retrogression in most metamorphic rocks. For example, garnets are often rimmed by small amounts of chlorite and quartz, indicating that limited quantities of water were available for the reverse of the reaction during cooling.

Bibliography

• Bucher, K., & Grapes, R. (2011). Petrogenesis of metamorphic rocks. Springer Science & Business Media.
• Fossen, H. (2016). Structural geology. Cambridge University Press.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Passchier, Cees W., Trouw, Rudolph A. J: Microtectonics (2005).
• Philpotts, A., & Ague, J. (2009). Principles of igneous and metamorphic petrology. Cambridge University Press.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.
• Vernon, R. H. (2018). A practical guide to rock microstructure. Cambridge university press.

Foto
Quartz (colorless) and glaucophane crystals embedded in chlorite. PPL image, 2x (Field of view = 7mm)	Quartz (colorless) and glaucophane crystals embedded in chlorite. PPL image, 2x (Field of view = 7mm)	Quartz and glaucophane crystals embedded in chlorite. XPL image, 2x (Field of view = 7mm)
glaucophane crystals altered by chlorite. PPL image, 10x (Field of view = 2mm)	glaucophane crystals altered by chlorite. PPL image, 10x (Field of view = 2mm)	glaucophane crystals altered by chlorite. PPL image, 10x (Field of view = 2mm)
glaucophane crystals altered by chlorite. XPL image, 10x (Field of view = 2mm)	glaucophane crystals altered by chlorite. PPL image, 10x (Field of view = 2mm)	glaucophane crystal altered by chlorite. PPL image, 20x (Field of view = 1mm)