1. (2pts) What is meant by prograde metamorphism? Retrograde metamorphism? Pleas
ID: 120365 • Letter: 1
Question
1. (2pts) What is meant by prograde metamorphism? Retrograde metamorphism? Please discuss the textural and mineralogical changes that the following rocks might undergo as a result of either prograde or retrograde metamorphism impure Prograde mudrock limestone ultramafic pure limestone Retrograde mudrock limestoneultramafic impure pure limestone 2. (2pts) What is an ACF diagram? An AFM diagram? Why would one use/prefer one of these two diagrams over the other? In Table 21.1 (handout from class on Wednesday), in the first column under the header of Mafic Compositions, hornblende is noted as a mineral from the Epidote-Amphibolite facies up to the Granulite facies. How is this possible? And how could I determine the correct grade of metamorphism given that hornblende occurs in all five of these metamorphic grades listed?Explanation / Answer
Simply
Prograde Metamorphism - General metamorphism that is caused by rise in temperature& Pressure.
Retrograde Metamorphism -General metamorphism that is caused by decrease in temperature and pressure.
Mudrocks:
Mudrocks are sedimentary rocks composed of at least 50% silt- and clay-sized particles. These relatively fine-grained particles are commonly transported as suspended particles by turbulent flow in water or air, and deposited as the flow calms and the particles settle out of suspension. Most authors presently use the term "mudrock" to refer to all rocks composed dominantly of mud. Mudrocks can be divided into - siltstones (composed dominantly of silt-sized particles), - mudstones (subequal mixture of silt- and clay-sized particles), - and claystones (composed mostly of clay-sized particles).
Prograde metamorphism:
Prograde metamorphism involves the change of mineral assemblages (paragenesis) with increasing temperature and (usually) pressure conditions. These are solid state dehydration reactions, and involve the loss of volatiles such as water or carbon dioxide. Prograde metamorphism results in rock characteristic of the maximum pressure and temperature experienced. Metamorphic rocks usually do not undergo further change when they are brought back to the surface.
Retrograde metamorphism:
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 reexposure at Earth’s surface; in this case, metamorphic rocks would never be seen in outcrop. Two factors mitigate against complete retrogression of metamorphic rocks, however, during their return to Earth’s surface. First is 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
Fe9Al6Si5O20(OH)16+ 4 SiO2 => 3Fe3Al2Si3O12+8H2O
Ohlorite Quartz Gamet Water
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. The second reason that 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, and metamorphic assemblages indicative of even extremely high temperatures or pressures or both are found exposed throughout the world. 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 given above to proceed during cooling. Retrograde features such as these reaction rims can be mapped to yield information on pathways of fluid migration through the rocks during uplift and cooling. In other rocks, such as high-temperature gneisses, mineral compositions often reflect temperatures too low to be in equilibrium with the preserved mineral assemblage; in these samples, it is clear that certain exchange reactions operated in a retrograde sense even when the net-transfer reactions were frozen in during prograde metamor
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.