Metamorphic Agents and Changes

Four types of Metamorphic agents

In regular continental crust, a typical geothermal gradient within the first 3 to 5 kilometers (2 or 3 miles) of Earth’s surface is about 25°C/km.
Promotes recrystallization/crystal growth.
Unstable mineral/s will decompose to form new stable mineral/s.
Devolatilization, dehydration, and decarbonization reactions may take place.
Temperature causes the rocks to easily overcome kinetic barriers toward attaining a new equilibrium.
Most metamorphism takes place between a temperature of around 150 to 850oC.
However, metamorphism can occur very locally at temperatures up to the solidus of dry basalt (>1000oC).

Here, pressure refers to confining pressure, equal in all directions.
There are pressure limits to the stability of minerals and mineral associations, just as there are temperature limits.
Most metamorphism takes place at pressures generally below 10 kbar corresponding to depths of 37 km depth, based on an average continental crustal density of 2.7 g/cm3.
However, in subduction zones near trenches and continental collision zone depths can reach 100 km or more (ultrahigh P >=30 kbar).

This occurs most commonly in orogenic belts, extending rifts, or in shear zones (i.e., generally at or near plate boundaries).
affects the textures and structures in rocks but not the equilibrium mineral assemblage.
may also provide the impetus to overcome kinetic barriers to reactions.
Generally causes mineral grains to be aligned in a preferred orientation, resulting in foliation and lineation in metamorphic rocks.

Intergranular metamorphic fluids are usually dominated by H2O, but CO2 may also be present in some rocks.
Fluids affect the P-T stability condition for minerals; this is particularly important for low P-T conditions. Zeolite stability is strongly dependent on fluid composition. Zeolites may not appear if the fluid is too rich in CO2.
The motion of fluids may transport various chemical species over considerable distances.
The material exchange may occur between fluids and rocks; if substantial exchange occurs, this is “Metasomatism.”

Types of Metamorphism