Fluids and Metamorphism
At least a small volume of water and other fluids is present in virtually all rocks, and because water allows materials to move more easily, the presence of fluids makes metamorphism more easy. Conversely, a dry rock is very hard to get to change. Without the fluids chemical changes are just harder to take place.The numerous open spaces between grains in a sedimentary rock and the tiny fractures in igneous and metamorphic rocks are called pores. All pores are filled with a watery fluid. The fluid is not pure water. The fluid typically has ions dissolved in it. The composition of the fluid is determined by the composition of the mineral grains with which it has direct contact.
At higher temperatures this fluid exists in a vapor form. When the temperature and pressure of a rock undergoing metamorphism changes, the composition of the fluid changes also. Some of the dissolved ions move from the fluid to the new minerals that are growing in the metamorphic rock.
Other dissolved ions move out of the minerals in the rock and into the fluid. The intergranular fluid serves as a transporting medium to speed up the chemical reactions taking place.
When fluids are absent, metamorphic changes are slow because the constituents must diffuse through a solid without the assistance of the liquid transport medium. As the pressure increases due to burial of the rock, the amount of pore space is reduced and the intergranular fluid is driven off. Hydrous minerals such as amphibole change to anhydrous minerals such as pyroxene. Therefore, higher grade Metamorphic rocks have fewer hydrous minerals.
Fluids and Melting:
Fluids are also important in melting. The more fluids there are the easier it is for the rock to melt. Fluids allow the chemicals to move more quickly and easily, and the increased mobility makes for easier melting. We observe that melting starts in water rich layers of a metamorphic rock, even though the adjacent dry layers show no evidence of melting.
Thus, when only a tiny amount of water is present, only a small amount of melting occurs, or the melting takes place at higher temperatures.
It is also important to recognize that because a rock is composed of several minerals it does not melt completely at one specific temperature. Minerals with the lowest melting temperatures melt first so it is possible to have a partial melt (or a fractional melt), a liquid portion (magma) with a mush of unmelted crystals.
A rock that is composed of an unmelted metamorphic portion and a melted igneous portion is called a MIGMATITE (click picture for enlargement).
If, however, the melted igneous portion (the magma) is separated from the unmelted crystals we have the process of fractional melting.
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