To prove this, fractional crystallization models would be produced to test the hypothesis that they share a common parental melt.
The whole gamut of mechanisms for differentiation has been referred to as the FARM process, which stands for fractional crystallization, assimilation, replenishment and magma mixing.
Fractional crystallization in silicate melts (magmas) is a very complex process compared to chemical systems in the laboratory because it is affected by a wide variety of phenomena.
The partial pressure of volatile phases in silicate melts is also of prime importance, especially in near-solidus crystallization of granites.
[5][6] More recent research has shown, however, that assimilation has a fundamental role in altering the trace element and isotopic composition of magmas,[7] in formation of some economically important ore deposits,[8] and in causing volcanic eruptions.
In such a case, the liquid line of descent is interrupted by the injection of a fresh batch of hot, undifferentiated magma.
Magma mixing also tends to occur at deeper levels in the crust and is considered one of the primary mechanisms for forming intermediate rocks such as monzonite and andesite.
Here, due to heat transfer and increased volatile flux from subduction, the silicic crust melts to form a felsic magma (essentially granitic in composition).
Often near the margins of a magma chamber which is convecting, cooler and more viscous layers form concentrically from the outside in, defined by breaks in viscosity and temperature.
With reference to the definitions, above, a magma chamber will tend to cool down and crystallize minerals according to the liquid line of descent.
When this occurs, especially in conjunction with zonation and crystal accumulation, and the melt portion is removed, this can change the composition of a magma chamber.
In fact, this is basically fractional crystallization, except in this case we are observing a magma chamber which is the remnant left behind from which a daughter melt has been extracted.
Of these water is the principal, and was formerly believed to have percolated downwards from the Earth's surface to the heated rocks below, but is now generally admitted to be an integral part of the magma.
Many peculiarities of the structure of the plutonic rocks as contrasted with the lavas may reasonably be accounted for by the operation of these gases, which were unable to escape as the deep-seated masses slowly cooled, while they were promptly given up by the superficial effusions.
It is conceivable that in the final stages the still uncrystallized part of the magma has more resemblance to a solution of mineral matter in superheated steam than to a dry igneous fusion.
Its late formation shows that in this case it arose at comparatively low temperatures and points clearly to the special importance of the gases of the magma as determining the sequence of crystallization.
They are powerful agents in attacking the minerals of the rocks which they traverse, and instances of their operation are found in the kaolinization of granites, tourmalinization and formation of greisen, deposition of quartz veins, and the group of changes known as propylitization.