But at the same time, it will be enriched in the elements contained in the later forming minerals, namely sodium and potassium.Further, the silicon content of the melt becomes enriched toward the latter stages of crystallization.
The mineral makeup of an igneous rock is ultimately determined by the chemical composition of the magma from which it crystallized.
As the crystallization process continues, the composition of the melt (liquid portion of a magma, excluding any solid material continually changes.
For example, at the stage when about 50 percent of the magma has solidified, the melt will be greatly depleted in iron, magnesium, and calcium, because these elements are found in the earliest formed minerals.
Lava erupting earlier would come from the top of the magma chamber, and lava erupting later would come from lower down.
A number of processes could cause the parent substance to be depleted at the top of the magma chamber, or the daughter product to be enriched, both of which would cause the lava erupting earlier to appear very old according to radiometric dating, and lava erupting later to appear younger.
Bowen also demonstrated that if a mineral remained in the melt after it had crystallized, it would react with the remaining melt and produce the next mineral in the sequence shown in Figure 3.6.