Dating very old rocks
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.
The following quote from The Earth: An Introduction to Physical Geology by Tarbuck & Lutgens, pp.
55-57, (1987), gives us an idea of the tremendous complexity of the processes that occur when magma solidifies.
The general idea is that many different minerals are formed, which differ from one another in composition, even though they come from the same magma.
As the crystallization process continues, the composition of the melt (liquid portion of a magma, excluding any solid material continually changes.Further, the silicon content of the melt becomes enriched toward the latter stages of crystallization.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.This belief in long ages for the earth and the existence of life is derived largely from radiometric dating.
These long time periods are computed by measuring the ratio of daughter to parent substance in a rock and inferring an age based on this ratio.
This calls the whole radiometric dating scheme into serious question.