This commonly occurs if the system has been heated or otherwise disturbed, causing a loss of some of the lead daughter atoms.Because Pb are chemically identical, they are usually lost in the same proportions. Wetherill has shown that the two points on the Concordia curve intersected by this straight line will represent the time of initial crystallization and the time of the subsequent lead loss.
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.
The amount of radiogenic lead from all these methods must be distinguished from naturally occurring lead, and this is calculated by using the ratio with Pb ages should agree.
If this is the case, they are concordant and the age determined is most probably the actual age of the specimen.
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.