The U-Th-Pb dating system

Complications due to inheritance, Pb loss, and disequilibrium highlighted in the above discussion can be particularly problematic for bulk or chemical measurement methods, but other techniques have their own drawbacks. For example, U-Pb ion microprobe data can be subject to calibration shifts that move a concordant data point 

Note that Th- ° Pb dating of monazite is insensitive to problems arising from disequilibrium of Pb because secular equilibrium among the intermediate daughter 

The single long-lived isotope of thorium, Th, decays to ° Pb (Table 1) with a half-life of 14 Ga through a series of intermediate daughters via the relationship  

Unlike uranium, intermediate daughter isotopes from Th decay all have short half-lives such that secular equilibrium is achieved across the chain within 30 years. 

Michot and Deutsch (1970) invoked Pb loss via recrystallization and episodic metamorphism to explain age discrepancies. 

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