U at the time of sample formation must be known or calculated. With time, Thorium 230 accumulates in the sample through radiometric decay.
Minerals, too, are predictable chemical compounds that can be shown to form at...
Of all the isotopic dating methods in use today, the uranium-lead method is the oldest and, when done carefully, the most reliable.
, method of establishing the time of origin of a rock by means of the amount of common lead it contains; common lead is any lead from a rock or mineral that contains a large amount of lead and a small amount of the radioactive progenitors of lead— isotope thorium-232.
The important characteristic of common lead is that it contains no significant proportion of radiogenic lead accumulated since the time that the mineral or rock phase was formed.
This age is in good agreement with the age of the meteorites and the age of the Moon as determined independently.
As each dating method was developed, tested, and improved, mainly since 1950, a vast body of knowledge about the behaviour of different isotopic systems under different geologic conditions has evolved.
In rubidium–strontium dating, micas exclude strontium when they form, but accept much rubidium.
In uranium–lead (U–Pb) dating of zircon, the zircon is found to exclude initial lead almost completely.
The three main parameters that have to be set are the original amount of uranium and lead in the sample, the rate at which uranium and lead enter and leave the sample, and how much the rate of decay changes.