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In order to use these methods, we have to start out with a system in which no daughter element is present, or else know how much daugher element was present initially so that it can be subtracted out.
We also need to know that no parent or daughter has entered or left the system in the meantime.
Potassium is about 1/40 of the earth's crust, and about 1/10,000 of the potassium is potassium 40.
Uranium decays to lead by a complex series of steps. Thus we obtain K-Ar dating, U-Pb dating, and Rb-Sr dating, three of the most common methods.
For potassium 40, the half-life is about 1.3 billion years.
In general, in one half-life, half of the parent will have decayed.
After study and discussion of this question, I now believe that the claimed accuracy of radiometric dating methods is a result of a great misunderstanding of the data, and that the various methods hardly ever agree with each other, and often do not agree with the assumed ages of the rocks in which they are found.We can assume that the Precambrian rocks already existed when life began, and so the ages of the Precambrian rocks are not necessarily related to the question of how long life has existed on earth.The Cambrian period is conventionally assumed to have begun about 550 million years ago.At the start, let me clarify that my main concern is not the age of the earth, the moon, or the solar system, but rather the age of life, that is, how long has life existed on earth.Many dating methods seem to give about the same ages on meteorites.