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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.

For isochrons, which we will discuss later, the conditions are different.If these conditions are not satisfied, the error can be arbitrarily large.In two half-lives, half of the remainder will decay, meaning 3/4 in all will have decayed.In general, in n half-lives, only 1/(2^n) of the original parent material will be left.Potassium 40 (K40) decays to argon 40, which is an inert gas, and to calcium.

Potassium is present in most geological materials, making potassium-argon dating highly useful if it really works.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.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.Assuming we start out with pure parent, as time passes, more and more daughter will be produced. A ratio of infinity (that is, all daughter and no parent) means an age of essentially infinity.