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Back to top Radioactive elements decay gradually into other elements.The original element is called the parent, and the result of the decay process is called the daughter element.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.By measuring the ratio of daughter to parent, we can measure how old the sample is. Each radioactive element has a half-life, which tells how long it takes for half of the element to decay.For isochrons, which we will discuss later, the conditions are different.If these conditions are not satisfied, the error can be arbitrarily large.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.
The Cambrian period is conventionally assumed to have begun about 550 million years ago.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.
When it is stated that these methods are accurate to one or two percent, it does not mean that the computed age is within one or two percent of the correct age.