Uranium 238 dating rocks

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.Uranium-lead dating uses four different isotopes to find the age of the rock.The reason for stopping at lead is because lead is not radioactive and will not change into a different element.It may sound straight-forward, but there are many variables that have to be considered.The half-life is the amount of time it takes for one half of the initial amount of the parent, radioactive isotope, to decay to the daughter isotope.Thus, if we start out with 1 gram of the parent isotope, after the passage of 1 half-life there will be 0.5 gram of the parent isotope left.

What makes this fact useful is that they occur at different rates, as expressed in their half-lives (the time it takes for half the atoms to decay).Carbon 14 with a half life of 5,700 years can only be used to date fossils of approximately 50,000 years. If 50% of pure uranium' is left in a sample the sample is assumed to be 4.5 billion years old.( This is assuming that the original sample was 100% uranium and no Uranium 238 has been eroded or lost in 4.5 billion years old.Most fossils are thought to be much older than 50,000 years. The fossilized remains have been mineralized where the original organic material has been replaced and turned into stones containing no carbon. If a fossil has only 25 % of the Uranium 238 the sample has an estimated age of 3.2 Billion years.The four isotopes are uranium-235, uranium-238, lead-207, and lead-206.The process of dating finds the two ratios between uranium-235 and lead-207; and uranium-238 and lead-206.

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This would be the estimated age of the earliest life or fossils. Uranium 238 is only found igneous or volcanic rocks. Because of the huge differences in the half lives of Carbon 14 and Uranium238 that cannot be used together.

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