Radioactive dating

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i am wondering if we know decays like Samarium-neodymium and Rubidium-strontium with half-lives of 106 and 49 billion years, why scientists consider the age of earth based on Uranium-lead dating with half-life of 4.47 billion years? i mean if there wasn't earth 100 billion years ago, where these Samarium comes from?
 

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  • #2
Borek
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They are produced in radioactive decay from other elements/isotopes. As they are relatively stable, their amount grows.
 
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thanks
 
  • #4
Drakkith
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Read more on half life here.
http://en.wikipedia.org/wiki/Half_life

Keep in mind that it is a probability that something will decay within this time frame. See #1 paragraph in the article.
 
  • #5
Also to date a rock their must be detectable amounts of both the parent and the daughter products, and you must be able to tell that the daughter products came from the parent product in the rock. Potassium/Argon dating comes to mind. Potassium is an element that can be found in quite a few minerals, especially the "K-feldspars" or potassium feldspars. K40 decays to Ca40 and Ar40. Only about 10.9% decays to argon but that is the element that geologists are interested in. The reason why is that there are usually other sources of calcium that make it very difficult to detect the calcium from radioactive decay. Argon on the other hand does not usually appear in rocks as part of the elements that make up its minerals so it is fairly safe to assume that it is a radioactive product. Potassium/Argon therefore is a very useful dating tool. You have an element that can be found in almost any rock and a daughter product that is not. The one concern is that you want material that has not been exposed to the atmosphere, since that can be a source of argon.

The main reason that I can see that samarium/neodymium is not used can be seen by what they call these elements. I know off of the top of my head that neodymium is a rare earth, and I am certain that samarium is one too. A daughter product that is rare is a good thing, that increases the odds that any daughter product you see is a result of decay, a parent material that is rare is a bad thing, that means it is highly unlikely that you will have any product to decay in the first place.
 
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Vanadium 50
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Rare earths are not that rare. There are more rare earths than copper, zinc or chromium. There is more samarium than tin.

You are exactly right, though, that you want to pick an isotope with a half-life close to the age of your sample. If the half-life is too short, you have mostly daughter nuclei and not many parents, and if it's too long, you have mostly parents and not many daughters.
 

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