can radiometric dating be used to find the absolute age of any material?

Question

I need listed some characteristics that would make some materials unsuitable for this dating method......I have been reading my science book over and over and cannot find the answer, thanks :)

Answer 1

Radiometric dating is based on the decay of a particular radioactive isotope - and there are quite a few to choose from (Uranium-238, Uranium-235, Thorium-232, Carbon-14...the list goes on). To calculate an age you need to know the following:

1) how much of the radioactive isotope is present now - this we measure.
2) how quickly the radioactive isotope decays - for things like uranium, carbon-14, the decay rate (or halflife) has been measured .
3) how much of the radioactive isotope was present to begin with - this is the most difficult bit to work out.

If you know all three of these, you can work out how much of the radioactive isotope has decayed, and calculate an age.

Not all things can be dated - here are some of the reasons why not:
A) Not everything contains suitable radioactive isotopes in large enough quantities to measure.
B) the item that you are trying to date is either too old or too young. Things like carbon-14 decay quite quickly, so you can only use it to date things on the order of thousands of years. If you wanted to try and date say something that was 20 million years old using carbion dating, it wouldn't work, becase virtually all of the carbon-14 would have decayed, so there wouldn't be enough left to measure (i.e. you would be missing no. 1 from the first list). Conversely, things like uranium-238 only decay very slowly, so you cannot use it to date very young things, because the amount of decay will be too small to measure. This relates back to point A. basically what you are trying to date must contain a radioactive isotope with a decay rate of the same order of magnitude of the age of the item you are trying to date.
C) Another reson for not being able to obtain an age is not being able to determine how much of the radioactive isotope was there to begin with (i.e. point 3). There are several reasons for this, but I'll just give a simple example. Uranium-238 decays into lead-206, so if you know how much lead-206 is in the sample, then in theory you can work out how much uranium-238 has decayed.....Simple!...except if some of the lead-206 has escaped from the ssample it all goes horribly wrong, and you cannot calculate an age!!

Answer 2

Radiometric dating is never absolute; it is most accurate when the age is about one half-life of the isotope being measured. Different isotopes, if present, are selected for different age ranges.

Crystaline materials are best because they had a known pure chemical composition when formed; products of radioactive decay are foreign to the material and could only have gotten there after the crystals formed.

Sometimes it is possible to combine radiometric dating with other techniques to get a more accurate result. For example, carbon dating can place a sample of petrified wood in a certain era; tree ring data for that era, if available, can then narrow it down to one year.