When does radiometric dating does not work?

Question

When does radiometric dating does not work?

Answer 1

I expect if the sample is contaminated with more recent stuff it is impossible to perform radiometric dating.   Carbon 14 dating only works with things that are less than seventy thousand years old.  Older than that and the remaining C-14 is not measurable.    Since most human history is more recent than that it is great for dating carbon containing substances like wood, human remains, etc.

There are of course other methods of radiometric dating that allow for assigning dates with varying levels of confidence to rocks.  Uranium isotopes decay in a long series of reactions leading to lead.  The whole sequence takes billions of years.  Using methods too complex to explain here, the age of rocks can be estimated right back to the formation of the earth.

Answer 2

In geology, radiometric dating can only be used on rocks that contains minerals that have radioactive isotopes - so igneous rocks can be dated using K-Ar or U-Pb from potassium feldspar or zircons etc. These minerals crystallized out  from the original magma, and therefore gives an absolute date. If the rocks have been metamorphosed the 'clock' is reset through melting, and so gives an inaccurate date. Because sedimentary rocks are composed from minerals that have weathered from previous igneous rocks, they cannot be accurately radiometrically dated.