Radiometric dating is a very useful tool, but it does have limits: The material being dated must have measurable amounts of the parent and/or the daughter isotopes. Radiometric dating can be done on only some materials. It is not useful for determining the age of sedimentary rocks.

What are some limitations in the use of 14c for radioactive dating?

Despite its usefulness, radiocarbon dating has a number of limitations. First, the older the object, the less carbon-14 there is to measure. Radiocarbon dating is therefore limited to objects that are younger than 50,000 to 60,000 years or so.

Why is U-238 used to date the Earth instead of I 131?

Uranium 238 has a half life of 4.5 billion years. Uranium can be used to date the age of the earth. If 50% of pure uranium is left in a sample the sample is assumed to be 4.5 billion years old. So no fossils can be dated directly using U 238.