Answer: Carbon 14 and Uranium 238 are not used together to determine fossil ages.
Explanation:
Carbon 14 with a half life of 5,700 years can only be used to date fossils of approximately 50,000 years. Most fossils are thought to be much older than 50,000 years. Also most fossils no longer contain any Carbon. The fossilized remains have been mineralized where the original organic material has been replaced and turned into stones containing no carbon.
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.( 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. If a fossil has only 25 % of the Uranium 238 the sample has an estimated age of 3.2 Billion years. This would be the estimated age of the earliest life or formation of fossils.
Note no fossils contain Uranium 238. Uranium 238 is only found in igneous or volcanic rocks. So no fossils can be dated directly using U 238.
Because of the huge differences in the half lives of Carbon 14 and Uranium238 they cannot be used together. Carbon 14 can only be used to date fossils of a very recent age. Uranium 238 can only be used to date volcanic rocks of a very old age.
