What isotope pair used to determine the age of fossils

Carbon-14, the radioactive isotope of carbon used in carbon dating has a half-life of 5730 years, so it decays too fast. It can only be used to date fossils younger than about 75,000 years.

Carbon 14 and Potassium - 40

Explanation:

Radioactive isotope of carbon - 14 is used in carbon dating. The half-life of C-14 isotope is 5730 years. It is an efficient isotope which can be used to determine the age of fossil which are younger than 75 thousand year old. Isotopes of other element are also used for dating such as Potassium - 40 (half life = 1.25 billion years).