Examples. carbon, potassium. As seen in the tables above, there are three isotopes of uranium. Of these, U-238 is by far the most abundant 99.
It consists mostly of two isotopes with masses 39 and 41, but a third isotope, of mass 40, is weakly radioactive. One of the products of its decay is argon, an inert gas that makes up about 1 percent of the atmosphere. The potassium of mass 40 has a half-life of 1. It is a constituent of many minerals in the most common rocks, both igneous and sedimentary. Required conditions for the potassium-argon clock to work are the same as explained above. The potassium must be free of argon when the clock is started, that is, when the mineral is formed.
Chronological Methods 9 - Potassium-Argon Dating Potassium-Argon Dating Potassium-Argon dating is the only viable technique for dating very old archaeological materials. Geologists have used this method to date rocks as much as 4 billion years old. It is based on the fact that some of the radioactive isotope of Potassium, Potassium-40 K-40 ,decays to the gas Argon as Argon-40 Ar-40.