Natural lead is a mixture of four stable isotopes. The fourth, lead 204, is not the result of radioactive decay. This means that all of the lead 204 on the Earth has been around since the formation of the Earth. Based on extensive sampling of the Earth's crust, scientists determined the present-day abundances of the four isotopes of lead relative to each other and to the parent isotopes that produced three of them. Because the original abundances of lead on the planet cannot be measured, scientists use meteorites to get at the Earth's original lead composition.
Some meteorites contain the four lead isotopes but no uranium or thorium parents.
This means that the lead composition in these meteorites has not changed since their formation, and scientists believe this is a reasonable approximation of the composition of the Earth's original lead, the so-called primordial lead. Comparing the amounts of the four lead isotopes in primordial lead to their present amounts, scientists can determine how much lead has been added by radioactive decay since the Earth was formed. These calculations also yield an age of about 4.6 billion years for the Earth, which is consistent with the ages determined from meteorites and lunar rocks.