Radiometric dating allows us to determine
Therefore, all one has to do in general terms is to find a radioactive mineral which has been a closed system since the time of mineralization, and for which the amount of the daughter product at the beginning is known, the so-called primordial amount, and the absolute age may be calculated from the present amount of parent and daughter isotopes in the mineral.
Briefly, the weakest points in this method are that (a) truly closed systems probably do not exist in nature, “As in the case with radiometric ages determined from almost any rock unit it is impossible to establish unequivocally that the ages reported here reflect the time of original crystallization or emplacement of the bodies from which they are derived.” Before we consider the actual lead/lead isotope data there is one other comment that needs to be made regarding extrapolation of present rates. Five billion years is five million times greater than one thousand years. 2 is 2.5 cm, five million times greater is about 125 km.
Before 1955, ages for the Earth based on uranium/thorium/lead ratios were generally about a billion years younger than the currently popular 4.5 billion years. old Earth is reviewed and deficiencies of the uranium/lead method are discussed.
The basic theory of radiometric dating is briefly reviewed.
In fact, there is no large body of concordant data.Lead-206 and lead-207 are also believed to be present in primordial lead since there is insufficient uranium to account for all the lead.Just how much lead-206 and 207 were present at the beginning, nobody knows. As a uranium ore ages, the ratio of lead-206 to lead-204 increases as does the ratio of lead-206 to lead-207.Up until 1972 these could be explained as being contaminated with radiogenic lead from uranium and thorium decay.In 1972, however, Gale showed unequivocally that there is by no means sufficient uranium and thorium to account for what could previously have been called radiogenic lead.