Radiometric dating of rocksfossils
They estimated the age of the Earth by substituting the lead isotope ratios of certain meteorites in the Holmes-Houtermans equation.In this equation the primordial lead ratios are required.That difficulty aside, they were selected because they contain very little uranium and thorium and are therefore unlikely to contain significant radiogenic lead.However, it is even more surprising to learn that the lead isotope ratios chosen by Patterson Most meteorites have lead isotope ratios similar to those of present day common lead.There is a large body of discordant data but concordant data are scarce.
In 1972 this assumption was shown to be highly questionable.
rely heavily on the uranium/thorium/lead radiometric dating methods.
Because it is not generally appreciated that the assumptions on which the radiometric estimates are based are a virtually impossible sequence of events, let us refresh our minds on the fundamentals of the method by turning to the hourglass analogy (Fig. This system of measuring time works well providing that: Since radioactive decay constants are believed to be unalterable, the requirement of an absolutely reproducible rate is hopefully met.
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.