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.

radiometric dating and the geological time scale-7

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 case the significance of these results is ignored, a few sentences from the Gale “ …

it is not widely appreciated, outside the ranks of those who work directly in geochronology or meteoritics that, judged by modern standards, the meteoritic lead-lead isochron is very poorly established.

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.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.The values they assumed were based on the lead isotope ratios observed for three meteorites.Since meteorites have not proved to be the ancient objects from the sky that one might imagine, it is surprising that they should be assumed to give the primordial lead composition on Earth.Despite this, the momentum gained in the two decades prior to 1972 has made 4.5 b.y.