Radiometric dating uranium 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.Despite this, the momentum gained in the two decades prior to 1972 has made 4.5 b.y.a popularly accepted “universal constant” even though the foundations on which it was based have been virtually removed.magazine has been continuously published since 1978, we are publishing some of the articles from the archives for historical interest, such as this.For teaching and sharing purposes, readers are advised to supplement these historic articles with more up-to-date ones suggested in the Related Articles and Further Reading below.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.
It has assumed something of the status of a universal constant to which all other data must be fitted, thus it has become common practice to assume that data which does not fit this result is either wrong or unintelligible. Lead-204, a minor isotope of common lead, has no radioactive parent and is believed to be primordial lead.
It is probably because of this type of evidence for extensive mixing in the alteration zone that Patterson “In view of the evidence for extensive mixing, it would seem contrary to the facts to postulate differing frozen lead/uranium ratios that have existed for billions of years.
The requirements of the assumptions in the lead ore method are so extreme it is unlikely that it should give a correct age.” So they took a different approach.
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.
The values they assumed were based on the lead isotope ratios observed for three meteorites.