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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.Since 1955 the estimate for the age of the Earth has been based on the assumption that certain meteorite lead isotope ratios are equivalent to the primordial lead isotope ratios on Earth.In 1972 this assumption was shown to be highly questionable.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.The radiometric dating method is basically an extrapolation of the form shown in Fig. If the decay constant is known with great accuracy, an extrapolation over one or two thousand years may be regarded as quite reasonable. It should be obvious that the further one projects present rates, the more likely one is to be quite wrong. era started about 1955 with the publication of a classic paper by Patterson In spite of cautions and scepticism advised by the authors this number has been widely and enthusiastically accepted and is usually quoted as if the evidence was decisive and conclusive. Lead-206 and lead-207 are known daughter products from the decay of uranium-238 and uranium-235, respectively.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.

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.

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.

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.

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