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.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.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.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.

They estimated the age of the Earth by substituting the lead isotope ratios of certain meteorites in the Holmes-Houtermans equation.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.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.For example, He is only retained in apatite when the crystal cools below about 60–70°C, and in zircon when it cools below ~185°C.Because the earth gets hotter as you go deeper, the cooling history of a rock often tracks its path from the deep earth up to the surface.The basic theory of radiometric dating is briefly reviewed.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.Therefore, when you calculate the (U-Th)/He age of a rock or mineral, you are in effect calculating the time that has elapsed since the system switched from open (hot) to closed (cool) behavior.The transitional temperature is called the closure temperature, and is different depending on what mineral you are interested in.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.These ratios for many lead ores are plotted in Fig. The lowest ratios are taken to be the most ancient ores, formed at the beginning, billions of years ago and separated from further radiogenic enrichment. They show that widespread contamination and differentiation from various sources of lead have occurred during the more than one thousandfold concentration into the present lead ore deposits. There is no discontinuity whatever between results lying in the time clock zone and those lying in the alteration zone. Since there is no reason why the alteration zone should not extend into what is classified as the time clock zone (apart from a belief in 4.5 b.y.), the majority of the data can be explained as indicating a history of geochemical alteration.