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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.Higher ratios are formed as the lead is fed by ageing uranium ore bodies. old lead fed continuously by uranium occurs at a lead-206 to lead-204 ratio of 18.5, which is taken as the present ratio for common lead. 3 since they have negative ages, that is, ages extending billions of years into the future, in some cases. Therefore the ores lying in the time clock zone are not necessarily any more a reflection of age than those lying in the alteration zone and ones lying in the alteration zone cannot possibly be time indicators.
The values they assumed were based on the lead isotope ratios observed for three meteorites.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.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.