Age of a rock using radiometric dating
They estimated the age of the Earth by substituting the lead isotope ratios of certain meteorites in the Holmes-Houtermans equation.
In this equation the primordial lead ratios are required.
In 1955 a symposium on radiometric dating was held from which the following was given in the summary: “Radioactive ‘dating’ has been perhaps the most widely publicised of geochemical techniques, but of several known dating methods based on radioactivity, only C-14 dating has developed to the point where it yields consistently reliable ages.
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.
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.
magazine has been continuously published since 1978, we are publishing some of the articles from the archives for historical interest, such as this.