Teaching radiometric dating


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.

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.

Since the lead in meteorites can no longer be ascribed to uranium/thorium decay, it may also be taken to represent primordial lead.