ABSOLUTE DATING 37 



ical analysis, because the mass spectrometer counts each atom of the 

 sample. Under ideal circumstances age determinations can now be 

 made on quantities of 1 ^g (a microgram, or one millionth of a 

 gram). 



Mass spectrometry has consequently made possible the use of the 

 rubidium — strontium and the potassium — argon decay for absolute 

 dating. It also gave a new impetus to the lead method. At present it 

 is possible to measure separately the ^'^^thorium — ^'^^lead, the ^'"^^ura- 

 nium — ^'^"lead and the ^''^^uranium — ^^^*^lead amounts in uranium 

 minerals. Moreover, it is possible to check the amount of these three 

 radiogenic isotopes of lead against the non-radiogenic ^^"^lead in the 

 sample. This, how^ever, still asks for chemical assaying of the amounts 

 of thorium, uranium and total lead. But in using so-called lead — lead 



206pb 



ages, derived from quotients such as-^— | — , age determinations can 



be made in which the total amount of parent element no longer 

 enters. 



This is very important, because in this way independent checks 

 can be made of a possible loss of certain elements during the radio- 

 active decay. Both of the uraniums and thorium have gaseous inter- 

 mediate steps in their decay series, such as radon gas, which can 

 easily escape over geologic periods. Moreover, the 'stable' end, the 

 daughter isotopes of lead, are not very stable in the crust of the 

 earth, but tend to segregate elsewhere through the influence of 

 various processes over the millions of years. The amount of loss of 

 radiogenic lead and other components can be checked by measuring 

 various quotients of the lead isotopes. Only when all values deter- 

 mined by the different lead methods concur, is there a probability 

 that no serious losses of radiogenic products have occurred. In this 

 case, the so-called 'concordant lead age' has proven as reliable a 

 method of dating as those based on the other decay series. 



Reliability of absolute dating 



To sum up this digression on the methods of absolute age deter- 

 mination, it follow^s that the techniques have shown a stupendous 

 development over the past decades. Moreover, the scientists engaged 

 in this work now^adays try to date one sample by as many techniques 

 as possible. Because of the very small quantities now required of the 



