ABSOLUTE DATING 



TABLE I 

 DECAY PROCESSES OF NATURAL RADIOACTIVE ELEMENTS USED IN 



ABSOLUTE GEOLOGIC DATING 



31 



4(» 40 = atomic weight 



K denotes : ^K ■= atomic symbol 



= atomic number 



1!» 



19 



Of these various methods the uranium-lead and thorium-lead 

 measurements are the oldest. Only after World War II has better 

 instrumentation, e.g. more accurate and more dependable mass 

 spectographs and spectrometers, led to techniques suitable for meas- 

 uring the other decay series. Moreover, the lead methods have also 

 been immensely improved. 



Isotopes 



Before the war, a sample had to contain uranium, thorium and 

 lead in large quantities for it to be suitable material for an absolute 

 age determination. Such samples, e.g. pitchblende, a highly complex 

 and partly amorphous uranium mineral, were analysed for their 

 uranium, thorium and lead contents. From the respective decay rates 

 of the uranium and thorium series, it was calculated what percentage 

 of the lead present ought to be attributed to the decay of uranium 



