Uranium-Thorium Dating

The Uranium-238 Decay Series

Gamma Decays Not Shown

Atomic
Number		 82 83 84 85 86 87 88 89 90 91 92
                  234Th
 238U
                    234Pa
  
  214Pb 218Po 222Rn 226Ra 230Th 234U
    214Bi
                  
  210Pb 214Po                
    210Bi
                  
  206Pb 210Po                

Uranium-Thorium dating is based on the detection by mass spectrometry of both the parent (234U) and daughter (230Th) products of decay, through the emission of an alpha particle.

The decay of Uranium 234 to Thorium 230 is part of the much longer decay series begining in 238U and ending in 206Pb.

For Uranium-Thorium dating, the initial ratio of 230Th/234U at the time of sample formation must be known or calculated. With time, Thorium 230 accumulates in the sample through radiometric decay. The sample age is based on the difference between the initial ratio of 230Th/234U and the one in the sample being dated. The method assumes that the sample does not exchange 230Th or 234U with the environment (i.e., that it is a closed system.) The method is used for samples that can retain Uranium and Thorium, such as carbonate sediments, bones and teeth. Ages between 1000 and 300,000 years have been reported.


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