Uranium radiometric dating

U238 —> —> Pb206 4.47 x 109y Uranium-lead radiometric dating (1 of 2)... 

A radiometric dating technique uses the decay of U-238 to Pb-206 (the half-life for this process is 4.5 billion years) to determine the age of the oldest rocks on Earth and by implication the age of Earth itself. The oldest uranium-containing rocks on Earth contain approximately equal numbers of uranium atoms and lead atoms. Assuming the rocks were pure uranium when they were formed, how old are the rocks ... 

A second radiometric dating technique, used to measure much longer periods of time, involves U-238. U-238 decays with a half-life of 4.5 X 10 yr through a number of intermediates, and eventually ends as lead. As a result, all rocks on Earth that contain uranium also contain lead. If a rock is assumed to have been only uranium when it was formed (which can be determined by the relative amounts of different lead isotopes in the rock), the ratio of uranium to lead can be used to date the rock. For example, if one half-life has passed, the rock would contain 50% uranium atoms and 50% lead atoms. After two half-lives, the rock would be 25% uranium atoms and 75% lead atoms. 

Traditionally foram records have not been datable by radiometric means (Delaney and Boyle, 1983 Henderson and O Nions, 1995). Generally ages are established by orbital tuning (Imbrie et al., 1984). However, in specific instances where forams have been separated from uranium-rich carbonate bank sediments, the bank sediments have been dated by °Th methods, yielding radiometric chronologies for the foram 180/1 0 record (Slowey et al., 1996 Henderson and Slowey, 2000 Robinson et al., 2002). 

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