Evaluation of the decay constant of uranium 235 from lead isotope ratios

The decay constant of U^(235) has been evaluated from the radiogenic Pb^(207)/Pb^(206) ratios of several cogenetic fractions of zircon and uranothorite, analyzed by conventional mass spectrometric methods. Variation of the Pb^(206)/U^(238) apparent ages among the mineral fractions demonstrates that some form of isotopic disturbance has occurred during their geologic history. Arguments are presented to show that the method of calculating λ_(235) is relatively insensitive to such disturbance because of the young geologic age of the samples and the probability that zircon and uranothorite experience isotopic disturbance by partial loss of Pb. Inaccuracy in the calculated value is further reduced by deducing, from geologic and isotopic considerations, the most likely limits to the apparent ages to which the Pb^(207)/Pb^(206) ratios should correspond. Although the uncertainties in the method do not permit a unique derivation of the decay constant, the results suggest that the currently accepted value is probably accurate to within 1%, relative to the accepted values for the decay constant of U^(238) and the isotopic composition of natural U.