Uranium-lead
From EvoWiki
The uranium-lead method is a radiometric dating method which uses the decay of uranium isotopes into lead isotopes. Two decay series from uranium isotopes into lead isotopes exists:
- 238U → ..... → 206Pb (half-life 4.5 billion years)
- 235U → ..... → 207Pb (half-life 0.704 billion years)
If the concentrations of the uranium isotopes and the corresponding lead daughter isotopes are determined, the ages can be calculated using the following formulas:
t = ln(1+206Pb/238U) / lambda238
t = ln(1+207Pb/235U} / lambda235
lambda238=1.55125 10-10 and lambda235=9.8485-10 are the decay constants.
There is also a third way to determine the age by only determining the 207Pb/206Pb ratio of a sample and using the formula:
207Pb/206Pb = 235U/238U (exp(lambda235*t)-1)/(exp(lambda238*t)-1)
The known 235U/238U ratio of 1/137,88 is homogeneous within the earth. The advantage of this third method is that isotope ratios of the same element can be determined more accurately than ratios of isotopes from different elements. Another advantage of the uranium-lead method is the well known decay constants of the uranium isotopes, which is better known than the decay constants of isotopes of other elements which are used for other dating methods.
