Uranium to lead dating
Most scientists today believe that life has existed on the earth for billions of years.
This belief in long ages for the earth and the existence of life is derived largely from radiometric dating.
In a paper published this week in Science, geochemist Roland Mundil of the Berkeley Geochronology Center (BGC) and his colleagues at BGC and UC Berkeley report that uranium/lead (U/Pb) dating can be extremely accurate - to within 250,000 years - but only if the zircons from volcanic ash used in the analysis are specially treated.
To date, zircons - known to many as a semiprecious stone and December's birthstone - have often produced confusing and inaccurate results.
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.
The age of a rock can be calculated if its ratio of uranium to lead is known.
However, in reality, there is more lead than uranium.
It could be assumed that an undetermined quantity of lead 206 formed directly when the rocks originated.
Finally, ages can also be determined from the U-Pb system by analysis of Pb isotope ratios alone. Clair Cameron Patterson, an American geochemist who pioneered studies of uranium-lead radiometric dating methods, is famous for having used it to obtain one of the earliest accurate estimates of the age of the Earth.
Therefore we can assume that the entire lead content of the zircon is radiogenic.
Where this is not the case, a correction must be applied.
U 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.
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