by Andrew Snelling As with other radiometric ‘dating’ methods, the U-Pb and Pb-Pb isochron methods have been questioned in the open literature, because often an excellent line of best fit between ratios obtained from a set of good cogenetic samples gives a resultant ‘isochron’ and yields a derived ‘age’ that has no geological meaning.
At the Koongarra uranium deposit, Australia, there is ample evidence of open system behaviour, or repeated migration, of U and Pb — ore textures, mineral chemistry, supergene alteration, uranium/daughter disequilibrium, and groundwater and soil geochemistry.
The energies involved are so large, and the nucleus is so small that physical conditions in the Earth (i.e. The rate of decay or rate of change of the number N of particles is proportional to the number present at any time, i.e.
The half-life is the amount of time it takes for one half of the initial amount of the parent, radioactive isotope, to decay to the daughter isotope.
Planetesimals were already forming by ∼1 Myr after CAI formation, consistent with their growth predominantly through the accretion of chondrules.
Nebular chondrule formation was completed by ∼5 Myr after CAI formation when the impact-generated Cba chondrules formed after the disk was cleared of gas and dust.
To see how we actually use this information to date rocks, consider the following: Usually, we know the amount, N, of an isotope present today, and the amount of a daughter element produced by decay, D*.
By definition, D* = N-1) (2) Now we can calculate the age if we know the number of daughter atoms produced by decay, D* and the number of parent atoms now present, N.
As such, we point out the risks inherent in comparing Pb–Pb ages produced by different approaches in different laboratories at the level of the internal errors of individual ages.We recognize that there is some debate over the age of the Solar System, but conclude that an age of 4567.30 ± 0.16 Ma based on four CAIs dated individually by the same method in two different laboratories is presently the best constrained published value.We further conclude that nebular chondrules dated by the Pb–Pb method require that they formed contemporaneously with CAIs and continued to form for at least ∼4 Myr, a conclusion that implies heterogeneous distribution of the short-lived Al nuclide in the protoplanetary disk.We anticipate that the ability to obtain precise and accurate in situ age and isotopic tracer information for USGS minerals-related projects will continue to be vitally important in the future. Our LA-SC-ICPMS has the capability of providing cost-effective in situ geochronological and mineral isotope tracer data, but the analytical techniques need to be developed further to reach their full potential. Yet U-Th-Pb isotopic studies of the uranium ore, host rocks and soils have produced an array of false ‘isochrons’ that yield ‘ages’ which are geologically meaningless.