U th dating
The primary uranium mineralised zone in cross-section is a series of partially coalescing lenses, which together form an elongated wedge dipping at 55Â¡ to the southeast within the host quartz-chlorite schist unit, sub-parallel to the reverse fault.True widths average 30 m at the top of the primary mineralised zone but taper out at about 100 m below the surface and along strike.
In the conclusion to a recent paper exposing shortcomings and criticising the validity of the popular rubidium-strontium (Rb-Sr) isochron method, Zheng wrote: ‘…
some of the basic assumptions of the conventional Rb-Sr isochron method have to be modified and an observed isochron does not certainly define a valid age information for a geological system, even if a goodness of fit of the experimental data points is obtained in plotting Sr ratios, even an excellent line of best fit between ratios obtained from good cogenetic samples, and yet the resultant isochron and derived ‘age’ have no distinct geological meaning.
Zheng documented the copious reporting of this problem in the literature where various names had been given to these anomalous isochrons, such as apparent isochron, mantle isochron and pseudoisochron; secondary isochron, inherited isochron, source isochron, erupted isochron, mixing line, and mixing isochron.
Put simply, none of these assumptions can have been observed to have always been true throughout the supposed millions of years the radioactive elements have presumed to have been decaying.
Of the various radiometric methods, uranium-thorium- lead (U-Th-Pb) was the first used and it is still widely employed today, particularly when zircons are present in the rocks to be dated.