U decay constants and half-lives have been made using direct counting experiments and geological age comparisons, as well as by critical reviews and reevaluations of those determinations. “Natural Reactor Studies.” In Uranium in the Pine Creek Geosyncline. By 1971 the direct counting experiments had provided U half-life values with small uncertainties which ever since have been the recommended values used in all U-Pb age calculations. Carbon dating is used to determine the age of biological artifacts up to 50,000 years old.This technique is widely used on recent artifacts, but educators and students alike should note that this technique will not work on older fossils (like those of the dinosaurs alleged to be millions of years old).Levels of carbon-14 become difficult to measure and compare after about 50,000 years (between 8 and 9 half lives; where 1% of the original carbon-14 would remain undecayed).The question should be whether or not carbon-14 can be used to date any artifacts at all? There are a few categories of artifacts that can be dated using carbon-14; however, they cannot be more 50,000 years old.
We can always try U-Pb dating using the isochron method, but this often doesn't work: the compositions of the minerals involved, when plotted on an isochron diagram, fail to lie on a straight line. First of all, the straight-line property of the isochron diagram is destroyed when the isotopes involved get shuffled between minerals.
But there have still been repeated calls for more modern, more accurate direct counting experiments to more precisely determine the U half-life by forced agreement of Rb-Sr, Lu-Hf, Re-Os, Sm-Nd, K-Ar, and Ar-Ar ages respectively with U-Pb ages obtained for the same rocks, minerals and meteorites, none of these decay half-lives are really known accurately. “New Average Values for n(U) Isotope Ratios of Natural Uranium Standards.” International Journal of Mass Spectrometry 295 (1–2): 94–97.
Therefore, without accurately known decay half-lives, all radioisotope ages cannot be accurately determined or be considered absolute ages.
In this article we shall discuss the basis of the U-Pb and Pb-Pb methods, and also fission track dating.
The reader will find this article much easier to grasp if s/he has already mastered the material in the articles on K-Ar dating, Ar-Ar dating, and Rb-Sr dating.
Now since all rocks are somewhat porous, and since we are pretty much obliged to date rocks from near the surface, it's hard to find instances in which uranium has not been lost.