“Some information from the book Uranium Geochemistry, Mineralogy, Geology provided by Jon Covey gives us evidence that fractionation processes are making radiometric dates much, much too old. Geology contributing author Massimo Cortini cites a very interesting anomaly regarding the U 238 decay chain, which is U-238, U-234, Th-230, Ra-226, Rn-222, Po-218 Po-214, Po-210, Pb-210, Bi-210, Pb-206. The half life of U-238 is 4.47 x 10^9 years and that of Ra-226 is 1.6 x 10^3 years. Thus radium is decaying 3 million times as fast as U-238. At equilibrium, which should be attained in 500,000 years for this decay series, we should expect to have 3 million times as much U-238 as radium to equalize the amount of daughter produced. Cortini says geologists discovered that ten times more Ra-226 than the equilibrium value was present in rocks from Vesuvius. They found similar excess radium at Mount St. Helens, Vulcanello, and Lipari and other volcanic sites. The only place where radioactive equilibrium of the U-238 series exists in zero age lavas is in Hawiian rocks. Thus instead of having 1/(3 million) as much radium as uranium, which we should expect, there is ten times as much, or 1/(300,000) times as much radium as uranium.
We need to consider the implications of this for radiometric dating. How is this excess of radium being produced? This radium cannot be the result of decay of uranium, since there is far too much of it. Either it is the result of an unknown decay process, or it is the result of fractionation which is greatly increasing the concentration of radium or greatly decreasing the concentration of uranium. Thus only a small fraction of the radium present in the lava (at most 10 percent) is the result of decay of the uranium in the lava.
This is interesting because both radium and lead are daughter products of uranium. If similar fractionation processes are operating for lead, this would mean that only a small fraction of the lead is the result of decay from the parent uranium, implying that the U-Pb radiometric dates are much, much too old. Cortini, in an article appearing in the Journal of Volcanology and Geothermal Research also suggests this possibility. He says:
“The invalidity of the Th-230 dating method is a consequence of the open-system behaviour of U and Th. By analogy with the behaviour of Ra, Th and U it can be suggested that Pb, owing to its large mobility, was also fed to the magma by fluids. This can and must be tested. The open-system behaviour of Pb, if true, would have dramatic consequences….” J Vol Geotherm Res 14 (1982) 247-260.
On the other hand, even if such a process is not operating for lead, the extra radium will decay rapidly to lead, and so in either case we have much too much lead in the lava and radiometric dates that are much, much too ancient! So this is a clue that something is not right with U-238/Pb-206 radiometric dates. It is also a convincing proof that some kind of drastic fractionation is taking place, or else an unknown process is responsible. Since most lavas have excess radium today, it is reasonable to assume this has always been true, and that all U-238/Pb-206 radiometric dates are much, much too old. Cortini says high Ra-226/U-238 ratios are a common feature of primitive magmas, which magma-generating processes produce. He says this is inexplicable in a closed-system framework and certainly invalidates the Th-230 dating method. And it is also possible that something similar is happening in the U-235 decay chain, invalidating U-235 based radiometric dates as well.”
Read more about the details here: http://www.cs.unc.edu/~plaisted/ce/dating2.html