Evidence to make you question the reliability of radiometric dating:
“The following is a quote from The Earth: An Introduction to Physical Geology by Tarbuck & Lutgens, pp. 55-57, (1987).
“For example, at the stage when about 50 percent of the magma has solidified, the melt will be greatly depleted in iron, magnesium, and calcium, because these elements are found in the earliest formed minerals. But at the same time, it will be enriched in the elements contained in the later forming minerals, namely sodium and potassium.”
A geologist writes:
“Uranium and thorium ARE strongly fractionated during magmatic processes and tend to be concentrated in the silicic/felsic part of a magma hence granites and rhyolites tend to have a much higher average uranium and thorium concentration (3-5 ppm U) compared to basalts (less than 1 ppm U).”
From the above quotes and references, uranium is concentrated in granite, which is depleted in magnesium and iron. The magnesium and iron rich minerals come from the mantle (subducted oceanic plates), while granite comes from continental sediments (crustal rock). The mantle part solidifies first, and is rich in magnesium, iron, and calcium. The silicic/felsic part of a magma typically becomes granite and solidifies later, enriched in uranium, thorium, sodium, and potassium. So it is reasonable to expect that initially, the magma is rich in iron, magnesium, and calcium and poor in uranium, thorium, sodium, and potassium. Later on the magma is poor in iron, magnesium, and calcium and rich in uranium, thorium, sodium, and potassium. It doesn’t say which class lead is in. But lead is a metal, and to me it looks more likely that lead would concentrate along with the iron. If this is so, the magma would initially be poor in thorium and uranium and rich in lead, and as it cooled it would become rich in thorium and uranium and poor in lead. Thus its radiometric age would tend to decrease rapidly with time, and lava emitted later would tend to look younger.”
Read more about the details here: http://www.cs.unc.edu/~plaisted/ce/dating2.html
