JonF writes:
I feel compelled to comment on knowing the initial isotopic state of the sample. Several modern methods do require that be known but the laws of physics give us the information we need. The most widely used is, of course, the aforementioned knowledge that zircons readily incorporate uranium and strongly reject lead so the initial P/U ratio is essentially zero.
I completely agree. However, creationists think that it is assumption. It isn’t.
The size of the Pb 2+ ions ensure that they can’t be incorporated into the tetrahedral crystal lattice of zircon (or the polymorphs of zircon). They are simply too big.
However, U2+ fit in nicely. So, at the formation of tetrahedric zircon crystals, we know that the Pb incorporated into the crystal lattice will essentially be zero.
Another Pb isotope, Pb6+, is not found in tetrahedral crystals for obvious reasons. They can't do tetrahedral crystals for the same obvious reasons. In chemistry the words "tetra" and "hexa" mean something...
It’s not an assumption. It’s basic chemistry.
Edited by Pressie, : No reason given.