General unreliability of radiometric dating
The main point at issue is fractionation and its relationship to U/Pb and Th/Pb dating.
Most scientists today believe that life has existed on the earth for billions of years. This belief in long ages for the earth and the existence of life is derived largely from radiometric dating. These long time periods are computed by measuring the ratio of daughter to parent substance in a rock and inferring an age based on this ratio. This age is computed under the assumption that the parent substance (say, uranium) gradually decays to the daughter substance (say, lead), so the higher the ratio of lead to uranium, the older the rock must be. Of course, there are many problems with such dating methods, such as parent or daughter substances entering or leaving the rock, as well as daughter product being present at the beginning.
Here I want to concentrate on another source of error, namely, processes that take place within magma chambers. To me it has been a real eye opener to see all the processes that are taking place and their potential influence on radiometric dating. Radiometric dating is largely done on rock that has formed from solidified lava. Lava (properly called magma before it erupts) fills large underground chambers called magma chambers. Most people are not aware of the many processes that take place in lava before it erupts and as it solidifies, processes that can have a tremendous influence on daughter to parent ratios. Such processes can cause the daughter product to be enriched relative to the parent, which would make the rock look older, or cause the parent to be enriched relative to the daughter, which would make the rock look younger. This calls the whole radiometric dating scheme into serious question.
Jon Covey cited some references about this, and it will take a lot of work to understand what is going on from a creationist viewpoint. But this is another factor that could be causing trouble for radiometric dating. If there is a proof that this could not be so, then I have missed it. I would not want to use a scale that might be right and might be wrong. This looks like the situation with U/Pb and Th/Pb dating so far. Another issue is selective reporting, and also an uncertainty as to how often U/Pb and Th/Pb dates agree with the expected ages of their geologic periods. And I'm curious to see how discordia relate to the possibility of fractionation -- I did look into them at one time. But this point is sufficiently complicated that I can't see the implications right away. In general, when an area is so complicated that I can just barely understand it, then there may be problems with the area that are more complicated still. But my inclination is to think that the same kinds of mixing processes that produce isochrons can also produce discordia.
Furthermore, if there are special circumstances that invalidate the method, then this raises questions about the method in general. It's been an eye opener to me to see all the processes that lead to segregation of different minerals in the magma. We have gold appearing pure at times, silver pure at times, etc., and no one says this is due to radiometric decay. The geological processes at work have a tremendous ability to separate different kinds of elements and minerals. And yet we expect that uranium-lead ratios are determined by radiometeric decay alone (or at least sometimes)!
There are so many complicated phenomena to consider like this that it calls the whole radiometric dating scheme into question.
We haven't even considered the fact that uranium is highly water soluble and lead is not, which could make the dates too old, too. Another factor to consider.
We now have so many things that can make radiometric dating go wrong, and isochrons don't remedy the situation at all, that I think the weight of evidence of radiometric dating is nullified.
The original has much more technical detail in a university technical paper: https://www.cs.unc.edu/~plaisted/ce/dating2.html
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