In The earth: How old does it look? (Creation Ex Nihilo 23(1):8-13) at the end of the article, the sixth fact stated about radiometric dating said, “If a ‘radiometric’ date and a ‘fossil’ (evolutionary) date conflict, the radiometric date is always discarded.” I remember reading about recent discoveries of fossils and artifacts whose radiometric ages where older than previous datings, and scientists have adjusted their timelines accordingly.
The instances you quote are ones where the radioactive date and the fossil date do not conflict. In these cases the radioactive age is usually adopted. It gives a very strong impression that the dating is precise and scientific. However when there is a conflict, the radioactive date is rejected.
Let me explain briefly how it works with your artefact illustration. In evolutionary thinking there would be a range of acceptable ages for a human artefact, say from 50,000 years to 500,000 years (depending on the location and lots of other factors). The artefact may be assigned an age of 100,000 years, say, based on the type of artefact and the position where it was found.
Later someone may want to date the rock using a radioactive dating method. Remember, he is looking for a radioactive dating method that gives a number somewhere between 50,000 and 500,000 years. From the outset that will determine the sort of method he chooses. He would not use the Sm-Nd method because that would not work with such a short age. He may use the Ar-Ar method, or the uranium disequilibrium method, or the fission track method. Each of these methods is known to have certain characteristics. After he found a suitably related volcanic rock, he would need to decide what minerals in the rock he would analyse. He knows that different minerals behave in different ways.
If his analysis yielded a radioactive age of 210,000 years he would be delighted. The assigned age of the artefact would almost certainly be adjusted accordingly. He would give good reasons why the 100,000 years was too low, and the 210,000 years is correct. Such a change would cause no problem and not conflict with the evolutionary dating scheme. Indeed, the change would actually strengthen confidence that scientific dating was getting better and more reliable.
If however, the radioactive age turned out to be 3.5 million years he would be disappointed after all his work. The age of the artefact would not be adjusted because it would no longer fit with the evolutionary scheme. It would be easy for others to suggest reasons why the radioactive age was far too high.
We are not suggesting that the scientists are dishonest. This illustrates how a person's perceptions and expectations affect the way they do their science.
Although this is a hypothetical example, there are many cases of how this process works out in practice. The book Bones of Contention by Marvin Lubenow gives an excellent example in the appendix of how a conflict between radioactive dating and fossil dating was resolved by eventually rejecting the radioactive dates. In addition, the book The Mythology of Modern Dating Methods by John Woodmorappe explains the dating process in more detail.
Dr Tas Walker