I questioned a geologist about this a few years ago. His answer helped with #1 is included.
Maybe Richard will help.
"The isotope concentrations can be measured very accurately, but isotope concentrations are not dates. To derive ages from such measurements, unprovable assumptions have to be made such as:
1. The starting conditions are known (for example, that there was no daughter isotope present at the start, or that we know how much was there)."
Igneous rocks are the place these starting conditions are present.
In the heat and subsequent cooling of magma or lava, "parent" and "daughter" elements actually separate.
You find this pattern in many mineral combinations. Plus, geologist have the ability to observe and test the behavior of present day minerals under the same conditions-- magma, lava, and cooling.
The amount of non-separation that is likely to have occurred should be established as a range...which creates your margin of error estimate.
Sam's second condition:
"2. Decay rates have always been constant."
True, it's very difficult to vouch 100% for things like that. Still, we're only proceeding on the best information we have. There seems less reason to doubt this than to doubt certain miracles we're often told about. It's only fair that you subject your own ideas to the same tests and doubts you propose for others. You make some assumptions about things you did not witness. You assume documents were created in the same manner and that all the stuff in those documents are true. It appears that you
choose to assume that. I think the spirit of believing in Christ is the spirit of believing in Truth...and that it is okay for our understanding of the natural world to grow through our best efforts. The assumption that the decay rates were the same is not a giant leap of faith, but a starting point...one which does not seem to have much, if any, evidence against it.
Sam's Third Condition:
"3. Systems were closed or isolated so that no parent or daughter isotopes were lost or added.
If any of these assumptions is wrong, the age of the rock is wrong. How can they know if these assumptions are correct?"
The age of the rock is only correct to the extent their calculation is correct. If you can add to the accuracy of the science, great!