I agree Dating is all about run-of-the-mill decay since the elements formed. We really are clueless to the original parent elements when the earth was formed but the helium trapped when the granites formed is one of the methods creationists use to date not the age of the elements but the age of the earth.
Except that Helium
isn't trapped when the granite forms. It is free to move in and out since there will not be any pressure differential between "in" and "out" of any given crystal.
The argument, as far as I can tell, (please correct me if i have this wrong. I haven'r followed it in great detail)is that the RATE project stated that the rate of helium diffusion
from an individual Zircon crystal could be used to date it and that it is more reliable than the U/Pb ratio in the same crystal.
Since we know (from laboratory experiments) that zircon crystals chemically reject the presence of Pb and actively accept Uranium while they are crystalizing, that means that
at the time of formation of the crystal there was zero lead in it so any lead now in it must have gotten there later. The only possible source is the decay of the Uranium, which is also known (from laboratory experiments) to happen. The pathway is exceedingly well documented.
I agree with you that we don't know the chemical makeup of much of anything at the time the earth was first formed but we certainly do know with total certainty that when that Zircon crystal formed, it contained
zero lead. there are no correction equations here. No assumptions. just the absolute fact that Zircon formation utterly rejects Lead.
What we don't know is how much Helium it had at that time. How much was dissolved in the liquid magma?
What we don't know is how much Helium got into it afterward or moved out later or any number of other details.
What we do also know is that Helium
does form (in quite high quantities) as a by-product of the uranium decay series.
Oh and one more thing we
DO know pretty well is the way that gas diffusion behaves at different pressures. It is Chemistry 101 gas laws.
We also know that you cannot compress the inter-atomic distances between the atoms in rock and since those are the pathways that very light gasses travel, it means that we know that the rate of diffusion is independent of pressure under these conditions since there literally isn't any pressure of the type that would effect gasses in the rock.
remember we aren't talking about gas pressures here. That would effect diffusion rates for sure. We are talking about compression pressures which are a different thing entirely.
The experiment that you say hasn't been done, hasn't been done for a reason. We already know the answer from myriads of other similar experiments and observations. Doing an experiment to measure diffusion rates of helium through underground rock would be a little akin to holding your head underwater and seeing if you can breathe still. Pointless and a complete waste of time.