If those rocks really have been sitting around on the moon for billions of years, I suspect that the the wide range of physical and chemical processes which occurred over that time period had a much more profound effect on the uncertainty of the age determination.This is best illustrated by the radioactive age of a sample of diamonds from Zaire.
However, it’s important to note that some radioactive dates (like those that come from carbon-14) don’t use the isochron method, so they aren’t affected by this particular flaw.
As a young-earth creationist, I look at this issue in a different way.
The isochron is supposed to take care of such issues.
Essentially, rather than looking at the amounts of Rb-87 and Sr-87, we look at their compared to Sr-86.
The amount of Sr-87 that was already in the rock when it formed, for example, should be proportional to the amount of Sr-86 that is currently there.
Since the data are divided by the amount of Sr-86, the initial amount of Sr-87 is cancelled out in the analysis.
Most likely, the effect will be dependent on the age.
I would think that the older the sample, the larger the overestimate.
Since a neutron has no charge, it must become positively charged after emitting an electron. Of course, there are all sorts of uncertainties involved.
How much Sr-87 was in the rock when it first formed?
As I have stated previously, we just don’t know a lot about radioactive decay.