Radiometric isochron dating
A plot is constructed of Pb-206/Pb-204 versus Pb-207/Pb-204.
If the solar system formed from a common pool of matter, which was uniformly distributed in terms of Pb isotope ratios, then the initial plots for all objects from that pool of matter would fall on a single point.
For example, Henry Morris says: He lead to similar results, i.e., a rate virtually identical to the estimated production flux.
Another possible escape mechanism is direct interaction of the solar wind with the upper atmosphere during the short periods of lower magnetic-field intensity while the field is reversing.
(I believe this argument was originally put forth by Mormon young-Earther Melvin Cook, in a letter to the editor which was published in .) But helium can and does escape from the atmosphere, at rates calculated to be nearly identical to rates of production.
In order to obtain a young age from their calculations, young-Earthers handwave away mechanisms by which helium can escape.
Those which appear the most frequently in talk.origins are reproduced below: Note that these aren't necessarily the "best" or most difficult to refute of young-Earth arguments.
The current level of helium in the atmosphere would accumulate in less than two hundred thousand years, therefore the Earth is young.
) and they are historically the ones posted to talk.origins more than any others.
The young-Earth argument goes something like this: helium-4 is created by radioactive decay (alpha particles are helium nuclei) and is constantly added to the atmosphere.
And from the slope of the line we can compute the amount of time which has passed since the pool of matter became separated into individual objects.
See the Isochron Dating FAQ or Faure (1986, chapter 18) for technical detail.