The open-system behaviour of Pb, if true, would have dramatic consequences Anyway, to me it seems unlikely that this chain of events would occur. Obviously, then, the minimum error in that measurement is 1. A pioneering investigation into the crystallization of magma was carried out by N. Chemical Geology. Lead has a low melting point, so it will melt early and enter the magma. Dergachev
Since you are exposed to the atmosphere and contain carbon, if you get oils from your skin onto an archeological artifact, then attempting to date it using radio carbon will fail because you are measuring the age of the oils on your skin, not the age of the artifact.
Two extensive studies done more than 25 years ago involved analyzing the isotopic composition of argon in such flows to determine if the source of the argon was atmospheric, as must be assumed in K-Ar dating Dalrymple26 flows; Krummenacher19 flows. These include the assumption that decay rates have never changed. Older materials can be dated using zirconapatitetitaniteepidote and garnet which have a variable amount of uranium content. Then the surrounding crystal surface would continue to grow and close up the gap, incorporating a tiny amount of magma. I believe that the above shows the 3 source mixing to be natural and likely.