resistance is futile, you will be assimilated

[SubtleVirtue]

if most of the tests are based on 'stardust' isotopes... how does that date the rocks in an eruption?

The radiometric "clocks" in a mineral don't start "ticking" until the mineral forms. And they don't actually start "ticking" until they cool below something called the "closure temperature", which can be a problem if the rock gets heated up again and ceases being "closed". Put simply, we're measuring two important quantities when we date any mineral: abundance of parent isotope and abundance of the daughter isotope. So how much 235-U and how much 207-Pb? Above the closure temperature the system behaves as if it were "open" and the daughter product diffuses out of the mineral, resetting the age to zero. So yes, the primordial isotopes do decay before the mineral ever forms, but until the mineral starts accumulating the daughter product the radiometric clock doesn't start ticking.

OK, that isn't quite true, you can use the decay of radiometric systems in an open system along with some interesting assumptions to produce something called a "model age", but that's a whole other can of worms. No point in going down the rabbit hole of Sm-Nd systematics and TDM just yet.

These minerals form from the cooling of magma?!

Are you claiming that heated atoms don't radioactively decay?

Can you prove that?