How do we know the initial parent to daughter isotope ratio in radiometric dating?

[blah]

You can date a rock by measuring the ratio of a parent isotope and its daughter. But you can only determine when the rock was formed if you know the ratio of parent to daughter isotopes when it was formed. So how is that ratio known?

 

[standing lenticular]

you assume that all daughter product is radiogenic, formed from radioactive decay process.

 

[TRCSF]

Take potassium-argon dating. Potassium 40 decays into argon 40, a gas, which can diffuse through lava and escape. Newly formed igneous rocks has essentially no argon 40. It's only when the rock solidifies that the argon becomes trapped and begins to accumulate.