All the unprovable assumptions ultimately depend on an assumed deep time history.Its rejection is recognized as fatal to the earth’s claimed age of billions of years.
He showed that there is still some uncertainty in what the values for these measures of the Rb decay rate differ when Rb-Sr ages are calibrated against the U-Pb ages of either the same terrestrial minerals and rocks or the same meteorites and lunar rocks.
Ironically it is the slow decay rates of isotopes such as Sm used for deep time dating that makes precise measurements of their decay rates so difficult.
Nor can the measured Pb isotope ratios be used to somehow decide what proportions of them are the initial Pb without recourse to unprovable assumptions about the mineral or rock’s history or their interpreted U-Th-Pb ages within an assumed deep time history.
Nevertheless, the ultimate foundation of this U-Pb dating methodology is the assumption that the earth formed from the solar nebula.
Once radioactive decay of U and Th started after creation, daughter Pb isotopes were added inside the earth.