The decay of Pb, respectively, forms the basis for one of the oldest methods of geochronology (Dickin 2005; Faure and Mensing 2005).
While the earliest studies focused on uraninite (an uncommon mineral in igneous rocks), there has been intensive and continuous effort over the past five decades in U-Pb dating of more-commonly occurring trace minerals.
There is also primordial Pb that the earth acquired when it formed, its isotopic composition determined as that of troilite in the Canyon Diablo iron meteorite.
Subsequently new crustal rocks formed via partial melts from the mantle.
Once radioactive decay of U and Th started after creation, daughter Pb isotopes were added inside the earth.
Then catastrophic plate tectonics during the Flood stirred the mantle and via partial melting added new rocks to the crust.
Various methods have been devised to determine this initial or common Pb, but all involve making unprovable assumptions.
Zircon does incorporate initial Pb when it crystallizes. It cannot be proven that the Pb in apparently cogenetic U- or Th-free minerals is only initial Pb, and that it is identical to the initial Pb in the mineral being dated.
Many in both the scientific community and the general public around the world thus remain convinced of the earth’s claimed great antiquity.U-Pb radioisotope dating is now the absolute dating method of first choice among geochronologists, especially using the mineral zircon.A variety of analytical instruments have also now been developed using different micro-sampling techniques coupled with mass spectrometers, thus enabling wide usage of U-Pb radioisotope dating.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.