Uranium lead dating meteorites
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.
Also, because Earth formed as part of our sun’s family of planets – our solar system – scientists use radiometric dating to determine the ages of extraterrestrial objects, such as meteorites.
These are space rocks that once orbited our sun, but later entered Earth’s atmosphere and struck our world’s surface.
You’ve seen these rock layers if you’ve ever observed a cut-away section of a mountain, perhaps because a highway runs through it. In the early part of the 20th century, scientists still weren’t sure.
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.
He concluded that Earth was born 20 to 400 million years ago.
Today’s scientists believe that answer is incorrect, but Kelvin’s calculations were in being based on logical thinking and mathematical calculation.