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I am reading studies about the age of the earth and U-238 to Pb-206 comes up a lot.
My question is:
In meteorites, or really anywhere, is lead 206 natural or does it always come from radioactive decay?
If it is natural is the potential quantity of that daughter isotope capable of skewing the results and predicted age? So if there was already some of that isotope there it would seem as if it had been decaying for longer and therefore thought to be older?
While lead 206 does occur naturally, unless a zircon (a zirconium silicate crystal) is contaminated with lead or has been around a long time, it will contain no lead. Zirconium, uranium, and thorium have similar chemistries. Lead has a dramatically distinct chemistry. The chemical reactions that form zircons can accommodate uranium or thorium replacing zirconium, but not lead.
Assuming no contamination, all of the lead in a zircon could only have resulted from radioactive decay. This is an overly strong assumption. Researchers do have to address the issue of possible contamination. But the basic concept remains: Zircons cannot accommodate lead.
The four stable isotopes of lead and their relative abundance on Earth are: 204Pb (1.4%), 206Pb (24.1%), 207Pb (22.1%) and 208Pb (52.4%). Lead-204 is a primordial nuclide and is not a radiogenic nuclide. 206Pb, 207Pb, and 208Pb form as part of the radioactive decay chains of Uranium and Thorium and they also occur as primordial nuclides that were made in supernovae. As David Hammen points out, uranium-lead dating is done with zircons, and they reject the inclusion of lead when forming. So, usually any lead in a zircon is from radioactive decay. Zircon is commonly found in igneous rocks, metamorphic rocks and sedimentary rocks.
