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Radiometric dating is based on heavier elements decaying into lighter elements, for example uranium decaying to lead. But wasn't all uranium in the Solar System created in one or more supernova? And floated around in space for millions or billions of years before accreting to form the Earth and its meteorites.

How come the isotopes reflect the time of the accretion of the Earth? How come the accretion affected the ratios of different isotopes, changed the nuclei of atoms? Why doesn't radiometric dating just reflect the age of the supernova in which the isotopes were once created by high energy nuclear fusion processes?

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    $\begingroup$ Because clocks are reset when rock solidifies. See also Wikipedia on radiometric dating. $\endgroup$ – gerrit Feb 17 '16 at 10:28
  • $\begingroup$ @gerrit I can imagine that C14 (in CO2) or Argon isotopes are created by cosmic rays in the upper atmosphere and has an equilibrium concentration there. When trapped inside a rock as it cooled from lava, the clock of decay starts to change that concentration. But what about uranium which is not found in the atmosphere? Why would uranium start to decay (or form) differently when rocks solidify? $\endgroup$ – LocalFluff Feb 17 '16 at 10:44
  • $\begingroup$ I don't know the details. If I did, I would post an answer… $\endgroup$ – gerrit Feb 17 '16 at 14:41
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Suppose you purify a sample of uranium 238, removing all of the lead. Half of those U-238 atoms will have decayed into lead 4.468 billion years from now. If the lead is removed from that half-uranium/half-lead sample 4.468 billion years from now, half of the U-238 atoms in that purified sample will have decayed into lead in yet another 4.468 billion years. The clock is reset when the decay products are removed. It doesn't matter whether those U-238 atoms were created in one of the very first supernova over ten billion years ago, a supernova shortly before the Earth formed 4.6 billion years ago, or even a supernova that occurred after the Earth formed. Those U-238 atoms do not have a memory.

Some natural processes do this purification and reset the clock when a rock first forms as a solid object. I'll address two; there are others. Zircons readily bond with uranium and thorium but strongly reject lead. Any lead present in zircons now is a result of the radioactive decay of uranium and/or thorium encapsulated within the zircon when it first formed. Potassium-40 sometimes decays into argon-40. The argon simply bubbles out If that decay happens in molten rock. They argon remains trapped if the decay happens inside a solidified crystal. Any argon present in a rock crystal now is a result of the radioactive decay. In both cases, (zircons containing uranium-238 and rock crystals containing potassium-40), the clock was reset when the rock first formed.

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  • $\begingroup$ "The clock is reset when the decay products are removed." Thank you! I think I've seen the light now. Nuclear decay does change also the chemical properties, because atomic nuclei lose not only neutrons but also protons (by alpha decay I suppose) and thus electrons. Chemistry doesn't change when deuterium or C14 decay, which are so commonly referred to in dating, because they only lose neutrons and that's why I haven't realized this connection between nuclear physics and chemistry before. $\endgroup$ – LocalFluff Feb 17 '16 at 22:09
  • $\begingroup$ On neat aspect of using zircon crystals for radiometric dating is that they include uranium-238, uranium-235, and thorium each of which decays into unique lead isotopes. This allows a kind of cross-checking ability to confirm the dating is accurate; if the amounts of each lead isotope agree with the dating results, you have a high level of confidence in the accuracy. $\endgroup$ – BillDOe Feb 17 '16 at 22:59

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