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How do scientists calibrate radiometric dating? How do they get original amount of parent isotope?

Carbon 14 dating can be calibrated with tree rings and that can go back about 100,000 years. For other methods like Potassium Argon or Uranium, rocks can be dated back billions of years. How is it that they can determine the original amounts of the parent and daughter isotopes?

Answer
For 40K/40Ar, the radiometric “clock”, as it were, is re-set whenever the parent- mineral is heated to a very high temperature…ordinarily, tests are conducted on volcanic extrusives, so that the target-date, for practical purposes, is actually the time of magma-cooling. Of course this provides a very convenient way for empirically establishing the relative amount of parent and daughter-isoptopes in the high-temp. petrology laboratory, as well as on samples of freshly-extruded magmas, coming soon to a volcano near you. The three isotopes, 39K, 40K and 41K occur at around 93.2581%, 0.0117% and 6.7302%, respectively (I admit that I just copped those from Wiki, though I’m sure they’re correct, since I recollect cross-checked that reference a year ago for something else). Ordinarily, micas like muscovite or other phyllosilicate minerals are employed for the analyses, as I understand it, because the geometry of the crystalline lattice is convenient for trapping the gaseous Argon. Sample selection is, of course, quite critical. The method has been refined a great deal, and independently confimed thousands of times.

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