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

Gavin Cato: 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?

Answers and Views:

Answer by georock1959
Tree rings going back 100,000 years? I think you should check your source for that information.

C-14 has a half life of about 5700 years, so the oldest that an object can be, to be reliably dated, using radiocarbon methods is about 50,000 years.

To determine the amount of original parent radioisotope in a sample when that particular radioisotope was “locked in” is calculated by determining the amount of parent radiosisotope and all of its daughter products by mass spectrometry. Add them up, and you’ve got the original amount before the radioactive decay process started.

The half life of a particular radioisotope (such as U235, C14, K40 etc) can be calculated by measuring the amount of energy emitted by beta particles, gamma rays, and especially alpha particles (helium nucelii) released during the radioactive decay process. If you can measure the amount of alpha particles being emitted from a known quantity of a radioisotope, then its a relatively straightforward mathematical calculation using the decay constant (k) to determine the half life, with the amount of energy released as alpha particles being inverse to the half life.

Radiometric dating is a perfectly sound concept of physics, and is difficult to argue against.

Answer by scooter
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.