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As I understand it, radioactive dating measures time by what portion of a radioactive isotope has decayed. Weren't all the natural radioactive isotopes created during the formation of the solar system? So how does radioactive dating age a specific rock, instead of just the number of years since the formation of the solar system?

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Weren't all the natural radioactive isotopes created during the formation of the solar system?

The half life of carbon 14 is 5730 years, orders of magnitude less than the age of the solar system. Carbon 14 is constantly being created in the upper atmosphere by neutron bombardment of nitrogen 14. On the other hand, the other isotopes used in radioactive dating were created in the end stages of the life of a star, well before the solar system formed. The formation of the solar system did not create radioactive isotopes.

So how does radioactive dating age a specific rock, instead of just the number of years since the formation of the solar system?

Carbon dating is perhaps the easiest to understand because this technique only looks at the ratio of carbon 14 to the stable isotopes of carbon. Plants take up carbon 14, along with carbon 12 and carbon 13, as they grow. Herbivores eat plants, and carnivores eat herbivore. The carbon 14 decays, leaving less and less of it as time passes.

One problem with carbon dating is that the amount of carbon 14 in the atmosphere is not constant. Fortunately, there are trees that are almost 5000 years old; the varying amounts carbon 14 captured in their rings provides a direct calibration. Humans have used wood for construction for a long, long time, enabling calibrations to go back even further in time. Even older chunks of wood enable calibrations to go back 50000 years.

Another form of radioactive dating is uranium-lead dating. This relies on the very different chemistries of uranium and lead. Lead does not combine chemically nearly as readily as does uranium with other elements. Uranium's chemistry is similar to that of zirconium, enabling uranium to replace zirconium in zircons. Lead is instead chemically rejected when zircons form. This means zircons have essentially no lead content when they form. The lead content in a zircon indicates how long ago the zircon formed.

Other forms of radioactive dating rely on similar concepts. The different chemistries of parent and daughter isotopes is key in many of those techniques.

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There are other, mostly chemical processes which alter the isotope ratios. Isotope dating uses a combination of them. This is why it can not be used always, for any radioactive isotopes, only in special circumstances. It also requires very sensitive measurements (below in the second example, close to induvidually count lead atoms in microscopic zircon crystals).

For example, in carbon dating, C14 is continuously generated in the atmosphere due to the cosmic radiation, and builds in the biosphere, where it decays. If a plant stops accumulating carbon ( = it dies), its C14 ratio starts to decay.

Another example, this time about isotopes which were created really with the Solar System. The mineral zircon ($\rm{ZrSiO_4}$) accumulates uranium and thorium atoms in it crystal structure, but strongly rejects lead. This rejection can happen as it freezes. After it is solid, the lead atoms created by U/Th decay do not go away any more. Thus, all the lead in the zircon crystals of the soil of Moon, were created since it frozen. This makes possible to know, when did that zircon solidify ($\approx$ when formed the crust of the Moon).

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  • $\begingroup$ Would the phrase cools and solidifies be more appropriate than the word freezes when describing what happens with zircon? $\endgroup$ – Fred Oct 9 '20 at 21:58
  • $\begingroup$ @Fred I tried to say when its temperature decreases below its melting point, making the object solid (crystallize). Only "cooling" would miss the content, that the cooling should start from a liquid phase and in solid phase. Thank you very much the fixes! $\endgroup$ – peterh Oct 10 '20 at 13:49
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Radioactive dating works on specific isotopes we use for specific time frames. Rubidium–strontium dating methods (because this substance has a half life of 50 billion years) to date extremely old geological samples as well as space samples like lunar rocks.

Another issue is the quantity of synthetic isotopes in varying samples. Alot of these isotopes do not occur in nature or don't occur in any sufficient amount capable of being used. Thus the only sources are manmade radioactive isotopes; such as the atomic bomb blasts such as Plutonium-238, Americium-241, etc, thus any high concentration found in an artifact or rock/soil is indicitive of atomic tests, steel/iron contaminated with these nuclides are a tell tale sign of any steel/alloy made after 1945. radioactive trace elements in teeth, bones and remains also let us know if a person even deceased was born Before or after Post WWII.

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