Here is one example of an isochron, based on measurements of basaltic meteorites (in this case the resulting date is 4.4 billion years) [Basaltic1981, pg. Skeptics of old-earth geology make great hay of these examples.

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Love-hungry teenagers and archaeologists agree: dating is hard.

But while the difficulties of single life may be intractable, the challenge of determining the age of prehistoric artifacts and fossils is greatly aided by measuring certain radioactive isotopes.Until this century, relative dating was the only technique for identifying the age of a truly ancient object.By examining the object's relation to layers of deposits in the area, and by comparing the object to others found at the site, archaeologists can estimate when the object arrived at the site.There is a basic pattern that occurs in the decay of radioactive substances.In each of these disintegration systems, the parent or original radioactive substance gradually decays into daughter substances.Though still heavily used, relative dating is now augmented by several modern dating techniques.