What assumptions are necesssary for radiometric dating to be useful and accurate?

Question

Radiometric dating is used to give absolute dates for various substances. What assumptions are necessary for this to be useful and reasonably accurate?

Answer 1

There's not a simple answer to that question. Radiometric dating encompasses a wide range of techniques, with varying strengths and weaknesses -- and widely varying assumptions.

For example, in carbon dating (discussed by a previous answerer), the dated creature is assumed to have the atmospheric C14/C12 ratio at the time the dated part was alive. This assumption may be weak in some cases, strong in others, depending on external evidence.

Isochron dating, in contrast, makes no assumption about initial parent and daughter concentrations. See this link for a lengthy discussion:
http://www.talkorigins.org/faqs/isochron-dating.html
However, it involves other requirements (such as having multiple samples from a single object with a good spread of parent-to-daughter initial ratios, and having the multiple samples come from an isotopically homogeneous source).

Further, there's a bit of a fuzziness around the term "assumption," which is sometimes abused by people trying to score debating points in a less than honest manner. Consider the statement: radioactive decay rates are constant. That's a requirement for radiometric dating to work properly. But is it an "assumption" -- as if it were a position held without any evidence for or against? In the light from distant supernovae, and many other direct and indirect means, we can assess potential change over time to decay rates. These assessments find zero change to within measurement accuracy (about one part in 10,000,000,000 per year). So, it is a mere assumption that radioactive decay rates relevant to geologic dating are constant?

See this portion of the Age of the Earth FAQ for more discussion on constancy of decay rates:
http://www.talkorigins.org/faqs/faq-age-of-earth.html#constant

Answer 2

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Answer 3

The ratio of C-12 and C-14 is set. When an animal or plant is alive it takes in carbon(CO2 for plants, food for animals). When it dies it stops taking in carbon. The radioactive decay of C-14 continues at its own rate, half life, so the ratio of C-12 to C-14 changes. This is how the age of bones or burned wood in a camp fire can be determined exactly.