Why don't we use radio carbon dating for dinosaur bones?    

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A radioisotope will decay at a fixed rate as a value known as its half-life. In this case, the parent isotope will eventually decay and form a daughter isotope. It does this by losing nuclear particles over time. Radioisotopes are unstable but they eventually decay into a stable isotope. By observing the ratio of isotopes in a fossil, the number of half-lives can be calculated and thus the approximate age of the fossil determined.

Dinosaurs went extinct 65 million years ago. To determine the age of a dinosaur fossil, carbon dating can never be used. Carbon-14 only works for fossils less than 75,000 years old. By the time a dinosaur fossil was found, any carbon-14 the organism would have taken up and incorporated into its tissues during its life, would have decayed too much to be useful as a tool to determine its age. Carbon-14 has a half life of 5730 years and this radioisotope eventually decays into Nitrogen-14. 

However, the isotope uranium-235 has a half-life of 704 million years. It decays to lead-207. Therefore, in a dinosaur fossil, only a small fraction of U-235 would have decayed and the age of the fossil could be determined by the ratio of U-235 to Pb 207 found in the specimen.

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