Physical Geology 2002

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  • Types of Radiometric Dating

 

Common Types of Radiometric Dating

Carbon 14 Dating

As shown in the diagram above, the radioactive isotope carbon-14 originates in the Earth's atmosphere, is distributed among the living organisms on the surface, and ceases to replenish itself within an organism after that organism is dead. This means that lifeless organic matter is effectively a closed system, since no carbon-14 enters the organism after death, an occurrence that would affect accurate measurements. In radiometric dating, the decaying matter is called the parent isotope and the stable outcome of the decay is called the daughter product.

Since the half-life of carbon-14 is 5730 years, scientists can measure the age of a sample by determining how many times its original carbon-14 amount has been cut in half since the death of the organism. For example, an object with a quarter of its original amount (2x1/2) should be roughly 11,460 years old.

In all radiometric procedures there is a specific age range for when a technique can be used. If there is too much daughter product(in this case nitrogen-14), age is hard to determine since the half-life does not make up a significant percentage of the material's age. The range of practical use for carbon-14 dating is roughly a few hundred years to fifty thousand years.

The equation (called the 'age equation') below shows the relationship of parent/daughter atoms to half-lives in all types of radiometric dating:

Potassium-Argon Dating

The isotope potassium-40 (k-40) decays into a fixed ratio of calcium and argon (88.8 percent calcium, 11.2 percent argon). Since argon is a noble gas, it would have escaped the rock-formation process, and therefore any argon in a rock sample should have been formed as a result of k-40 decay. The half-life of this process is 1.25 billion years, meaning that it can date significantly older samples.

Rubidium-Strontium Dating

In rubidium-strontium dating a rubidium-87 isotope becomes the daughter product strontium-87. In an igneous rock formation, the entirety of the cooled rock will have the same ratio of strontium-87 and strontium-86 (another stable isotope). This means that as the rubidium-87 decays and more strontium-87 is formed, the ratio will change. The half-life of rubidium-87 is 48.8 billion years, meaning it can accurately measure rocks as old as the Earth itself.

Uranium-Lead Dating

Uranium-lead dating is one of the most complicated of all dating techniques. This is in part because uranium and lead are not retained in rocks as easily as some others, and in part because the parent isotopes and daughter products are not even directly related. For the isotopes uranium-235 and uranium-238 to respectively become lead-207 and lead-206, they must first undergo a serious of highly unstable transformations into isotopes with very short half-lives. However, if one knows the scientific formula for interpreting these transitions, the results can be "highly precise" according to paleontologist Guy Narbonne (Kerr, 789). The half-life of uranium-235 is 704 million years, while the half-life of uranium-238 is 4.5 billion years.

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Earlham · Geosciences Department · Geociences 211: Physical Geology

Copyright ©-2001 Earlham College. Revised 25 February 2002. Send corrections or comments to parkero@earlham.edu