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Copyright  Dennis Mitton


Radiometric Dating

Reotgen Hand X-Rays

Ms. Roentgen’s hand

As the nineteenth century rolled into the twentieth, just a decade after Roentgen photographed his wife’s hand and wedding ring using x-rays, radiochemists Boltwood and Rutherford began work on a novel idea: if a rock contains some amount of a radioactive element, and if we know the rate of decay, and if we have some knowledge of the original composition, then we should be able to calculate the age of said rock based on the amount of daughter product present. Whew. They were correct, of course, and their idea, called radiometric dating, is the standard method of dating artifacts older than a couple of thousand years old. But that was one long sentence. Let’s parse it down.

Radiometric dating relies on what is called the half-life of a radioactive element. The half-life is an unchanging statistical property of a radioactive element. Radioactive elements are those that exist in an unstable state compared to their more relaxed non-radioactive brethren. You can’t tell from looking, but within a piece of uranium there is energy wanting to escape. And as with all natural things, the rock seeks a ground state – a place where there is no more energy to lose. In the case of uranium, this occurs via radiation and transformation to lead. By releasing energy, uranium slowly transforms its fundamental nature to become another element. This is something akin to a red-hot fireplace poker releasing heat to return to its ground state where the metal is cool.

We can’t know for sure when the next emission will occur – radioactive emissions are random – but we can reliably know how long it takes for all or half of the ‘parent’ element to transform into the ‘daughter’ element. Some half-lives are exceptionally long: the half-life for uranium-235 is about 700 million years. Many half-lives are easily measurable under normal methods such as iodine-131 at about 8 days. Many, too, are only seconds long. An isotope’s half-life appears to be one of the most stable elemental characteristics in all of nature. No natural process that we know of changes it.

So, if we know the amount of original material and the amount of presently existing daughter product we can, using simple algebra and the half-life, determine the age of the piece. A common confounding question asks how we know what the make-up of the original rock was – after all we weren’t there a million years ago to measure it. But we don’t have to due to what is called the diffusion of isotopes in molten rock. Simply put, daughter- products (what the parent-product turns into via half-life)  turns to gas in molten lava and escape the substrate at different rates than do the parent-products,  effectively setting the geoclock back to zero.

Rb-Sr Isochron

Example of a simple isochron.

Because of the inherent variables in dating, researches often use several isotopic dating schemes as a cross check for their samples. Analyzing several samples from the same rock bed helps by ensuring that the rock is homogeneous. A single rock or sample can be checked using different instruments to ensure that they reliably reveal the same results. When the original composition cannot be confidently known, an isochron is developed by plotting the ratios the amount of parent vs. daughter present. Each mineral’s putative age in plotted on an X-Y graph. The result should be linear with the slope representing the sample’s age. (Go here for a good explanation with graphics.)

A few facts:

A Rule of Thumb

A rule of thumb is that radioactive species are generally considered decayed and stable after seven half-lives. (If you begin with a dollar and divide by two seven times you end up with less than a penny…) This is why C-14, with its relatively short half-life, is only used to date artifacts up to about 30,000 years old.

Isotopes

There are dozens of isotopes used for dating. Each has its own useful range and it’s common to combine methods as a cross-check. See here at Wiki for an overview of radiometric dating and a listing of useful isotopes.

Sedimentary Rock

Dating sedimentary rocks reveals the age of the constituent components but not necessarily when the sediment is laid down.

The Age of The Earth

The oldest rocks on the earth that we know of date to just older than four billion years old. The moon appears to be slightly older, and we have meteorites that are older than the earth.

Fossils

Fossils are most commonly found in sedimentary rock making it difficult to provide absolute dates for them individually. Use of the geologic column flanked by dating helps put them into a geologic context.

Fossil_Fern

 

Something To Look Out For

When cruising through the web, looking for information about dating, you’ll assuredly come across Young-Earth Creationist website who play fast and loose with radiometric dating. More than once, I’ve seen a date based on C-14 dating rather than on an appropriate isotope that is just plain goofy. Results from the same sample can be enormously scattered and might show dates from a billion years old to two weeks old. This is because all parent-daughter radioisotope pairs work for a very precise date range. As stated above, radioactive material is considered decayed and stable after seven half-lives. An isotope that is fully decayed after about 40,000 years (C-14) cannot be used to date a fragment that is millions of years old.  And carbon, because of its relationship to living things, is normally used only for fossil plant and animals, not minerals. Any radiation technologists knows this which is why I’m especially suspect when I see it written up as such on Creationist websites.

Cheers!

See here at Wiki for an excellent overview of radiometric dating.

How half-lives of long-lived isotopes are calculated?  See here at the Health Physics Society.

Educational dating page at the Smithsonian here.


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