Paper spotlights key flaw in widely used radioisotope dating technique

To conduct radioisotope dating, scientists evaluate the concentration of isotopes in a material. The number of protons in an atom determines which element it is, while the number of neutrons determines which isotope it is. For example, strontium-86 has 38 protons and 48 neutrons, whereas strontium-87 has 38 protons and 49 neutrons. Radioactive elements, such as rubidium-87 (but not strontium-86 or strontium-87), decay over time. By evaluating the concentrations of all of these isotopes in a rock sample, scientists can determine what its original make-up of strontium and rubidium were. Then, by assessing the isotope concentrations of rubidium and strontium, scientists can back-calculate to determine when the rock was formed.

The three isotopes mentioned can be used for dating rock formations and meteorites; the method typically works best on igneous rocks.

But it's not quite that straight-forward. The data from radioisotope analysis tends to be somewhat scattered. So, researchers "normalize" the data by making a ratio with strontium-86, which is stable -- meaning it doesn't decay over time.

Dividing the isotope concentrations of all the forms of strontium and rubidium by the isotope concentration of strontium-86 generates something called the "isochron." The isochron is then plugged into a model, which uses it to turn the overall radioisotope data into a clear, linear function. This function is able to tell researchers how old a sample is. Or it's supposed to.

But there's a wrinkle in the process that has been overlooked.

The ratios of strontium-86 to rubidium and strontium-87 are thought to only be influenced by the radioactive decay of the rubidium-87 into strontium-87. The current model of radioisotope dating is based on that idea.

But that model doesn't account for differential mass diffusion -- the tendency of different atoms to diffuse though a material at different rates. And atoms of strontium-86 can diffuse more readily than atoms of strontium-87 or rubidium, simply because atoms of strontium-86 are smaller.

"It's a slow process, but not necessarily a negligible one when you're talking about geological time scales," says Robert Hayes, an associate professor of nuclear engineering at NC State and author of a paper describing the work.

"The rate of diffusion will vary, based on the sample -- what type of rock it is, the number of cracks and amount of surface area, and so on," Hayes says. "So, there's not a simple equation that can be applied to every circumstance. Researchers will need to evaluate samples individually, then apply the relevant physics accordingly.

"It's a pain in the neck, but it will make our estimates significantly more accurate," Hayes says. "If we don't account for differential mass diffusion, we really have no idea how accurate a radioisotope date actually is. It's worth noting that the issues raised here do not apply to carbon dating, which does not utilize isotopic ratios."