Saturday, 4 February 2017

New Research Spots a Key Flaw in Radioisotope Dating Methods

Younger that previously assumed? Image courtesy of Meteorite Recon, Creative Commons (CC BY-SA 3.0).

Joel Kontinen

New research published in the journal Nuclear Technology challenges a basic assumption of radiometric dating. Science Daily gives a brief overview of the problem:

An oversight in a radioisotope dating technique used to date everything from meteorites to geologic samples means that scientists have likely overestimated the age of many samples.”

The article describes the issues involved in rubidium-strontium dating. Rubidium-87 decays to Strontium-87 over time, so the amounts of isotopes in a rock sample or other objects should make it possible for scientists to calculate the age of the sample.

This should be more or less straight-forward. In theory, it is. But in practice, it’s a bit more complicated:

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.

Associate Professor Robert Hayes, the author of the paper, says that different types of rocks will give differing results and things like “the number of cracks and amount of surface area” will also have an effect on the rate of diffusion, and hence the assumed age.

This is not the first time that researchers doubt the accuracy of radiometric methods. Several other papers have likewise cast doubt on the reliability of dating techniques. See details here, here, here and here.


North Carolina State University. 2017.Paper spotlights key flaw in widely used radioisotope dating technique. Science Daily. (31 January).