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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.
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.
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.
Although without the ratios, the data are inherently noisy.
But that model doesn’t account for differential mass diffusion – the tendency of different atoms to diffuse though a material at different rates.By taking into consideration the isotope effect (differential mass diffusion rates) when measuring isotopic ratios from very old samples, the distribution dependency in the coefficient ratios will cause a bias if isotopic diffusion rates are not identical throughout a sample.The isotope effect being that isotopes having a smaller atomic mass will diffuse faster throughout a medium than will their heavier counterparts causing concentration gradients of their ratios even when there are no contributions from radioactive decay.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.
Then, by assessing the isotope concentrations of rubidium and strontium, scientists can back-calculate to determine when the rock was formed.