After one half-life has elapsed, one half of the atoms of the nuclide in question will have decayed into a "daughter" nuclide, or decay product.
The results have now been published in Geology, the journal of the Geological Society of America.
Radiometric dating, often called radioactive dating, is a technique used to determine the age of materials such as rocks.
It is based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates.
However, such methods are far from perfect: it is difficult to gain accurate depositional ages for sedimentary rocks, and matters can be further complicated when millions of years of geologic and environmental forces cause erosion of fossil-bearing strata.
Fossils can even migrate from their original positions within strata.