This first time of shaking represents one half life, and all those pieces of candy that have the printed M facing up represent a change to the daughter isotope. U is the parent isotope of Pb, which is the daughter isotope. Accurate radiometric dating generally requires that the parent has a long enough half-life that it will be present in significant amounts at the time of measurement except as described below under "Dating with short-lived extinct radionuclides"the half-life of the parent is accurately known, and enough of the daughter product is produced to be accurately measured and distinguished from the initial amount of the daughter present in the material. Using the same reasoning about proportions as in Part 2b above, students can determine how old the pegmatite and the granite are. This scheme has application over a wide range of geologic dates.
Zircon also forms multiple crystal layers during metamorphic events, which each may record an isotopic age of the event.
In these cases, usually the half-life of interest in radiometric dating is the longest one in the chain, which is the rate-limiting factor in the ultimate transformation of the radioactive nuclide into its stable daughter. This predictability allows the relative abundances of related nuclides to be used as a clock to measure the time from the incorporation of the original nuclides into a material to the present. Therefore, the slate that contains the acritarch and bacteria is between million years and million years old, because the pegmatite is million years old and the granite is million years old. After the graphs are plotted, the teacher should guide the class into thinking about: The above equation makes use of information on the composition of parent and daughter isotopes at the time the material being tested cooled below its closure temperature.