Radioactive elements are unstable; they breakdown spontaneously into more stable atoms over time, a process known as radioactive decay.Radioactive decay occurs at a constant rate, specific to each radioactive isotope.Lead isotope geochemistry is useful for providing isotopic dates on a variety of materials.Because the lead isotopes are created by decay of different transuranic elements, the ratios of the four lead isotopes to one another can be very useful in tracking the source of melts in igneous rocks, the source of sediments and even the origin of people via isotopic fingerprinting of their teeth, skin and bones.Hence, during melting of the mantle, rhenium is stripped out, and prevents the osmium–osmium ratio from changing appreciably.This locks in an initial osmium ratio of the sample at the time of the melting event.During photosynthesis, organisms using the C Nitrogen ratios are frequently linked to agricultural activities.Nitrogen isotope data has also been used to measure the amount of exchange of air between the stratosphere and troposphere using data from the greenhouse gas N They are particularly useful to understand mixing processes between different components, because (heavy) radiogenic isotope ratios are not usually fractionated by chemical processes.
An example of this application is to the evolution of the Earth's crust and Earth's mantle through geological time.
Because of their unique properties, it is useful to distinguish them from the conventional radiogenic isotope systems described above. Some He is not affecting the concentration or noble gas ratios of the mantle.
Helium-3 is created by cosmic ray bombardment, and by lithium spallation reactions which generally occur in the crust.
Rhenium and osmium are siderophile elements which are present at very low abundances in the crust.
Rhenium undergoes radioactive decay to produce osmium.