Lead is perhaps the best example of a partly radiogenic substance, as all four of its stable isotopes (204Pb, 206Pb, 207Pb, and 208Pb) are present primordially, in known and fixed ratios.
However, 204Pb is only present primordially, while the other three isotopes may also occur as radiogenic decay products of uranium and thorium.
Some nitrogen-14 is radiogenic, coming from the decay of carbon-14 (half-life around 5700 years), but the carbon-14 was formed some time earlier from nitrogen-14 by the action of cosmic rays.
Helium-3 is almost entirely primordial (a small amount is formed by natural nuclear reactions in the crust).
Such nuclides are formed in supernovas, but are known as extinct radionuclides, since they are not seen directly on the Earth today.
It is found in meteorites that condensed from the primordial Solar System dust cloud and trapped primordial iodine-129 (half life 15.7 million years) sometime in a relative short period (probably less than 20 million years) between the iodine-129's creation in a supernova, and the formation of the Solar System by condensation of this dust.
Units used in this tableGyr = gigayear = 109 yearsMyr = megayear = 106 yearskyr = kiloyear = 103 years Radiogenic heating occurs as a result of the release of heat energy from radioactive decay[4] during the production of radiogenic nuclides.