nuclear reactions of other types, such as neutron absorption).

For radiogenic isotopes that decay slowly enough, or that are stable isotopes, a primordial fraction is always present, since all sufficiently long-lived and stable isotopes do in fact naturally occur primordially. An additional fraction of some of these isotopes may also occur radiogenically.

Lead is perhaps the best example of a partly radiogenic substance, as all four of its stable isotopes

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