When considering stable isotopes, it is worth bearing in mind that most naturally occurring nuclides are themselves actually stable. Only a small number, of the order of 34, are known to be radioactive with sufficiently long half lives that they are ‘primordial’ and hence still available on the Earth. Many of these have such long half lives (e.g. 209Bi, 4.6 × 1019 yr; 180W, 1.8 × 1018 yr – whereas the age of the universe is only ~13.7 × 109 yr!) that they are for all intents and purposes stable. Hence, of all the natural nuclides we observe on the earth, we are left with only a small number which are effectively radioactive and of use in the geosciences (see Chapter 5). The result of this is that stable isotopes comprise the vast majority of systems we have available to interrogate.

As described in detail in Section 2.2, a range of processes are capable of fractionating isotopes from one another. Stable isotope fractionation effects can take place on isotopes of any element (except of course if an element is monotopic (or mono-isotopic), such as F, Na, P, Au, etc.); however, stable isotope fractionation effects are most clearly developed in the light elements. This is because the relative mass difference is greater between the isotopes of light elements than it is between heavy elements. Remember that both equilibrium and kinetic fractionation are controlled by relative masses between species displaying otherwise identical chemistry. Therefore the greater the relative difference in masses, the greater the fractionation. Hence there will be greater fractionation between 18O and 16O in reaction than between 17O and 16O. In contrast, although there is still a 2 amu difference between 142Nd and 144Nd, the relative mass difference is only ~0.5 per cent (compared with ~13 per cent between 18O and 16O), and hence the stable isotopic fractionation due to kinetic and equilibrium processes is correspondingly smaller.

Until relatively recently, it was generally assumed that mass fractionation for isotopes heavier than ~80 amu (i.e. isotopes of Sr) was effectively non-existent. This assumption still plays an important role in many systems of radiogenic isotopic measurement, since the fractionations are indeed minuscule relative to the radiogenic isotopic effects.