Physical and Chemical Controls on Stable Isotope Fractionation

Understanding the physical and chemical controls on stable isotope fractionation is vital to a correct interpretation of measured stable isotope ratios. When the fractionation processes are well understood, the measured δ-values can be used to correctly identify the source of the element in question and the geological processes involved. In this section, we examine briefly the chief controls on stable isotope fractionation.

Temperature Control on Isotopic Fractionation

There is an important temperature control on isotopic fractionation. This has already been described in the equation:

1000 ln αmineral1-mineral2 = A (106 / T2) + B

This equation has an obvious application in isotopic thermometry. Relative volume changes in isotopic exchange reactions, on the other hand, are very small except for hydrogen isotopes, and therefore there is a minimal pressure effect. Clayton (1981) showed that at pressures of less than 20 kb, the effect of pressure on oxygen isotope fractionation is less than 0.1‰ and lies within the measured analytical uncertainties. The absence of a significant pressure effect on stable isotope fractionation means that isotopic exchange reactions can be investigated at high pressures where reaction rates are fast, and the results can be extrapolated to lower pressures.

Kinetic Effects in Isotope Fractionation

Some isotope fractionations, notably those in biological systems, are primarily controlled by kinetic effects. For example, the bacterial reduction of seawater sulfate to sulfide proceeds 2.2% faster for the light isotope ³²S than for ³⁴S. For the reactions:

34SO42- → H2S
32SO42- → H2S

The rate constant k₁ is greater than the rate constant k₂, and the ratio k₁ / k₂ = 1.022. The effects of this fractionation in a closed system may be modeled using the Rayleigh fractionation equation. When isotopic fractionation takes place as a result of diffusion, the light isotope is enriched relative to the heavy one in the direction of transport. Diffusion-controlled isotopic fractionation can be important when interpreting the results of oxygen isotopes as thermometers. A related process to that of diffusion is the microfiltration effect in which isotopes are fractionated by adsorption onto clay minerals in sediments. It is thought that isotopically lighter hydrogen, oxygen, and sulfur may be preferentially adsorbed onto clay, leading to isotopic enrichments in formation waters (Ohmoto and Rye, 1979).

Distillation Processes

During distillation, the light isotopic species is preferentially enriched in the vapor phase according to the Rayleigh fractionation law. This process applies to the evaporation and condensation of meteoric water and accounts for the marked fractionation of δ¹⁸O and δD in rainwater and ice.

Effect of Major Element Chemistry on Isotope Fractionation

Generally, major element chemistry has a very small effect on stable isotope fractionation. However, Taylor and Epstein (1962) observed that the oxygen isotope fractionation in silicate minerals can be accounted for in terms of the average bonding arrangement of the mineral structure, and heavy isotopes associated with elements with high ionic potential. This is seen in the fractionation of δ¹⁸O between quartz and magnetite, where quartz containing the small, highly charged Si⁴⁺ is enriched in δ¹⁸O, whereas magnetite with the large Fe²⁺ ion is δ¹⁸O-deficient. In the case of feldspars, the oxygen isotope composition of plagioclase is a function of An content.

Crystallographic Control on Isotope Fractionation

Heavy isotopes are concentrated in more closely packed crystal structures. The fractionation of carbon isotopes between diamond and graphite is well known, and there are smaller changes between calcite and aragonite. There is also a small change in δ¹⁸O between α- and β-quartz.

Recently, it was discovered that there is a crystallographic control on the fractionation of oxygen and carbon isotopes in calcite. Dickson (1991) found that in a single crystal of calcite, crystal faces from different crystallographic forms have different isotopic compositions. This observation indicates that different surfaces in the same crystal have slightly different bonding characteristics, which are sufficient to fractionate the isotopes of oxygen and carbon.