Physical and chemical controls on stable isotope fractionation

3 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, for when the fractionation processes are fully understood, then the measured d-values can be used to identify correctly 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. 

There is an important temperature control on isotopic fractionation. This has already been described in Eqn [7.5] and has an obvious application in isotopic thermometry (see Sections 7.2.2., 7.4.6 and 7.5.2). 

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 

Some isotope fractionations, notably those in biological systems, are primarily controlled by kinetic effects. For example, the bacterial reduction of seawater sulphate to sulphide proceeds 2.2 % faster for the light isotope than for S. -/For the reactions 

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