Geochemistry isotopic equilibrium

For isotope exchange reactions in geochemistry, the equilibrium constant K is often replaced by the fractionation factor 6c. The fractionation factor is defined as the ratio of the numbers of any two isotopes in one chemical compound A divided by the corresponding ratio for another chemical compound B ... 

Thermodynamic energy terms (and equilibrium constants) may differ for compounds containing different isotopic species of an element. This effect is described in theoretical detail by Urey (1947), and applications to geochemistry are discussed by Broecker and Oversby (1971) and Faure (1977). A good example is the case of the vapor/liquid equilibrium for water. The vapor pressure of a lighter isotopic species, H2 0, is higher relative to that of heavier species, (or HD O), and others. 

Despite the vast amount of work on 14C dating which has already been accomplished and despite the fact that it is the best developed method available today, numerous difficulties still exist with its application. First, carbonate geochemistry which helped control 14C concentrations in the past is not simple to reconstruct. Carbonate minerals are commonly in a state of near equilibrium with groundwater, and only slight changes in water temperature or chemistry will promote either dissolution or precipitation of carbonate ions. In this way, the proportion of modern carbon in the water can be changed and some isotope... 

Of special interest in stable isotope geochemistry are evaporation-condensation processes, because differences in the vapour pressures of isotopic compounds lead to significant isotope fractionations. For example, from the vapour pressure data for water given in Table 1.2, it is evident that the lighter molecnlar species are preferentially enriched in the vaponr phase, the extent depending upon the temperature. Such an isotopic separation process can be treated theoretically in terms of fractional distillation or condensation under equilibrium conditions as is expressed by the Rayleigh (1896) equation. For a condensation process, this equation is... 

Chacko, T., Cole, D.R., and Horita, J. (2001) Equilibrium oxygen, hydrogen and carbon isotope fractionation factors applicable to geologic systems. In Reviews in Mineralogy and Geochemistry Stable Isotope Geochemistry (Valley, J.W., and Cole, D.R., eds.), Vol. 43, pp. 1-81, Mineralogical Society of America, Chantilly, VA. 

There were several key observations, which were made almost around the same time, that led to studies in the fields of stable isotope geochemistry and cosmochemistry. First, it was the physicochemical formalism for isotope effects, particularly the determination of the position of equilibrium in isotopic exchange reactions. Urey (1947) and Bigeleisen and Mayer (1947) demonstrated that the position of isotope exchange in a chemical reaction may be calculated with high precision. The difference in chemical behavior for... 

Taylor BE, O Neil JR (1977) Stable isotope studies of metasomatic Ca-Fe-Al-Si skams and associated metamorphic and igneous rocks, Osgood Mts, Nevada. Contrib Mineral Petrol 63 1-49 Taylor BE, Bucher-Nurminen K (1986) Oxygen and carbon isotope and cation geochemistry of metasomatic carbonates and fluids-Bergell aureole, N. Italy. Geochim Cosmochim Acta 50 1267-1279 Thompson JB Jr (1959) Local equilibrium in metasomatic process. In Abelson PH (ed) Res Geochem 1 427-457... 

---