At this point, with an appreciation of what an isotope actually is with respect to a chemical element and/or an ion, it is useful to investigate the processes that can cause variations in the relative proportions of the isotopes of an element. At heart, this is the key to the isotope geochemistry, as it is the changes in isotopic ratios that inform geoscientists of rates and process.

Fractionation: chemical vs isotopic

In essence, isotope geochemistry is an understanding of fractionation, a term that is used in a range of applications and situations. Fractionation is used in several confusing contexts in geochemistry, and it is important to distinguish between chemical fractionation and isotopic fractionation.

Although it is often clear from the context, chemical fractionation is the process by which a mixture is separated into smaller quantities of differing compositions. That is, changing the chemical composition through successive operations (e.g. crystallisation, boiling, precipitation), each of which removes one or other of the constituents. Such a process is driven by a gradient, generally thermal or chemical, but it can also be physical. Hence chemical fractionation drives differentiation of magmas (e.g. fractional crystallisation), or can occur during boiling and/or phase separation of hydrothermal fluids. Significantly, however, all chemical fractionation processes generally refer to changing chemical proportions or phases.

A commonly observed example of chemical fractionation in the earth sciences is the presence of the Eu anomaly in magmas crystallising plagioclase (Figure 2.1). Unlike the other rare earth elements (REE), Eu is unique in that it can form Eu2+ ions which are small enough to sit happily in the crystal lattice instead of the Ca2+ ions which make up the bulk of the crystal. In this case the Eu is said to substitute for Ca, and is extracted from the magma preferentially with respect to the other REE, which are otherwise chemically very similar to Eu. This has the result that the remaining liquid will not have as much Eu as expected, since it is locked up (fractionated) in the plagioclase. If the magma subsequently separates from the plagioclase and crystallises elsewhere, when we analyse it and observe the presence of the negative Eu anomaly we can infer the role of plagioclase fractionation. Thus chemical fractionation occurs in almost every geological process, and in essence ore deposits are the end product of the most impressive chemical fractionation processes operating in the crust.