Kinetic isotope fractionation, definition

Accordingly, isotopic equilibration for Cr and Se species is expected to be much slower than for the aqueous Fe(III)-Fe(II) couple, which reaches equilibrium within minutes in laboratory experiments (Beard and Johnson 2004). Additionally, Cr(III) and Se(0) are highly insoluble and their residence times in solution are small, which further decreases the likelihood of isotopic equilibration. In the synthesis below, isotopic fractionations are assumed to be kinetically controlled unless otherwise stated. However, definitive assessments of this assumption have not been done, and future studies may find that equilibrium fractionation is attained for some reactions or rmder certain conditions. 

Note that a. B may reflect either kinetic or equilibrium isotope partitioning between phases A and B. As discussed in the previous chapter, a -B for Fe/ Fe ratios typically varies between 0.997 and 1.003 (Chapter lOA Beard and Johnson 2004). In general, we will describe isotopic fractionations using A. b, following standard definitions ... 

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