CALCULATING THE PARTITIONING OF THE ISOTOPES OF MO BETWEEN OXIDIC AND SULFIDIC SPECIES IN AQUEOUS SOLUTION

Abstract

The fractionation of the isotopes of Mo between different geological environments has recently been determined to high accuracy using mass spectrometry (Barling et al., 2001). Fractionation is observed between Mo in seawater, where it exists primarily in the form of the Mo(VI) anion molybdate, MoO4⁻², and in oxic sediments, where the Mo is isotopically lighter than in sea water by ∼1.8‰ (in terms of the ⁹⁷Mo, ⁹⁵Mo isotope pair). EXAFS evidence exists for a five- or six-coordinate Mo environment in the Fe,Mn oxyhydroxides of ferromanganese nodules (Kuhn et al., 2003). In sediment regimes which are anoxic and sulfidic (sometimes referred to as euxinic), where the Mo(VI) is expected to exist as a sulfide, no fractionation is observed compared to seawater. This is presumably because of the stoichiometric conversion of the Mo from MoO4⁻² to MoS4⁻² (Erickson and Helz, 2000) and then to other sulfides. If the conversion is stoichiometrically complete, mass balance requires the same isotopic distribution in reactant and product. This is a result of the very high equilibrium constant for this reaction. Thus, to understand isotopic fractionation processes both the equilibrium constants for the isotopic fractionation reactions and the equilbrium constants for transformation of one chemical compound to another must be considered.