Molybdenum isotope fractionation during adsorption by manganese oxides

The isotopic composition of Mo (δ97/95Mo) in seawater is ∼2‰ heavier than Mo in marine ferromanganese crusts and nodules [Barling et al., Earth Planet. Sci. Lett. 193 (2001) 447–457; Siebert et al., Earth Planet. Sci. Lett. 211 (2003) 159–171]. To explore this phenomenon, we have conducted an experimental investigation into the mass-dependent fractionation of Mo isotopes during adsorption onto Mn oxyhydroxide. Two series of experiments were carried out: a ‘time series’, in which adsorption proceeded for 2–96 h; and a ‘pH series’ in which pH varied from 6.5 to 8.5. The extent of Mo adsorption by Mn oxyhydroxides decreases with increasing pH, a trend typical of anion adsorption, and takes 48 h to reach steady-state. Lighter Mo isotopes are preferentially adsorbed. Experimentally determined fractionation factors (αsoln–MnOx) exhibit no systematic variation with either time or experimental pH. The mean αsoln–MnOx for all experiments is 1.0018±0.0005 (2 S.D.). Comparison of the Mo isotopic data for experimental solutions and Mo adsorbed to Mn oxyhydroxide with predictions for ‘closed system’ equilibrium and Rayleigh fractionation models indicates that isotope fractionation occurs as a result of ‘closed system’ equilibrium exchange between dissolved and adsorbed Mo. The isotopic offset between dissolved and adsorbed Mo is comparable to that observed between Mo in seawater and Mo in ferromanganese nodules and crusts. It is therefore likely that adsorption of Mo to Mn oxyhydroxides is a significant factor in the fractionation of Mo isotopes in the oceans.