Nickel isotope fractionation during continental weathering

Abstract Recent literature has documented systematic variations in the Ni stable isotope compositions of several terrestrial and marine reservoirs of this bioessential trace metal. For example, dissolved Ni in the world's major rivers has been shown to be more enriched in heavy isotopes than the continental rocks from which that Ni was derived (Cameron and Vance, 2014, Geochim. Cosmochim. Acta, 128, 195–211). This observation implies that one or more chemical reactions occurring during weathering of Ni-rich rock drives stable isotope fractionation that results in retention of lighter isotopes of Ni associated with solid phases. We present new Ni isotope analyses of samples from various horizons of the Webster-Addie laterite, near Democrat, North Carolina, USA, as well as results from two sets of experiments designed to constrain the fundamental mechanism(s) driving the isotopic variation observed in that weathering profile. Bedrock samples (dunite and talc-rich, altered dunite) have δ60/58Ni values of + 0.06 to + 0.20‰, in excellent agreement with previous research. A sample of “yellow laterite” is enriched in lighter isotopes of Ni, with δ60/58Ni = − 0.21‰. Mixed-phase samples containing quartz and traces of serpentine, goethite, and smectite range from + 0.13 to + 0.35‰. Experimental leaching of Ni from olivine in dilute acid at ambient conditions results in no resolvable fractionation, indicating that initial release of Ni to solution from bedrock is not responsible for the isotope variation we observe in the natural samples. Sorption of Ni onto montmorillonite, a minor secondary weathering product present in laterites, is associated with a very small fractionation of + 0.11 ± 0.09‰, with lighter isotopes sorbed on the smectitic clay. Such adsorption can thus account for some, but not all of the isotope systematics observed in the weathering profile. Given previous evidence that sorption of Ni to Fe oxyhydroxides does drive a fractionation of appropriate sense and magnitude (Wasylenki et al., 2015, Chem. Geol., 400, 56–64), we infer that oxidation of Fe2+ released from bedrock and precipitation result in retention of a light pool of Ni in solid weathering products, thereby enriching dissolved Ni in heavier isotopes. Because much of the continental crust's Ni budget is hosted by mafic and ultramafic rocks, which weather very rapidly at Earth's surface, and for which Fe oxyhydroxides are extremely common weathering products, this Ni fractionation mechanism may be relevant for terrestrial weathering of Ni-bearing rocks in general, even though the overall mineralogy and chemistry of weathering profiles varies with parent lithology and environmental conditions. Secondary, hydrous, Ni-rich Mg-silicates from veins in the saprolite zone are enriched in heavy isotopes, with δ60/58Ni = + 0.29 to + 0.86‰. Similar Ni-rich Mg-silicate samples from several similar profiles around the world display a similar range from + 0.36 to + 0.90‰, suggesting that the processes responsible for isotopic fractionation at the North Carolina locality are general phenomena characteristic of laterites worldwide. We hypothesize that, in laterites, continual downward percolation of meteoric waters and progressive, chromatographic exchange with goethite-bound Ni progressively shifts the Ni contents of solids toward lighter and fluids toward heavier isotopic compositions. When fluids reach the Mg-rich, Si-rich saprolite zone, isotopically heavy garnierite minerals precipitate in veins. The phenomena we report and discuss here may long have been the dominant controls on the distribution of Ni isotopes in Earth's near-surface, terrestrial environment, at least since oxygenation of the atmosphere. Due to lack of abundant oxygen in air and water earlier in Earth history, the biogeochemical cycle of Ni isotopes was almost certainly rather different, such that riverine dissolved Ni was not necessarily enriched in heavy isotopes relative to continental crust, and the oceans may not have been so highly enriched in heavy isotopes of Ni as they are presently.