Geochemistry 81 (2021) 125714 Contents lists available at ScienceDirect Geochemistry journal homepage: www.elsevier.com/locate/chemer Ni isotope fractionation during coprecipitation of Fe(III)(oxyhydr)oxides in Si solutions Anna Neubeck a,*, Christoffer Hemmingsson b, Arjen Boosman c, Olivier Rouxel d, Madeleine Bohlin a a Department of Earth Sciences, Uppsala University, Sweden b Department of Geological Sciences, Stockholm University, Sweden c Department of Earth Sciences, Utrecht University, The Netherlands d Ifremer, Unit of Marine Geosciences, Brest, France ARTICLE INFO ABSTRACT Handling Editor: Astrid Holzheid The dramatic decline in aqueous Ni concentrations in the Archean oceans during the Great Oxygenation Event is evident in declining solid phase Ni concentrations in Banded Iron Formations (BIFs) at the time. Several ex­ Keywords: periments have been performed to identify the main removal mechanisms of Ni from seawater into BIFs, whereby Stable Ni isotopes adsorption of Ni onto ferrihydrites has shown to be an efficient process. Ni isotopic measurements have shown Ferrihydrite precipitation limited isotopic fraction during this process, however, most experiments have been conducted in simple solutions Co-precipitation experiment containing varying proportions of dissolved Fe and Ni as NO salts, as opposed to Cl salts which are dominant in Banded iron formation 3 Silica seawater. Further, Archean oceans were, before the advent of siliceous eukaryotes, likely saturated with amorphous Si as seen in the interlayered chert layers within BIFs. Despite Si being shown to greatly affect the Ni elemental partitioning onto ferrihydrite solids, no studies have been made on the effects of Si on the Ni isotope fractionation. Here we report results of multiple coprecipitation experiments where ferrihydrite precipitated in mixed solutions with Ni and Si. Ni concentrations in the experiments ranged between 200 and 4000 nM for fixed concentrations of Si at either 0, 0.67 or 2.2 mM. The results show that Si at these concentrations has a limited effect on the Ni isotope fractionation during coprecipitation of ferrihydrite. At 0.67 mM, the saturation con­ centration of cristobalite, the isotopic fractionation factors between the precipitating solid and experimental fluid are identical to experiments not containing Si (0.34 ± 0.17‰). At 2.2 mM Si, and the saturation concentration of amorphous silica, however, the Ni isotopic composition of the ferrihydrite solids deviate to more negative values and show a larger variation than at low or no Si, and some samples show fractionation of up to 0.5‰. Despite this seemingly more unstable fractionation behaviour, the combined results indicate that even at as high concen­ trations of Si as 2.2 mM, the δ60Ni values of the forming ferrihydrites does not change much. The results of our study implicate that Si may not be a major factor in fractionating stable Ni isotopes, which would make it easier to interpret future BIF record and reconstruct Archean ocean chemistry. 1. Introduction biosphere, indirectly leading to the onset of the ‘great oxygenation event’ (GOE) (Konhauser et al., 2015, 2009). It was suggested that Prior to the onset of large-scale global mantle convection after the successive removal of essential trace elements led to a dramatic decline Archean to Proterozoic Transition, low vertical mixing of the mantle in abundance of methanogens, paving the way for oxygen-producing restricted the capture and burial of elements (Andrault et al., 2017), cyanobacteria and the concomitant onset of atmospheric oxygenation. resulting in higher elemental concentrations in the Archean oceans. It Although valuable clues can be found in Ni/Fe variability in banded iron has been hypothesised that the depletion of nickel (Ni) in early Earth formations (BIFs) which may have formed as a consequence of a gradual oceans, as a result of the onset of global mantle convection, had a direct atmospheric oxygenation, anoxygenic photosynthesis or C-P-O-Fe negative influence on the strongly nickel-dependent methanogenic cycling, both the source of the early Earth atmosphere and its * Corresponding author at: Villavagen¨ 16, 752 36 Uppsala, Sweden. E-mail address: [email protected] (A. Neubeck). https://doi.org/10.1016/j.chemer.2020.125714 Received 12 August 2020; Received in revised form 19 November 2020; Accepted 20 November 2020 Available online 23 November 2020 0009-2819/© 2020 The Authors. Published by Elsevier GmbH. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). A. Neubeck et al. Geochemistry 81 (2021) 125714 composition is highly debated (Andrault et al., 2017; Bekker et al., 2010; Holland et al., 1986; Konhauser et al., 2017; Marakushev and Belono­ gova, 2019; Ozaki et al., 2019; Thibon et al., 2019). BIFs are chemical balance sedimentary deposits (usually of Archean and Paleoproterozoic age) characterized by layers of iron oxides alternating with chert layers. The mass mechanisms of formation of these deposits are still debated and several explanations have been put forward, such as the formation through 0.07 0.06 0.12 0.04 0.03 0.12 0.08 0.05 0.16 0.04 0.07 0.06 0.07 0.05 Isotopic ‰ nd ). oxidation of dissolved Fe as a consequence of photosynthetic, cyano­ ‰ bacterial production of molecular oxygen (Cloud, 1973). Other, purely 0.08 stdev abiotic explanations have been suggested, such as the oxidation of ± 2 ‰ nd 0.03 0.06 0.03 0.03 0.05 0.04 0.05 0.05 0.04 0.05 0.03 0.04 0.04 0.05 ferrous iron to ferrihydrite or magnetite through UV oxidation or by is reduction of carbon dioxide to methane (Thibon et al., 2019). Several abiotic processes show a preferential uptake of light Ni isotopes, such as the adsorption and coprecipitation of Ni with ferrihydrites (Eusterhues deviation et al., 2011; Twidwell and Leonhard, 2008; Wasylenki et al., 2015). solution-solid Ni 0.05 0.08 Ferrihydrite, a primary Fe(III)-oxide, was likely the firstFe-oxide phase 60 Δ ‰ nd 0.28 0.27 0.31 0.30 0.31 0.34 0.21 0.52 0.43 0.30 0.43 0.36 to form as the ferrous ocean oxidised and produced the extraordinary standard 2 BIF deposits (Posth et al., 2013; Wasylenki et al., 2015). The large specific surface area of ferrihydrites promote adsorption of trace ele­ stdev 2 ‰ nd 0.02 0.03 0.05 0.06 0.06 0.05 0.04 0.06 0.17 0.05 0.04 0.02 0.05 0.04 ments and possibly the fractionation of their stable isotopes. Several external experiments have been conducted to evaluate the magnitude of abiotic (the fractionation of Ni isotopes in ferrihydrites, through adsorption and Fluid Ni coprecipitation experiments (Gueguen et al., 2018; Wang and Wasy­ error 0.06 0.06 60 δ lenki, 2017; Wasylenki et al., 2015). The studies demonstrated a strong ‰ nd 0.08 0.07 0.04 0.22 0.08 0.17 0.13 0.21 0.17 0.13 0.07 0.26 coupling between iron oxide precipitation and light Ni, with fraction­ internal ation factors between the experimental solution and ferrihydrite of Fluid Ni the F ~+0.35‰. However, these fractionation factors were determined from 0.30 0.48 0.51 0.50 0.44 0.16 0.34 0.26 0.14 0.28 0.61 0.44 0.39 0.68 0.15 experiments conducted in solutions containing only Fe and Ni dissolved are in water or dilute NaNO3. The Archean ocean is hypothesised to have Fluid had dissolved silicon, Si, concentrations as high as 2.2 mM (Konhauser [Ni] et al., 2009; Jones et al., 2015), evidenced by the microcrystalline quartz nM 60 969 2028 99 176 94 344 513 562 56 244 264 385 1361 583 layers alternating with the iron bands within BIF deposits. The presence measurements of Si has been shown to have a large effect on the partitioning of Ni into Fluid ferrihydrites (Konhauser et al., 2009), but no studies have yet been made M μ 2.9 3.2 3.8 66.8 62.2 49.7 37.4 76.6 44.6 21.7 83.9 104.7 33.1 48.5 84.2 [Fe] Fluids on the effects on Ni isotopes during this process. Therefore, as a firststep isotopic to investigate the effects of Si on Ni isotope fractionation, we conducted of multiple coprecipitation experiments to investigate the isotopic frac­ stdev 0.07 0.03 0.06 0.03 0.03 0.05 0.04 0.05 0.05 0.04 0.05 0.03 0.04 0.04 0.05 2 tionation of Ni sorption when ferrihydrite forms in mixed solutions with ‰ Si and Ni. deviations Solid Ni 2. Experimental and analytical methods 0.09 0.20 0.20 0.27 0.01 0.08 0.23 0.16 0.08 0.31 0.26 0.17 0.37 0.10 60 0.02 δ ‰ standard 2.1. Mineral synthesis 2 Solid The Ni 0.70 0.52 0.49 0.50 0.56 0.84 0.66 0.74 0.86 0.72 0.39 0.56 0.61 0.32 0.85 All coprecipitation experiments were conducted in a clean lab at the F Swedish Museum of Natural History in Stockholm, Sweden. The exper­ ◦ Solid iments were performed at room temperature (~24 C) in acid cleaned solutions. plastic beakers (PP) with constant agitation by a Tefloncoated magnetic and 229 1710 3678 140 408 1092 1032 3091 8353 237 405 736 1309 1018 7597 [Ni] nM stirrer. Stock solutions of FeCl2, NiCl2 and Si (as sodium metasilicate nonahydrate) were prepared by diluting concentrated solutions and solids powders with Milli-Q (18.2 MΩ) water.