https://doi.org/10.1130/G48495.1 Manuscript received 22 July 2020 Revised manuscript received 12 October 2020 Manuscript accepted 6 February 2021 © 2021 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 15 April 2021 Ferric iron triggers greenalite formation in simulated Archean seawater Isaac L. Hinz1, Christine Nims1, Samantha Theuer1, Alexis S. Templeton2 and Jena E. Johnson1 1 Department of Earth and Environmental Sciences, University of Michigan, 1100 North University Avenue, Ann Arbor, Michigan 48109, USA 2 Department of Geological Sciences, University of Colorado–Boulder, 2200 Colorado Avenue, Boulder, Colorado 80309, USA ABSTRACT Estimates of dissolved iron concentrations in Sedimentary rock deposits provide the best records of (bio)geochemical cycles in the the pre-oxygenated ocean vary between 20 µM ancient ocean. Studies of these sedimentary archives show that greenalite, an Fe(II) silicate and 2 mM Fe(II) depending on constraints set with low levels of Fe(III), was an early chemical precipitate from the Archean ocean. To bet- by siderite (Holland, 1984; Canfield, 2005) or ter understand the formation of greenalite, we explored controls on iron silicate precipitation ferrous greenalite (Jiang and Tosca, 2019) satu- through experiments in simulated Archean seawater under exclusively ferrous conditions ration. These geochemical differences prevent or supplemented with low Fe(III). Our results confirm a pH-driven process promoting the direct translations between the high-concentra- precipitation of iron-rich silicate phases, and they also reveal an important mechanism in tion, high-pH synthesis experiments and ancient which minor concentrations of Fe(III) promote the precipitation of well-ordered greenalite processes in the Archean ocean. among other phases. This discovery of an Fe(III)-triggering iron silicate formation process A few previous studies explored iron silicate suggests that Archean greenalite could represent signals of iron oxidation reactions, poten- mineralization under reducing and geochemi- tially mediated by life, in circumneutral ancient seawater. cally relevant conditions plausible for the early ocean. Under reducing conditions with very low INTRODUCTION inclusions of early iron silicates (Rasmussen concentrations of iron and silica, Harder (1978) The early ocean shaped the evolution of mi- et al., 2015; Muhling and Rasmussen, 2020). produced Fe[II,III]-silica coprecipitates at pH croscopic life and supported the development of The best-preserved inclusions were character- 7–9 but did not include structural data to confirm the earliest (bio)geochemical cycles. Chemically ized as greenalite [Fe(II)3Si2O5(OH)4], the iron iron silicate formation. Later studies similarly precipitated sediments record the (bio)geochem- end member of the two-layer serpentine group, showed the anoxic formation of poorly crystal- istry of their aqueous environments (Derry and but with ∼10%–25% Fe(III) (Johnson et al., line iron-silica coprecipitates but were unable Jacobsen, 1990), and deeper marine strata from 2018). This small proportion of Fe(III) in Ar- to robustly characterize these phases (Farmer, the Archean Eon (4.0–2.5 Ga) are dominated chean greenalite suggests that partial iron oxida- 1991; Konhauser et al., 2007). Recently, Tosca by iron formations (silica-rich chemical sedi- tion could have favored iron-silica precipitation, et al. (2016) examined Fe(II)-silicate formation ments with >15% iron) (Simonson and Hassler, and indeed, metastable Fe[II,III] hydroxy salts in Archean seawater-like solutions and proposed 1996). The long-term, widespread deposition (green rust phases) were proposed as potential that poorly ordered experimental precipitates of iron formations suggests anoxic conditions precursors to iron formation silicates and oxides detected at pH ≥7.5 were precursors to green- and high iron concentrations in seawater, yet (Halevy et al., 2017). alite. In contrast, at pH 7, Halevy et al. (2017) the accumulation of iron-rich sediments was Understanding the formation process of iron observed that Fe[II,III] green rust formed upon likely triggered by a chemical change or (bio) silicates under ancient marine conditions should partial Fe oxidation under low concentrations of geochemical process that induced mineral pre- yield insights into Archean (bio)geochemistry. silica (100–250 µM), transforming into three- cipitation (Holland, 1984; Bekker et al., 2014; Existing iron silicate synthesis protocols have layer iron silicates in solutions upon 22 °C and Johnson and Molnar, 2019). relied on oversaturated iron and silica chemi- 50 °C aging for days to months. The identification of the original iron cal conditions set at high pH (8 to >12) (e.g., While this research demonstrates that iron- precipitate(s) is integral to unraveling the an- Decarreau and Bonnin, 1986; Baldermann et al., silica precipitates can form in simulated ancient cient process(es) that induced the deposition 2014; Chemtob et al., 2015; Dzene et al., 2018). seawater, no study has conclusively shown of Archean marine iron formations. Though However, two independent models of early Earth greenalite formation in predicted Archean ocean the iron oxides prevalent in iron formations marine geochemistry estimated Archean oce- chemistry. It is also unclear whether the Fe(III) are largely accepted as primary components anic pH between ∼6.5 and 7.5 (Halevy and in the ancient iron silicate inclusions signals a of these deeper marine sediments (Beukes and Bachan, 2017; Krissansen-Totton et al., 2018). role for iron oxidation in the precipitation of Gutzmer, 2008; Bekker et al., 2014; Sun et al., Geological observations suggest that dissolved these iron-silica phases. Therefore, we explored 2015), rigorous characterization of well-pre- silica was near or at saturation with amorphous the precipitation and crystallization of iron and served chert associated with ca. 3.5 to ca. 2.4 Ga silica (∼1.9 mM) in the Archean ocean (Siever, silica precipitates in artificial ancient seawater at iron formations documented nanometer-scale 1992; Maliva et al., 2005; Stefurak et al., 2015). the predicted Archean pH range of 6.5–7.5 under CITATION: Hinz, I.L., et al., 2021, Ferric iron triggers greenalite formation in simulated Archean seawater: Geology, v. 49, p. 905–910, https://doi.org/10.1130/G48495.1 Geological Society of America | GEOLOGY | Volume 49 | Number 8 | www.gsapubs.org 905 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/8/905/5362185/g48495.1.pdf by guest on 27 September 2021 both strictly ferrous conditions and in systems pH CONTROL ON IRON-RICH els. The quantity of solids at pH 7.5 in- with low levels of Fe(III). SILICATE FORMATION creased approximately fivefold with Fe(III) In measuring the mass of precipitated phas- (Table 1), producing deep-green precipitates EXPERIMENTAL METHODOLOGY es and the associated changes in solution chem- (Fig. 2A; Fig. S1). The 25 °C bulk solid phase Experiments were performed in 500 mL bo- istry (Table 1), we did not detect iron-silica was XRD-amorphous (Fig. S4) and >80% rosilicate glass bottles at 25 °C under anoxic precipitation, but observed possible colloidal Fe(II) (Table 1). SEM (Fig. S5A) and TEM conditions. Solutions were first supplemented silica at pH 6.5 and 7 under anoxic and ferrous (Figs. 2B–2E) imaging revealed abundant na- with 1 mM sodium orthosilicate and stirred for conditions (see the Supplemental Material). noscale tubular structures and poorly layered 24 h at their respective experimental pH (6.5, Low quantities of visible iron-silica coprecipi- curling structures. These were morphologi- 7, or 7.5), ensuring full silica depolymerization tate did form at the higher pH of 7.5 (Fig. S1). cally similar to the ferrous layered precipitate (Dietzel, 2000). We added seawater salts, then The mass of the observable precipitate at pH (Figs. S3D and S3E). HR-TEM also indicated ferrous or ferrous-ferric chloride and bicar- 7.5 was still not measurable, but changes in so- iron-oxide precipitates at 25 °C, including bonate buffer, and set the final pH of solutions lution chemistry after 5 days at 25 °C indicated euhedral crystals with magnetite lattice spac- (Table S1 in the Supplemental Material1). Given a decrease in Fe(II) from 1 mM to 0.78 mM ing (Fig. S5D). Even after 150 °C aging, XRD our focus on iron silicate formation, we omitted and a decrease in Si from 1 mM to 0.73 mM of the Fe[II,III] pH 7.5 experiment did not de- calcium in the media to preclude the precipita- (Table 1). We were unable to subsample the tect ∼7 Å silicates (Fig. S4), and TEM imag- tion of calcium carbonate. negligible pH 7.5 precipitate at 25 °C, so the ing showed curling layered structures instead After 5 days of low-temperature (25 °C) re- entire pH 7.5 precipitate was aged at 150 °C of well-ordered stacks (Figs. 2G–2I; Figs. action in the dark, the experiments were sub- and developed into a similar quantity of darker S6B–S6D). In HR-TEM, a lack of repeating sampled for aqueous and solid analyses and gray material (Fig. 1A). layers precluded definitive measurements, but secondary hydrothermal aging. For aging, sub- A nanoscale examination of this 150 °C– we estimated a basal layer spacing of ∼7.9 Å samples of 25 °C precipitates were sealed in aged ferrous pH 7.5 sample illuminated three (Fig. 2 K). While amorphous by selected area a gas-tight Parr vessel (22 mL) under anoxic distinct morphologies (Fig. 1). We observed ag- electron diffraction (SAED) (Fig. S3), the lay- conditions and heated at 150 °C for 5 days to gregates of layered structures, webs of amor- ered phases had elemental chemistry consis- enhance crystallization, simulating ∼100 m.y. phous gel, and rare hexagonal platelets (Fig. 1C; tent with a serpentine-group silicate similar of low-temperature diagenesis (Siever, 1983). Fig. S2C). The few hexagonal platelets were ten- to greenalite (Fig.