Figures

top

Introduction

Iron and other trace elements released from hydrothermal vents are critical contributors to marine biogeochemical cycles (Elderfield and Schultz, 1996

Elderfield, H., Schultz, A. (1996) Mid-Ocean Ridge Hydrothermal Fluxes and the Chemical Composition of the Ocean. Annual Review of Earth and Planetary Sciences 24, 191–224. https://doi.org/10.1146/annurev.earth.24.1.191

; Resing et al., 2015

Resing, J.A., Sedwick, P.N., German, C.R., Jenkins, W.J., Moffett, J.W., Sohst, B.M., Tagliabue, A. (2015) Basin-scale transport of hydrothermal dissolved metals across the South Pacific Ocean. Nature 523, 200–203. https://doi.org/10.1038/nature14577

). In low temperature hydrothermal systems, the mixing of Fe-rich fluids with oxygenated seawater, combined with biomineralisation by Fe(II)-oxidising bacteria, triggers rapid precipitation of amorphous and poorly crystalline Fe(III) oxyhydroxides (e.g., ferrihydrite). These metastable phases act as transient sinks for trace elements and nutrients, such as cobalt (Co), nickel (Ni), silicon (Si), and phosphorus (P), thereby regulating their fluxes into surrounding seawater (German et al., 1991

German, C.R., Campbell, A.C., Edmond, J.M. (1991) Hydrothermal scavenging at the Mid-Atlantic Ridge: Modification of trace element dissolved fluxes. Earth and Planetary Science Letters 107, 101–114. https://doi.org/10.1016/0012-821X(91)90047-L

). Yet, these initially formed Fe(III) oxyhydroxides undergo transformations during deposition and burial, with important implications for the mobility and fate of associated trace elements. While microbial mediated mineralisation processes in these setting have been relatively well studied (Konhauser and Riding, 2012

Konhauser, K.O., Riding, R. (2012) Bacterial Biomineralization. In: Knoll, A.H., Canfield, D.E., Konhauser, K.O. (Eds.) Fundamentals of Geobiology. Wiley-Blackwell, Oxford, 105–130. https://doi.org/10.1002/9781118280874.ch8

), the role of Fe(II)_aq — which is ubiquitous in hydrothermal fluids and porewaters — in driving subsequent mineral transformation remains poorly constrained within hydrothermal systems.

Fe(II)-induced transformation of Fe(III) oxyhydroxides is well documented in laboratory experiments using synthetic Fe minerals such as ferrihydrite and lepidocrocite (Liu et al., 2022

Liu, J., Sheng, A., Li, X., Arai, Y., Ding, Y., Nie, M., Yan, M., Rosso, K.M. (2022) Understanding the importance of labile Fe (III) during Fe (II)-catalyzed transformation of metastable iron oxyhydroxides. Environmental Science and Technology 56, 3801–3811. https://doi.org/10.1021/acs.est.1c08044

; Hua et al., 2023

Hua, J., Sun, J., Chen, M., Liu, C., Wu, F. (2023) Aqueous Fe(II)-catalyzed iron oxide recrystallization: Fe redox cycling and atom exchange, mineralogical recrystallization and contributing factor. Reviews in Environmental Science and Bio/Technology 22, 55–78. https://doi.org/10.1007/s11157-023-09646-3

). Extensive Fe atom exchange between Fe(II)_aq and structural Fe(III) has also been identified even without noticeable mineral transformation (Handler et al., 2014

Handler, R.M., Frierdich, A.J., Johnson, C.M., Rosso, K.M., Beard, B.L., Wang, C.M., Latta, D.E., Neumann, A., Pasakarnis, T., Premaratne, W., Scherer, M.M. (2014) Fe(II)-Catalyzed Recrystallization of Goethite Revisited. Environmental Science and Technology 48, 11302–11311. https://doi.org/10.1021/es503084u

; Chen et al., 2023

Chen, C., Dong, Y., Thompson, A. (2023) Electron Transfer, Atom Exchange, and Transformation of Iron Minerals in Soils: The Influence of Soil Organic Matter. Environmental Science and Technology 57, 10696–10707. https://doi.org/10.1021/acs.est.3c01876

). This Fe(II)-induced “cryptic” recrystallisation and transformation not only alters the mineralogy of Fe(III) oxyhydroxides, but also affects the stability of associated trace metals (Frierdich et al., 2011

Frierdich, A.J., Luo, Y., Catalano, J.G. (2011) Trace element cycling through iron oxide minerals during redox-driven dynamic recrystallization. Geology 39, 1083–1086. https://doi.org/10.1130/G32330.1

; Hua et al., 2023

Hua, J., Sun, J., Chen, M., Liu, C., Wu, F. (2023) Aqueous Fe(II)-catalyzed iron oxide recrystallization: Fe redox cycling and atom exchange, mineralogical recrystallization and contributing factor. Reviews in Environmental Science and Bio/Technology 22, 55–78. https://doi.org/10.1007/s11157-023-09646-3

). Additionally, it can drive Fe isotope fractionation among different mineral phases, complicating the interpretation of Fe isotope signatures to trace the origin and formation of Fe deposits (Johnson et al., 2020

Johnson, C., Beard, B., Weyer, S. (2020) Iron Geochemistry: An Isotopic Perspective. Springer, Cham. https://doi.org/10.1007/978-3-030-33828-2

; Fitzsimmons and Conway, 2023

Fitzsimmons, J.N., Conway, T.M. (2023) Novel Insights into Marine Iron Biogeochemistry from Iron Isotopes. Annual Review of Marine Science 15, 383–406. https://doi.org/10.1146/annurev-marine-032822-103431

). However, natural hydrothermal systems are a unique matrix of mineralogical and geochemical environments, with major and trace elemental compositions that may impede or redirect the transformation pathways of Fe-rich precipitates (Jones et al., 2009

Jones, A.M., Collins, R.N., Rose, J., Waite, T.D. (2009) The effect of silica and natural organic matter on the Fe(II)-catalysed transformation and reactivity of Fe(III) minerals. Geochimica et Cosmochimica Acta 73, 4409–4422. https://doi.org/10.1016/j.gca.2009.04.025

; Li et al., 2024

Li, J., Sun, M., Qi, W., Zhou, Z., Hohl, S.V., He, Z. (2024) Geochemical and Sr-Nd-Pb-Fe Isotopic Constraints on the Formation of Fe-Si Oxyhydroxide Deposits at the Ultraslow-Spreading Southwest Indian Ridge. Geochemistry, Geophysics, Geosystems 25, e2023GC011185. https://doi.org/10.1029/2023GC011185

). Studies relying solely on laboratory synthesised minerals may therefore fail to capture the full range of processes occurring in situ, underscoring the need to investigate Fe(II)_aq-induced processes using geochemically realistic samples from actual hydrothermal systems.

In this study, we address this knowledge gap by studying natural Fe-rich precipitates collected from the Longqi Hydrothermal Field on the Southwest Indian Ridge. These samples preserve the chemical and mineralogical complexity of natural systems, allowing us to explore Fe(II)_aq-induced processes under environmentally relevant conditions. By coupling ⁵⁷Fe(II) stable isotope tracer experiments with mass spectroscopy, scanning and transmission electron microscopy, X-ray diffraction, and Mössbauer spectroscopy, we tracked mineralogical and morphological changes in detail and identified Fe atom exchange and trace metal redistribution in laboratory conditions. Our findings highlight the often overlooked role of Fe(II)_aq in modern low temperature hydrothermal systems in addition to biomineralisation, providing additional constraints for interpreting sedimentary records in modern and ancient Fe deposits.

top

Materials and Methods

Sample collection. Fe-rich precipitates were sampled from the Longqi low temperature hydrothermal field on the Southwest Indian Ridge, near an active vent located at 49.6497° E, 37.7832° S, at a water depth of ∼2750 metres (Fig. 1). Ambient water temperatures ranged from 50–60 °C near the vent to 2–4 °C farther away, with pH values between 7.8–8.0. Sampling was carried out December 2, 2018, using the RV Tansuoyihao and the submersible “Shenghaiyongshi”. Immediately upon retrieval, the precipitate suspension (solid-liquid ratio was about 1:1) was preserved at −80 °C until further processing. Prior to commencing our experiments, an aliquot of the precipitate suspension was totally digested by 6 M HCl to determine the concentrations of Fe and other elements in the stock (see Table S-1).

Full size image

Experiments. All experiments were conducted inside an anoxic glovebox under N₂ atmosphere (O₂ < 0.2 ppm). Ultrapure water was deoxygenated by purging with N₂ and equilibrated within the glovebox for one week before preparing anoxic solutions. A stock solution of ⁵⁷Fe(II) was prepared by dissolving metallic ⁵⁷Fe (96 % of ⁵⁷Fe, ISOFLEX, California) in 1 M HCl (double distilled). This solution was then used to obtain 2 mM ⁵⁷Fe(II) in either HEPES buffer (25 mM, pH 8.0) or artificial seawater (ASW, pH 8.0, ionic strength 0.72 mol/L)(Kester et al., 1967

Kester, D.R., Duedall, I.W., Connors, D.N., Pytkowicz, R.M. (1967) Preparation of artificial Seawater. Limnology and Oceanography 12, 176–179. https://doi.org/10.4319/lo.1967.12.1.0176

), and subsequently exposed to the Fe-rich precipitates (containing 10 mM Fe). Control experiments were performed with only ⁵⁷Fe(II)_aq or only the precipitates in HEPES buffer and ASW, respectively. For the reaction medium, HEPES buffer was used to isolate Fe(II) driven processes under controlled conditions and investigate their mechanistic feasibility, whereas ASW provided a more environmentally relevant context simulating hydrothermal systems. Although we acknowledge that the experimental setups cannot fully replicate natural conditions, key parameters such as pH (8.0) and Fe(II)/Fe(III) ratios were chosen to reflect conditions relevant to low temperature hydrothermal systems.

Triplicate reactors were rotated in the dark and sampled over a 14 day reaction period. Aliquots were centrifuged, and the resulting solids were subjected to sequential extractions: 1 M MgCl₂ to isolate the exchangeable fraction, 0.5 M HCl to target the amorphous to poorly crystalline Fe fraction, and 6 M HCl for the crystalline Fe fraction. More detailed methods can be found in SI. Aqueous trace metals (Co, Ni, Zn, Cu, and Ba) were quantified separately by ICP-MS, using ¹⁰³Rh and ¹¹⁵In as internal standards. The count rate stability of the instrument was maintained below 3 % relative standard deviation (RSD).

Mineral characterisation. To track microstructural changes, aliquots of sediment suspensions were placed on a copper observation table and a copper net, dried in an anoxic glovebox, and characterised with a field emission scanning electron microscope (SEM Nano 450, FEI Co., USA) and a transmission electron microscope (TEM, JEOL JEM-F200, Japan). Due to limited sample volumes, unreacted and reacted samples were filtered (0.22 μm) and characterised using micro X-ray diffraction (μXRD, Bruker D8 Endeavor, Germany). The ⁵⁷Fe Mössbauer Spectroscopy (Wissel GmbH, Germany) was also applied to track the mineralogical changes. More detailed methods about mineral characterisation can be found in the Supplementary Information.

top

Results and Discussion

Characteristics of the hydrothermal Fe-rich precipitates. The Fe-rich precipitates investigated in this study primarily consist of amorphous to poorly crystalline Fe(III) phases (e.g., ferrihydrite), goethite, and lepidocrocite, as identified by HCl extractions (Fig. 2), TEM imaging (Fig. 3), XRD, and Mössbauer spectroscopy (Fig. 4). Approximately 25 % of Fe in the original precipitates was extractable with 0.5 M HCl, and 100 % was extractable with 6 M HCl, indicating the relatively low stability of these Fe(III) oxyhydroxides (Fig. 2). Morphologically, both amorphous and rod-like hollow structures were observed (Fig. 3a1,a2), suggesting that abiotic and microbially mediated Fe(II) oxidation both contributed to their formation (Chan et al., 2016

Chan, C.S., McAllister, S.M., Leavitt, A.H., Glazer, B.T., Krepski, S.T., Emerson, D. (2016) The Architecture of Iron Microbial Mats Reflects the Adaptation of Chemolithotrophic Iron Oxidation in Freshwater and Marine Environments. Frontiers in Microbiology 7, 796. https://doi.org/10.3389/fmicb.2016.00796

; Li et al., 2024

Li, J., Sun, M., Qi, W., Zhou, Z., Hohl, S.V., He, Z. (2024) Geochemical and Sr-Nd-Pb-Fe Isotopic Constraints on the Formation of Fe-Si Oxyhydroxide Deposits at the Ultraslow-Spreading Southwest Indian Ridge. Geochemistry, Geophysics, Geosystems 25, e2023GC011185. https://doi.org/10.1029/2023GC011185

). EDS analysis showed a very low abundance of Si (< 1%) within these structures (Fig. S-1). The crystallinity and structural order of these minerals were further elucidated by selected area electron diffraction (SAED) associated with TEM (Fig. 3a5), the full width at half maximum (FWHM) of identical peaks in XRD patterns (Table S-2), and magnetic ordering from ⁵⁷Fe Mössbauer spectra at 295 K (Fig. 4b). Collectively, these analyses reveal structural defects in the crystal structures, particularly in goethite with the nanoneedle morphology. Notably, the relatively high proportion of crystallised Fe minerals (goethite and lepidocrocite) combined with low Si content (< 1 %; Fig. S-1) distinguishes these precipitates from those described in our previous study, where only poorly crystalline Fe(III) oxyhydroxides, such as ferrihydrite, were identified (Li et al., 2024

Li, J., Sun, M., Qi, W., Zhou, Z., Hohl, S.V., He, Z. (2024) Geochemical and Sr-Nd-Pb-Fe Isotopic Constraints on the Formation of Fe-Si Oxyhydroxide Deposits at the Ultraslow-Spreading Southwest Indian Ridge. Geochemistry, Geophysics, Geosystems 25, e2023GC011185. https://doi.org/10.1029/2023GC011185

).

Full size image

Full size image

Full size image

Reactions with Fe(II). While microbial mediated mineral formation and transformation are crucial, this study intentionally focuses on abiotic processes mediated by Fe(II)_aq, which are comparatively understudied in hydrothermal systems. By reacting Fe-rich precipitates with Fe(II)_aq at an initial Fe(II)/Fe(III) ratio of 0.2, 91 % of Fe(II)_aq was removed over 14 days in the HEPES buffer, resulting in a final Fe(II)/Fe(III) ratio of 0.14 in the solid phase (Fig. 2a). Sequential extraction revealed a significant decrease in the total Fe content within the 0.5 M HCl extractable fraction, accompanied by an increase in the 6 M HCl extractable fraction, indicating a shift towards more stable and crystalline mineral phases. Rapid Fe isotope mixing was identified, with the fraction of ⁵⁷Fe in different phases reaching near equilibrium (⁵⁷Fe fraction ∼0.16) within two days (Fig. 2e). Visually, the solid phase exhibited a noticeable colour change from yellowish to blackish, signaling substantial mineralogical transformation.

In ASW, the removal of Fe(II)_aq proceeded more slowly, with only 50 % removed over 14 days, resulting in a final Fe(II)/Fe(III) ratio of 0.09 in the solid phase (Fig. 2b). This slower removal is mainly due to cation competition with Fe(II) for adsorption sites at high ionic strength. Correspondingly, both the Fe(II) and total Fe mass in the 0.5 M HCl extractable fraction decreased more gradually, and Fe isotope exchange proceeded at a slower rate compared to the reaction in HEPES buffer (Fig. 2d,f), mainly due to the lower Fe(II) loading in the solid phase. Although no visible colour change was observed as seen under the HEPES buffered conditions, microstructural alterations in the solid phase were nevertheless evident in both SEM and TEM analyses.

Morphological and mineralogical alterations. Compared to the control group containing only the original precipitates, the long rod-like hollow structures were generally shorter and showed signs of fragmentation after reaction with Fe(II)_aq in HEPES buffer (Fig. 3a1,b1). Additionally, new spherical particle agglomerates formed, indicating substantial mineral transformation likely driven by Fe(II)_aq (Fig. 3b2). In ASW, the rod-like hollow structure and nanoneedles were largely preserved, although an additional lath-shaped structure appeared, indicating significant modifications to the morphological features (Fig. 3c1,c2).

TEM, XRD and Mössbauer spectroscopy provided further insights into these transformations. After reaction with Fe(II)_aq in HEPES buffer, the abundance of non-crystalline phases in the original precipitates was diminished, corresponding to the formation of more crystallised Fe minerals (Fig. 4). In the XRD pattern, magnetite identical peaks appeared, consistent with the spherical particles observed in SEM images (Figs. 4a, 3b2). In contrast, a higher proportion of non-crystalline phases persisted in ASW (Fig. 4e, Table S-3), accompanied by the formation of secondary goethite and lepidocrocite (Fig. 4a). The ratio of lepidocrocite to goethite decreased from 0.45 to 0.31 after reaction with Fe(II)_aq, indicating the enrichment of goethite after mineralogical transformation.

Comparing the SAED of the nanoneedles (Fig. 3) and the FWHM values of the goethite [110] face (Table S-2), we observed an improvement in goethite crystallinity after reaction with Fe(II)_aq in both media. This observation is linked to extensive Fe atom exchange between Fe(II)_aq and structural Fe(III) within the crystalline Fe minerals (corresponding to the 6 M HCl extractable; Fig. 2e,f). These results suggest that Fe(II)-induced recrystallisation of goethite occurred, leading to a reduction in crystal defects by removing site vacancy or substitutions (Notini et al., 2018

Notini, L., Latta, D.E., Neumann, A., Pearce, C.I., Sassi, M., N’Diaye, A.T., Rosso, K.M., Scherer, M.M. (2018) The Role of Defects in Fe (II)–Goethite Electron Transfer. Environmental Science and Technology 52, 2751–2759. https://doi.org/10.1021/acs.est.7b05772

; Southall et al., 2018

Southall, S.C., Micklethwaite, S., Wilson, S., Frierdich, A.J. (2018) Changes in Crystallinity and Tracer-Isotope Distribution of Goethite during Fe(II)-Accelerated Recrystallization. ACS Earth and Space Chemistry 2, 1271–1282. https://doi.org/10.1021/acsearthspacechem.8b00100

). The increased concentration of trace metals immediately after reaction with Fe(II)_aq (Fig. S-4) also supports this interpretation (Frierdich et al., 2011

Frierdich, A.J., Luo, Y., Catalano, J.G. (2011) Trace element cycling through iron oxide minerals during redox-driven dynamic recrystallization. Geology 39, 1083–1086. https://doi.org/10.1130/G32330.1

).

Conversely, in ASW which better simulates natural conditions, the crystallinity of lepidocrocite decreased (Table S-2). This reduction in crystallinity may result from transformation of pre-existing lepidocrocite into secondary goethite, with the pre-existing goethite potentially serving as a template (Yin et al., 2025

Yin, M., Li, X., Guo, C., Zhong, Q., Li, X., Zeng, L., Zhou, Y., Yang, C., Dang, Z. (2025) Effects of coexisting goethite or lepidocrocite on Fe(II)-induced ferrihydrite transformation pathways and Cd speciation. Science of the Total Environment 959, 178321. https://doi.org/10.1016/j.scitotenv.2024.178321

). Additionally, the identified lepidocrocite could be newly formed, promoted by the “lepidocrocite favouring effect” during ferrihydrite transformation influenced by natural components (Chen et al., 2015

Chen, C., Kukkadapu, R., Sparks, D.L. (2015) Influence of coprecipitated organic matter on Fe²⁺(aq)-catalyzed transformation of ferrihydrite: implications for carbon dynamics. Environmental Science and Technology 49, 10927–10936. https://doi.org/10.1021/acs.est.5b02448

). The relatively short aging time of this secondary lepidocrocite likely accounts for its lower crystallinity. This interpretation is also supported by the emergence of newly formed lath-shaped structures (Fig. 3c1,c2). Overall, our findings based on laboratory experiments provide direct evidence that Fe(II)_aq can induce rapid mineral transformation and recrystallisation within Fe-rich precipitates derived from the low temperature hydrothermal systems.

Coupled trace element cycling. During Fe(II)-induced mineral transformation and recrystallisation, we observed a rapid release of Ni, Co, Cu, Zn and Ba at the beginning of the reactions, followed by near complete resorption of Ni within one day (Figs. S-3, S-4). The concentrations of Co, Cu, Zn and Ba were significantly higher in ASW compared to the HEPES buffer, likely due to higher background levels and cation desorption driven by the higher ionic strength of ASW (Fig. S-3). Most of the trace metal release occurred immediately after Fe(II)_aq adsorption, which could be attributed to cation exchange with Fe(II)_aq and a reduction in available binding sites within the newly formed secondary minerals (Liu et al., 2016

Liu, C., Zhu, Z., Li, F., Liu, T., Liao, C., Lee, J.-J., Shih, K., Tao, L., Wu, Y. (2016) Fe (II)-induced phase transformation of ferrihydrite: The inhibition effects and stabilization of divalent metal cations. Chemical Geology 444, 110–119. https://doi.org/10.1016/j.chemgeo.2016.10.002

). Importantly, this prompt Fe(II)-induced release of trace elements is crucial for their remobilisation, potentially allowing them to escape capture by Fe(III) oxyhydroxides within the hydrothermal plume (Lough, 2016

Lough, A.J.M. (2016) Trace metal chemistry of hydrothermal plumes. PhD Thesis, University of Southampton, Ocean & Earth Science. https://eprints.soton.ac.uk/403372/

). In natural environments, dynamic mixing of hydrothermal fluids with surrounding seawater further facilitates the efficient export of these remobilised trace elements, potentially enhancing their fluxes from hydrothermal systems to the ocean.

Implications for the Fe-rich precipitates formation processes. A number of studies have highlighted the role of Fe(II)-oxidising microorganisms in forming Fe-rich precipitates and characteristic morphological features in low temperature hydrothermal fields (Konhauser et al., 2017

Konhauser, K.O., Planaysky, N.J., Hardisty, D.S., Robbins, L.J., Warchola, T.J., Haugaard, R., Lalonde, S.V., Partin, C.A., Oonk, P.B.H., Tsikos, H., Lyons, T.W., Bekker, A., Johnson, C.M. (2017) Iron formations: A global record of Neoarchaean to Palaeoproterozoic environmental history. Earth-Science Reviews 172, 140–177. https://doi.org/10.1016/j.earscirev.2017.06.012

; Dong et al., 2022

Dong, A., Sun, Z., Kendall, B., Izon, G., Cao, H., Li, Z., Ma, X., Yin, X., Qiu, Z., Zhu, X.-k., Bekker, A., Poulton, S.W. (2022) Insights from modern diffuse-flow hydrothermal systems into the origin of post-GOE deep-water Fe-Si precipitates. Geochimica et Cosmochimica Acta 317, 1–17. https://doi.org/10.1016/j.gca.2021.10.001

; Li et al., 2024

Li, J., Sun, M., Qi, W., Zhou, Z., Hohl, S.V., He, Z. (2024) Geochemical and Sr-Nd-Pb-Fe Isotopic Constraints on the Formation of Fe-Si Oxyhydroxide Deposits at the Ultraslow-Spreading Southwest Indian Ridge. Geochemistry, Geophysics, Geosystems 25, e2023GC011185. https://doi.org/10.1029/2023GC011185

). Complementing this microbial perspective, our study underscores the importance of Fe(II)_aq in driving subsequent transformation and recrystallisation of Fe-rich precipitates in these environments.

Our experimental findings, together with previous studies, demonstrate that secondary minerals such as goethite, magnetite, and some intermediate minerals (e.g., lepidocrocite, green rust) can form rapidly when settling Fe(III) oxyhydroxides interact with Fe(II)_aq within a hydrothermal plume or during the hydrothermal alteration of Fe deposits (Zegeye et al., 2012

Zegeye, A., Bonneville, S., Benning, L.G., Sturm, A., Fowle, D.A., Jones, C., Canfield, D.E., Ruby, C., MacLean, L.C., Nomosatryo, S., Crowe, S.A., Poulton, S.W. (2012) Green rust formation controls nutrient availability in a ferruginous water column. Geology 40, 599–602. https://doi.org/10.1130/G32959.1

; Li et al., 2017

Li, Y.-L., Konhauser, K.O., Zhai, M. (2017) The formation of magnetite in the early Archean oceans. Earth and Planetary Science Letters 466, 103–114. https://doi.org/10.1016/j.epsl.2017.03.013

). While our experimental setup may not fully replicate all conditions of the natural environment, especially the system with HEPES buffer, the high Fe(II)/Fe(III) ratios achieved in the solid phase, a key factor for magnetite formation (Bauer et al., 2020

Bauer, K., Byrne, J., Kenward, P., Simister, R., Michiels, C., Friese, A., Vuillemin, A., Henny, C., Nomosatryo, S., Kallmeyer, J. (2020) Magnetite biomineralization in ferruginous waters and early Earth evolution. Earth and Planetary Science Letters 549, 116495. https://doi.org/10.1016/j.epsl.2020.116495

), is plausible within a hydrothermal plume or during post-depositional processes, such as those proposed for Precambrian hydrothermal systems (Kump and Seyfried, 2005

Kump, L.R., Seyfried, W.E. (2005) Hydrothermal Fe fluxes during the Precambrian: Effect of low oceanic sulfate concentrations and low hydrostatic pressure on the composition of black smokers. Earth and Planetary Science Letters 235, 654–662. https://doi.org/10.1016/j.epsl.2005.04.040

). These results suggest that Fe(II)-induced magnetite formation can occur not only in high temperature settings (Li et al., 2017

Li, Y.-L., Konhauser, K.O., Zhai, M. (2017) The formation of magnetite in the early Archean oceans. Earth and Planetary Science Letters 466, 103–114. https://doi.org/10.1016/j.epsl.2017.03.013

) but also in low temperature hydrothermal systems. Furthermore, the secondary lath-shaped structures identified in our experiments closely resemble those observed in natural hydrothermal deposits (Li et al., 2024

Li, J., Sun, M., Qi, W., Zhou, Z., Hohl, S.V., He, Z. (2024) Geochemical and Sr-Nd-Pb-Fe Isotopic Constraints on the Formation of Fe-Si Oxyhydroxide Deposits at the Ultraslow-Spreading Southwest Indian Ridge. Geochemistry, Geophysics, Geosystems 25, e2023GC011185. https://doi.org/10.1029/2023GC011185

), implying that such morphologies could serve as indicators for Fe(II)-induced mineral transformation under anoxic and Fe(II)-rich (“ferruginous”) conditions.

Additionally, the ⁵⁷Fe(II) tracer experiments revealed rapid Fe atom exchange between Fe(II)_aq and the ferric substrates of varying crystallinity (Fig 2e,f). The prevalence of crystal defects in naturally occuring minerals likely facilitated this efficient Fe atom exchange (Notini et al., 2018

Notini, L., Latta, D.E., Neumann, A., Pearce, C.I., Sassi, M., N’Diaye, A.T., Rosso, K.M., Scherer, M.M. (2018) The Role of Defects in Fe (II)–Goethite Electron Transfer. Environmental Science and Technology 52, 2751–2759. https://doi.org/10.1021/acs.est.7b05772

), enabling the Fe(II)-induced recrystallisation to play a part even in limited coexistence time of Fe(II)_aq and Fe(III) oxyhydroxides in the hydrothermal plume. This rapid Fe atom exchange also suggests an additional mechanism of Fe isotope fractionation, thus posing potential constraints on the interpretation of Fe isotope signals in hydrothermal systems (Johnson et al., 2020

Johnson, C., Beard, B., Weyer, S. (2020) Iron Geochemistry: An Isotopic Perspective. Springer, Cham. https://doi.org/10.1007/978-3-030-33828-2

). Furthermore, the concurrent redistribution of associated trace elements during mineral transformation and recrystallisation may promote their release into surrounding seawater (Frierdich et al., 2011

Frierdich, A.J., Luo, Y., Catalano, J.G. (2011) Trace element cycling through iron oxide minerals during redox-driven dynamic recrystallization. Geology 39, 1083–1086. https://doi.org/10.1130/G32330.1

; Liu et al., 2016

Liu, C., Zhu, Z., Li, F., Liu, T., Liao, C., Lee, J.-J., Shih, K., Tao, L., Wu, Y. (2016) Fe (II)-induced phase transformation of ferrihydrite: The inhibition effects and stabilization of divalent metal cations. Chemical Geology 444, 110–119. https://doi.org/10.1016/j.chemgeo.2016.10.002

; Gini et al., 2024

Gini, C., Jamieson, J.W., Reeves, E.P., Gartman, A., Barreyre, T., Babechuk, M.G., Jørgensen, S.L., Robert, K. (2024) Iron Oxyhydroxide-Rich Hydrothermal Deposits at the High-Temperature Fåvne Vent Field, Mohns Ridge. Geochemistry, Geophysics, Geosystems 25, e2024GC011481. https://doi.org/10.1029/2024GC011481

). Taken together, these findings highlight the need to take into account Fe(II)-induced processes when reconstructing geochemical cycling and the formation pathways for Fe-rich precipitates. More complex experimental setups better simulating natural conditions and covering greater geochemical gradients will be needed to further elucidate the implications of Fe(II)-Fe(III) interactions in the hydrothermal system.

Drawing parallels between modern hydrothermal systems and the depositional environments of ancient iron formations (IFs), our findings suggest that in addition to biomineralisation processes, the abiotic Fe(II)-induced processes could have also played a role in iron deposition on early Earth. During the Great Oxidation Event (GOE), redox stratification within the ocean water column may have permitted the coexistence of Fe(II)_aq and Fe(III) oxyhydroxides over a broad spatial and temporal scale (Zegeye et al., 2012

Zegeye, A., Bonneville, S., Benning, L.G., Sturm, A., Fowle, D.A., Jones, C., Canfield, D.E., Ruby, C., MacLean, L.C., Nomosatryo, S., Crowe, S.A., Poulton, S.W. (2012) Green rust formation controls nutrient availability in a ferruginous water column. Geology 40, 599–602. https://doi.org/10.1130/G32959.1

; Liang et al., 2025

Liang, X., Stüeken, E.E., Alessi, D.S., Konhauser, K.O., Li, L. (2025) A seawater oxygen oscillation recorded by iron formations prior to the Great Oxidation Event. Nature Geoscience 18, 417–422. https://doi.org/10.1038/s41561-025-01683-7

). Consequently, Fe(II)-induced mineral transformation of settling Fe(III) oxyhydroxides and concurrent recrystallisation could have enhanced the crystallinity of Fe minerals and fractionated Fe isotope compositions in IFs (Johnson et al., 2020

Johnson, C., Beard, B., Weyer, S. (2020) Iron Geochemistry: An Isotopic Perspective. Springer, Cham. https://doi.org/10.1007/978-3-030-33828-2

). Furthermore, Fe(II)-induced processes may have facilitated the release of micronutrients (e.g., Co, Ni, Cu, Zn) from amorphous to poorly crystalline Fe(III) oxyhydroxides formed during oxygenation, thus increasing nutrient availability for early marine microorganisms in ancient oceans.

top

Acknowledgements

Financial support was provided by the National Key Research and Development Program of China (2021YFF0501301), the National Science Foundation of China (42306052; 42230410; 42072333), and the Shanghai Pilot Program for Basic Research. We thank Shanghai Synchrotron Radiation Facility for the beam time granted under project NO.2024-SSRF-PT-510157. We thank Prof. Hu Wang, Dr. Lei Su, Yan Zhang and Yue Sheng for their help with sample preparations and background information. We would also like to acknowledge all the reviewers and editors for their constructive comments which significantly improved the quality of this work.

Editor: Juan Liu

top

References

top

Supplementary Information

The Supplementary Information includes:

• Supplementary Methods
• Tables S-1 to S-3
• Figures S-1 to S-4
• Supplementary Information References

Download the Supplementary Information (PDF)