Simulating Precambrian banded iron formation diagenesis

Abstract

Post-depositional diagenetic alteration makes the accurate interpretation of key precipitation processes in ancient sediments, such as Precambrian banded iron formations (BIFs), difficult. While microorganisms are proposed as key contributors to BIF deposition, the diagenetic transformation of precursor Fe(III) minerals associated with microbial biomass had not been experimentally tested. We incubated mixtures of ferrihydrite (proxy for biogenic ferric oxyhydroxide minerals) and glucose (proxy for microbial biomass) in gold capsules at 1.2 kbar and 170°C. Both wet chemical analysis and mineralogical methods (microscopy, X-ray diffraction and Mössbauer spectroscopy) were used to analyze the reaction products. Under these conditions, ferrihydrite (Fe III(OH)3) transforms to hematite (Fe2IIIO3), magnetite (FeIIFe2IIIO4), and siderite (FeIICO3). Silica-coated ferrihydrite prepared at conservative Si:Fe ratios (as predicted for the Precambrian oceans) and mixed with glucose yielded hematite and siderite, whereas magnetite could not be identified microscopically. Our results show that electron transfer from organic carbon to Fe(III) minerals during temperature/pressure diagenesis can drive the production of key BIF minerals. Our results also demonstrate that the post-depositional mineralogy of BIF does not directly archive the oceanic or atmospheric conditions present on Earth during their lithification. As a consequence, atmospheric composition regarding concentrations of methane and CO2 during the time of BIF mineral deposition cannot be directly inferred from BIF mineralogical data alone

Experimental diagenesis of organo-mineral structures formed by microaerophilic Fe(II)-oxidizing bacteria

“…Kennedy et al showed that pure ferrihydrite transformed to haematite at 80°C, whereas it remained stable when associated with bacteria at the same temperature 38 . Using experimental conditions similar to ours, Posth et al 29 demonstrated that, after 14 days at 170°C-120 MPa, ferrihydrite alone transformed to haematite while ferrihydrite associated with glucose completely transformed to haematite, magnetite and siderite 29 . Interestingly, biogenic ferrihydrite recovered at deep-sea hydrothermal vents was preserved after heating 20 min up to 400°C and transformed to haematite only at 500°C (ref.…”

Limited Zn and Ni mobility during simulated iron formation diagenesis

“…Capsules were subjected for 14 days at 170¼C and 1.2 kbar, approximately the diagenetic pressure and temperature (P-T) conditions of one of the largest and most studied iron formations on Earth, the Transvaal Supergroup BIF (Miyano and Klein, 1984;Miyano, 1987). Furthermore, these conditions mimic previous diagenetic capsule experiments (Posth et al, 2013b;Kšhler et al, 2013), and therefore, facilitate a ready comparison between experimental conditions. Our experimental duration and P-T parameters are obviously much abbreviated compared to natural burial diagenesis and metamorphism that takes thousands to millions of years; and are less than experienced by some IF which were subjected to amphibolite facies.…”

“…Diagenetic capsule experiments on synthetic Fe oxyhydroxide precipitates (Posth et al, 2013b) demonstrate that the Fe mineralogy observed in IF today can be reproduced after incubation of synthetic 2-line ferrihydrite in the presence or absence of glucose (as a proxy for biomass). Capsules were subjected for 14 days at 170¼C and 1.2 kbar, approximately the diagenetic pressure and temperature (P-T) conditions of one of the largest and most studied iron formations on Earth, the Transvaal Supergroup BIF (Miyano and Klein, 1984;Miyano, 1987).…”

“…Diagenetic experiments and analysis were performed at the University of TŸbingen using similar procedures and apparatus as per Posth et al (2013b were excluded from further analysis as the integrity of the seals were compromised during diagenetic treatments. Finally, the prefix C-denotes a control capsule (e.g., CZnFh1).…”

“…However, the extent to which Zn and Ni are mobilized when the precursor ferrihydrite underwent reduction and dehydration during diagenesis remains unclear. Our work builds on methodologies of previous simulated IF diagenesis experiments that focused on Fe-mineral transformations (Posth et al, 2013b;Kšhler et al, 2013), in order to evaluate trace element mobilization during diagenesis and the robustness of previous paleomarine Zn and Ni estimates. Additionally, due to the role of organic matter as a diagenetic reductant, we consider reactions between organic carbon and Fe(III) in our diagenetic experiments; with Zn or Ni sorbed to either synthetic 2-line ferrihydrite with added glucose in the capsule or sorbed directly to biogenic ferrihydrite.…”