Formation of diagenetic siderite in modern ferruginous sediments

Abstract

Ferruginous conditions prevailed in the world's deep oceans during the Archean and Protero zoic Eons. Sedimentary iron formations deposited at that time may provide an important record of environmental conditions, yet linking the chemistry and mineralogy of these sedimentary rocks to depositional conditions remains a challenge due to a dearth of information about the processes by which minerals form in analogous modern environments. We identified siderites in ferruginous Lake Towuti, Indonesia, which we characterized using high-resolution microscopic and spectroscopic imaging combined with microchemical and geochemical analyses. We infer early diagenetic growth of siderite crystals as a response to sedimentary organic carbon degradation and the accumulation of dissolved inorganic carbon in pore waters. We suggest that siderite formation proceeds through syntaxial growth on preexisting siderite crystals, or possibly through aging of precursor carbonate green rust. Crystal growth ultimately leads to spar-sized (>50 μm) mosaic single siderite crystals that form twins, bundles, and spheroidal aggregates during burial. Early-formed carbonate was detectable through microchemical zonation and the possible presence of residual phases trapped in siderite interstices. This suggests that such microchemical zonation and mineral inclusions may be used to infer siderite growth histories in ancient sedimentary rocks including sedimentary iron formations.

Vivianite formation in ferruginous sediments from Lake Towuti, Indonesia

“…2a). Thus, in sediments such as Lake Towuti's, siderite is an expected mineral phase, and siderite is indeed abundant in some of Towuti's sediments (Ordoñez et al, 2019;Vuillemin et al, 2019a). This implies that CO 2− 3 can compete with PO 3− 4 for available Fe 2+ .…”

“…Dissolved inorganic carbon (DIC) concentrations were calculated by solving the carbonate system using the pH and alkalinity profiles and borehole temperatures (Jenkins and Moore, 1977). The complete pore water dataset, inclusive of all major cations and anions (Vuillemin et al, 2019a), was used to calculate mineral saturation indices based on pH, alkalinity, pore water concentrations and borehole temperatures, using the PHREEQC v.3 software (Parkhurst and Appelo, 2013).…”

“…The formation of vivianite in sediments often results from smallscale microbially mediated reactions (Rothe et al, 2016), such as reduction of ferric Fe minerals, with partial dis-solution and/or precipitation of mineral phases (Rothe et al, 2014;Egger et al, 2015;Tamuntuan et al, 2015;Dijkstra et al, 2016), alongside OM decomposition (Gächter et al, 2003;Hupfer and Lewandowski, 2008). In Fe-rich, SO 2− 4 -poor, oligotrophic settings like Lake Towuti and Lake Matano, where HS − production is minimal (Vuillemin et al, 2016), the first authigenic Fe minerals expected to form via the reduction of ferrihydrite are mixed-valence iron oxides (e.g., green rust, magnetite) instead of sulfides (Crowe et al, 2008;Zegeye et al, 2012;Vuillemin et al, 2019a). In nearby Lake Matano, lake waters contain more than 40 nM Ni and are supersaturated with respect to millerite where sulfide accumulates to low µM concentrations (Crowe et al, 2008).…”

“…The onset of autotrophic methanogenesis is expected to reduce DIC activity in pore water. Moreover, Ca 2+ and Mg 2+ concentrations in pore water, which are predicted to control the solubility of PO 3− 4 in ferruginous systems (Jones et al, 2015), drop around these depths as divalent cations can precipitate during siderite formation (Vuillemin et al, 2019a), suggesting that PO 3− 4 can then outcompete CO 2− 3 for available Fe 2+ and thereby saturate pore water with respect to vivianite (Table 1) Vivianite formation is indeed reported to occur under methanogenic conditions, often initiating below the sulfate-methane transition zone (Reed et al, 2011;Dijkstra et al, 2016), presently located within the upper meter of sediment (Vuillemin et al, 2018). Since pore waters are sat-A.…”

“…After deposition, partitioning of the light Fe isotopes mainly transits through release to pore water (Henkel et al, 2016) implying a succession of mineral transformation and dissolution with internal diagenetic Fe redistribution during burial (Severmann et al, 2006;Scholz et al, 2014). For instance, mixed-valence iron oxides (e.g., green rust), which are authigenic phases that form initially under ferruginous conditions (Zegeye et al, 2012;Vuillemin et al, 2019a), can react with pore water HCO − 3 and HPO 2− 4 , and thereby transform into either siderite or vivianite as the sediment ages (Hansen and Poulsen, 1999;Bocher et al, 2004;Refait et al, 2007;Halevy et al, 2017). Because vivianite formation initiates in the sediment, we infer that vivianite crystals acted as additional traps for the reduced Fe 2+ released to pore water and that their light δ 56 Fe values are consistent with kinetic fractionation related to mi-crobial Fe reduction during early diagenesis, or eventually inherited from postdepositional dissolution of transient ferrous phases.…”