Ferruginous conditions prevailed in the world's deep oceans during the Archean and Proterozoic 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.
Bibliographical noteFunding Information:
Financial and logistical support was provided by the International Continental Scientific Drilling Program (ICDP); U.S. National Science Foundation (NSF); German Research Foundation (DFG); Swiss National Science Foundation (SNSF); PT Vale Indonesia; the Ministry of Research, Technology, and Higher Education of the Republic of Indonesia (RISTEK); Brown University; University of Minnesota; University of Geneva; GFZ German Research Centre for Geosciences; Natural Sciences and Engineering Research Council of Canada (NSERC). This study was supported by the DFG ICDP priority program (SSP 1006) through grants to Kallmeyer (KA 2293/8–1) and Vuillemin (VU 94/1–1), an SNSF grant to Vuille-min (P2GEP2_148621) and an NSERC Discovery grant (0487) to Crowe. Benning acknowledges support from the Helmholtz Recruiting Initiative fund (grant no. I-044-16-01). We thank the U.S. Continental Scientific Drilling and Coordination Office, U.S. National Lacustrine Core Repository, and DOSECC
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