Kinetic isotope effect in siderite growth: Implications for the origin of banded iron formation siderite

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The widespread occurrence of siderite (FeCO 3 ) at Earth's modern surface and in sedimentary rocks has led to its frequent use as a tool for palaeoenvironmental reconstruction. Isotopic studies of siderite associated with Archaean-Palaeoproterozoic banded iron formations show negative δ 13 C values, which coupled with δ 56 Fe values, have been considered to support an important role of dissimilatory iron reduction (DIR) in the genesis of iron formations. Facies-specific analyses show that texturally and petrographically syndepositional and/or early diagenetic, finely laminated microsparitic (≤10 µm in diameter) siderite exhibits δ 13 C between −3 and −7‰. This siderite δ 13 C range can be interpreted in three ways: (1) precipitation of siderite from dissolved inorganic carbon (DIC) produced by DIR coupled to partial oxidation of organic carbon with δ 13 C < −25‰, (2) precipitation from a mixed hydrothermal-seawater fluid bearing mantle-derived DIC with δ 13 C ≅ −6.5‰, and (3) precipitation from seawater-derived DIC and expression of a kinetic isotope effect (KIE) associated with siderite growth. We present the carbon isotopic composition of siderite formed in abiotic seeded growth experiments over a wide range of solution siderite saturation at 25 °C and 1 bar. With a complete set of chemical kinetic data, we develop and apply a model of disequilibrium siderite precipitation to constrain the carbon KIE as a function of growth rate. Sampling of model parameters from distributions that represent uncertainty in the parameter values, we find best-fit values of equilibrium and kinetic fractionation factors: 10 3 ln α eq = −0.2 and 10 3 ln α f = −14.9. These constraints allow us to assess the origins of δ 13 C values in microsparitic siderite in iron formations. For example, a moderate level of siderite supersaturation (e.g., Ω sid = 100) in the hydrothermal-seawater mixing fluids (source of dissolved iron) would have induced a carbon KIE of ∼−8‰ in fluid-buffered early diagenetic siderite growth; a range that encompasses essentially all negative δ 13 C values reported from microsparitic siderite in Campbellrand-Kuruman iron formations, Transvaal Supergroup, South Africa. We suggest that a straightforward, abiotic explanation for the range of δ 13 C values in microsparitic siderite in Archaean-Palaeoproterozoic banded iron formations is pulsed deposition of iron-rich sediments associated with intense hydrothermal activity. Specifically, the siderite-δ 13 C range is well explained by a partial expression of carbon KIE associated with siderite growth from supersaturated solutions, and from a bottom-water DIC reservoir with near-zero δ 13 C values (seawater with a possible contribution of mantle carbon).