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

Abstract

The widespread occurrence of siderite (FeCO3) 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 δ13C values, which coupled with δ56Fe 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 δ13C between −3 and −7‰. This siderite δ13C 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 δ13C < −25‰, (2) precipitation from a mixed hydrothermal-seawater fluid bearing mantle-derived DIC with δ13C ≅ −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: 103lnαeq = −0.2 and 103lnαf = −14.9.These constraints allow us to assess the origins of δ13C values in microsparitic siderite in iron formations. For example, a moderate level of siderite supersaturation (e.g., Ωsid = 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 δ13C 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 δ13C 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-δ13C 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 δ13C values (seawater with a possible contribution of mantle carbon).