Geochemical controls on the elemental composition of siderite: Implications for palaeo-environmental reconstructions

Abstract

The elemental composition of siderite (FeCO3) has been widely used as a palaeo-environmental proxy, and commonly utilised in the diagnosis of marine influences on Phanerozoic coastal wetland sediments. That siderite might reflect marine or freshwater sources is based on the premise that a marine origin should show an enrichment of Ca and/or Mg, while high concentrations of Mn should reflect precipitation from freshwater. However, the main controls on the elemental composition of siderite, and the degree to which siderite composition reflects parent water chemistry, are poorly understood. To address these issues, we conducted siderite nucleation and seeded growth experiments to examine element partitioning at a variety of saturation states and solution compositions (i.e., cation concentrations, pH, and pCO2). Results indicate a strong preference for Mn uptake over Ca and Mg, even when Mn is present at concentrations far below those of Ca or Mg. This uptake is enhanced at slower growth rates and lower saturation states, which may, in part, reflect surface structural controls on uptake and or a change in the dominant growth mechanism. Ca uptake only occurred during nucleation experiments, while measurable Mg uptake was not observed in experiments conducted at 20 °C, but increased slightly at 55 °C. These results suggest that the chemical composition of siderite does not directly reflect aqueous cation chemistry as previously assumed and is strongly influenced by kinetics rather than equilibrium behaviour. This leads us to conclude that variations in pore water chemistry alone cannot explain the range of Ca and Mg concentrations reported from geological siderites, and that other factors must be considered (such as physical admixtures of multiple carbonate minerals and/or the products of diagenetic recrystallization).