Equilibrium calcite-fluid Sr/Ca partition coefficient from marine sediment and pore fluids

Abstract

The equilibrium partition coefficient of strontium (KSreq) between aqueous solutions and calcite is still poorly known, even though it is a valuable parameter for studies involving the use of calcite trace element geochemistry for reconstructing paleoenvironments and fluid chemistry. In this paper we use pore fluid data from deep sea carbonate sediments to constrain KSreq at low temperature (5–17 °C) and show that the derived values are consistent with laboratory calcite precipitation experiments at 25 °C when the latter are corrected for kinetic effects. Using these low-temperature values, and experimental data available at higher temperature based on replacement reactions, we show that all of the data can be accounted for by a single formulation based on the thermodynamics of the relevant components of seawater-like fluids and the SrCO3–CaCO3 solid solution. The value of pore fluid data is that the fluids and carbonate sediment have been in contact for millions of years so local equilibrium is approached, the overall system can be treated with one-dimensional models, and the temperature is constrained. These natural systems provide an opportunity to investigate low-temperature equilibrium in the carbonate system that is difficult to probe in the laboratory because of sluggish exchange kinetics. We estimate values of KSreq using measured Sr and Ca concentrations in pore fluid and sediment solid calcite and a numerical model of sediment deposition, reaction and transport. The model is used to fit the Sr, Ca, and sulfate concentrations observed in pore fluids of several calcite-dominated sites that we believe are optimal for understanding Sr partitioning. Combining the pore fluid results with experimental measurements at higher temperature results in the following expression which applies for the temperature range 0–200 °C:KSreqT=0.025expΔGr,0R1298.15-1Twhere ΔGr,0 is the free energy change associated with the exchange reaction and temperature is in Kelvin. The uncertainty is approximately ±20%. Recently summarized thermodynamic data yield ΔGr,0 = 1.2 kcal/mol (5.0 kJ/mol) which fits well the lower limit of the high temperature data. The corresponding activity coefficient for the SrCO3 component in the calcite crystal structure is 5.4 for the lower value of ΔGr,0, and 3.17 for the higher value. The derived low-temperature values of KSreq of 0.020 to 0.025 (for 0 °C to 25 °C) have implications for models of marine carbonate diagenesis, and the interpretation of vein carbonate Sr/Ca in oceanic crust. Published data showing much higher KSr values in Mg-bearing solutions are not representative of equilibrium values for either Mg or Sr.