Effects of brine chemistry and polymorphism on clumped isotopes revealed by laboratory precipitation of mono- and multiphase calcium carbonates

AbstractCarbonate clumped isotopes are applied to an increasing number of geological archives to address a wide range of Earth science questions. However, the effect of changes in salinity on the carbonate clumped isotope technique has not been investigated experimentally yet. In particular, evaporated sea water and diagenetic fluids differ substantially from solutions used to calibrate the clumped isotope thermometer as they exhibit high ionic concentrations of e.g., Na+, Ca2+, Mg2+, and Cl−. High ionic concentrations are known to have an impact on δ18O values, and could potentially impact the successful application of clumped isotopes to the reconstruction of diagenetic processes, including precipitation temperatures and the origin of the diagenetic fluid.In order to address the potential influence of salt ions on the clumped isotope Δ47 value we precipitated CaCO3 minerals (calcite, aragonite and vaterite), hydromagnesite and mixtures of these minerals in the laboratory from solutions containing different salt ions (Na+, Ca2+, Mg2+, Cl−) at various concentrations and temperatures. The precipitation of some mineralogies was restricted to solutions with specific ionic concentrations, limiting direct comparability. NaCl-rich solutions mostly led to vaterite formation. In control experiments CaCO3 minerals (calcite and aragonite) were precipitated from a CaCO3 supersaturated solution without addition of any other ions.Our results show that calcium carbonates precipitated from high NaCl concentrations yield Δ47 values identical to our NaCl-free control solution. Although addition of Mg led to the formation of hydromagnesite, it also follows the same Δ47-T calibration as calcite. In contrast, Δ47 values increase together with increased CaCl2 concentrations, and deviate by a few 0.01‰ from expected equilibrium values.Overall, clumped isotope values of CaCO3 minerals precipitated between 23°C and 91°C (with and without NaCl addition) follow a line with a slope close to results from statistical thermodynamics. We conclude for calcium carbonate and hydromagnesite that the combined effect of salt ion concentration, acid fractionation and polymorphism is negligible for Cl− and Na+ with respect to clumped isotope geochemistry, but that offsets are possible in brines containing high concentrations of CaCl2