Co-precipitation of Sr²⁺ and Ba²⁺ with aragonite by membrane diffusion of CO₂ between 10 and 50 °C

Aragonite is precipitated by a new CO₂-diffusion technique from a Ca²⁺-Mg²⁺-Cl^- solution between 10 and 50 °C. Crystallisation of aragonite instead of calcite occurs by maintaining a [Mg²⁺]/[Ca²⁺] ratio of 2 in the fluid. The dissolved inorganic carbon (DIC) is received by diffusion of CO₂ through a polyethylene membrane (diffusion coefficient: D_CO2 =10^-6.4 cm² s^-1 at 19 °C). It is suggested that significant amounts of DIC may be transferred by diffusion of CO₂ in natural systems if the CO₂ gradient is high. The CO₂ -diffusion technique is used as a kind of simple mixed flow reactor for the co-precipitation of barium and strontium with aragonite. The distribution coefficients of Ba²⁺ and Sr²⁺ decrease from 10 to 50 °C according to D^* _Ba,a =2.42-0.03595 T (°C) and D^* _Sr,a =1.32-0.005091 T (°C). At 25 °C, the distribution coefficients are D^* _Ba,a =1.5±0.1 and D^* _Sr,a =1.19±0.03. The effect of temperature on D^* _Ba,a is about one order of magnitude higher versus that on D^* _Sr,a. Thus, Ba²⁺ may be a potential paleotemperature indicator if the composition of the solution is known.