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: DCO2=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 DBa,a*=2.42−0.03595T (°C) and DSr,a*=1.32−0.005091T (°C). At 25 °C, the distribution coefficients are DBa,a*=1.5±0.1 and DSr,a*=1.19±0.03. The effect of temperature on DBa,a* is about one order of magnitude higher versus that on DSr,a*. Thus, Ba2+ may be a potential paleotemperature indicator if the composition of the solution is known.