Mg isotope fractionation between inorganic aragonite and aqueous solution

Recent studies showed δ26Mg values of some species of Scleractinian corals, and aragonitic sponges and Scaphopod are inconsistent with the Mg isotope fractionation calibrated previously between inorganic aragonite and seawater. In this study, we explored Mg isotope fractionation between aragonite and aqueous solution under various experimental conditions, including salinity (mostly in fresh water), Mg/Ca ratio (3-5 in molar), Ca concentration (400-1500 ppm), temperature (25-55oC) and duration of experiments (3-21 days). Precipitation experiments were conducted using 'free-drift' method. The starting solution was made by mixing an appropriate amount of reagent-grade NaHCO3, CaCl2 and MgCl2 in deionized water, flushed with CO2 gas. The mixed solution was filtered after these reagents completely dissolved before any experiment, and then passively-degassed in a water bath kept at a constant temperature. Over the course of the experiments, pH and alkalinity of the aqueous solution were closely monitored. At the end of the experiment, the precipitates were cleaned, characterized by SEM and checked by XRD. The solution and precipitates were treated by acid. The supernatant was passed through two chromatographic columns to extract pure Mg, and measured for their δ26Mg values. In each case, a leaching procedure is employed to clean aragonite before their isotopic compositions were measured. Our preliminary results show that the Mg isotope fractionation between aragonite and solution varies with Ca concentration at a given Mg/Ca ratio, i.e., aragonite are strongly depleted in 26Mg in solution with high and low Ca concentration (e.g., Ca = 400ppm and 2000 ppm), but less depleted in solutions with intermediate Ca concentration (e.g., Ca= 1000 ppm). At a given Ca and Mg/Ca, the fractionation factor is temperature-dependent, defining linear relationship with 1/T (T is temperature in Kelvin) with temperature sensitivity of ~ 0.01‰/oC. These results seem to suggest a kinetic control of Mg isotope fractionation in inorganic aragonites, and provide a framework to explain biomineralization process of biogenic aragonite.