EXPERIMENTAL DETERMINATION OF MG ISOTOPE FRACTIONATION DURING PRECIPITATION OF INORGANIC CALCITE

The isotopic composition of Mg in biogenic and inorganic carbonate is of great interest in studies of paleoclimate and paleooceanography because of the potential for constraining temperatures, vital effects, and marine Mg fluxes. Growth of carbonate skeletons in marine organisms produces a wide range of isotopic compositions that are species dependent, where Δ^26/24Mg_carb-sol fractionations vary from -1 to -5 ‰ (e.g, Hippler et al., 2009 GCA). Constraining the Mg isotope fractionation during inorganic carbonate precipitation is important because this serves as the baseline for interpreting the Mg isotope compositions of biogenic samples. In this study we report Mg isotope fractionation factors between Mg-bearing calcite and Mg-Ca solutions under laboratory conditions. Eighteen free-drift synthesis experiments of carbonate were carried out at temperatures between 4 ºC and 45 ºC, using solutions with Mg:Ca molar ratio between 3:1 and 18:1, buffered at P_CO2 between 0.038% and 3%. Pure CaCO₃ seed crystals were used to promote the heterogeneous growth of solid form solution in the experiments and to reduce kinetic isotope effects associated with nucleation and crystallization. Under such conditions, calcite overgrowths that contained 1.28-14.9 mole percent Mg precipitated on the seed crystals over 1 to 58 days. The Mg isotope composition of Mg-bearing calcite and Mg-Ca solutions were measured to a precision of ±0.15‰ (2sd) in δ^26/24Mg after careful purification of these Ca-rich materials using multiple ion-exchange separation techniques. The measured Mg isotope fractionation factors between Mg-calcite and solution (Δ^26/24Mg_cal-sol) show a systematic and discernable temperature dependence, changing from -2.26 ‰ at 45 ºC to -2.76 ‰ at 4 ºC. The fractionation factors are not correlated with Mg content of the Mg-calcite overgrowth, P_CO2, or the composition of the Mg-Ca solution. A chemostat experiment was done conducted at 22 ºC, where the solution composition was maintained under steady-state conditions. The measured Δ^26/24Mg_carb-sol fractionation factor for the chemostat experiment is -2.54 ‰, which is identical within error to the 3 free drift experiments conducted at the same temperature.