Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
4698155 | Chemical Geology | 2016 | 15 Pages |
•First theoretical and experimental determination of equilibrium fractionation for 26Mg/24Mg among inorganic and organic aqueous Mg species.•Mg bicarbonate and carbonate complexes are strongly enriched in 26Mg compared to Mg2+(aq).•Mg carboxylate complexes can be either enriched in heavy or light Mg isotopes compared to Mg2 +(aq).•The interpretation of the Mg isotopic record of marine carbonates requires taking account of aqueous Mg speciation in the forming fluids.
The magnesium isotope compositions of sedimentary carbonates are widely used to investigate the geochemical cycling of this element in seawater. Density Functional Theory techniques and experimental data from this study and literature on Mg isotope fractionation between sedimentary minerals and fluids have been used to derive equilibrium fractionation factors and to quantify the impact of aqueous magnesium speciation on the isotopic composition of its aquo ion and Mg in the precipitated carbonates. Although aqueous Mg2 + undergoes hydrolysis to a lesser extent than divalent transition metals, it nevertheless forms relatively strong complexes with inorganic and organic ligands including bicarbonate/carbonate, sulfate and carboxylate. Furthermore, aqueous Mg2 + undergoes a significant contraction of its coordination sphere when it reacts with bicarbonate and carbonate ions to form the MgHCO3+ and MgCO3° complexes which favors the preferential partitioning of heavy Mg into these species. Calculated values of the reduced partition function ratios for Mg2 +(aq) and Mg2 + bound to a number of inorganic and organic ligands show a strong enrichment of 26Mg in MgCO3°, MgHCO3+ and MgSO4° compared to Mg2 + (i.e. 1000lnβ26/24MgCO3°-1000lnβ26/24Mg2 + = 5.2 at 25 °C), and either a significant enrichment (Mg(oxalate)2 −, Mg(oxalate)22 −, Mg(citrate)−) or depletion (Mg(EDTA)2 −, Mg(citrate)24 −) of 26Mg in Mg-carboxylate complexes compared to Mg2 +. Analysis of experiments from this study and the literature on Mg isotope fractionation between aqueous solution and Mg carbonate and hydroxide minerals validate the reduced partition functions for Mg2+, Mg bicarbonate/carbonate and carboxylate couples generated in the present work and confirms the significant impact of carbonate and carboxylic ligands on the isotope composition of precipitated Mg-bearing minerals. This study thereby provides new insights into the parameters controlling the isotope composition of aqueous Mg2+ in natural fluids as well as improved tools to reconstruct paleo-environmental conditions from the magnesium isotope compositions recorded in carbonate sediments.