The control of carbonate mineral Mg isotope composition by aqueous speciation: Theoretical and experimental modeling

Jacques Schott, Vasileios Mavromatis, Toshiyuki Fujii, Christopher R. Pearce, Eric H. Oelkers

Research output: Contribution to journalArticleResearchpeer-review

Abstract

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°-1000lnβ26/24Mg 2 + = 5.2 at 25 °C), and either a significant enrichment (Mg(oxalate)2 −, Mg(oxalate)2 2 −, Mg(citrate)) or depletion (Mg(EDTA)2 −, Mg(citrate)2 4 −) 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.

Original languageEnglish
Pages (from-to)120-134
Number of pages15
JournalChemical Geology
Volume445
DOIs
Publication statusPublished - 16 Dec 2016

Fingerprint

Carbonate minerals
Carbonates
Isotopes
isotope
carbonate
mineral
Chemical analysis
modeling
bicarbonate
ligand
Fractionation
Bicarbonates
magnesium
Magnesium
Minerals
fractionation
oxalate
Oxalates
Ligands
Bearings (structural)

Keywords

  • Carbonates
  • First principles calculations
  • Mg isotopes
  • Mg speciation
  • Seawater δMg

ASJC Scopus subject areas

  • Geology
  • Geochemistry and Petrology

Cite this

The control of carbonate mineral Mg isotope composition by aqueous speciation : Theoretical and experimental modeling. / Schott, Jacques; Mavromatis, Vasileios; Fujii, Toshiyuki; Pearce, Christopher R.; Oelkers, Eric H.

In: Chemical Geology, Vol. 445, 16.12.2016, p. 120-134.

Research output: Contribution to journalArticleResearchpeer-review

Schott, Jacques ; Mavromatis, Vasileios ; Fujii, Toshiyuki ; Pearce, Christopher R. ; Oelkers, Eric H. / The control of carbonate mineral Mg isotope composition by aqueous speciation : Theoretical and experimental modeling. In: Chemical Geology. 2016 ; Vol. 445. pp. 120-134.
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AB - 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°-1000lnβ26/24Mg 2 + = 5.2 at 25 °C), and either a significant enrichment (Mg(oxalate)2 −, Mg(oxalate)2 2 −, Mg(citrate)−) or depletion (Mg(EDTA)2 −, Mg(citrate)2 4 −) 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.

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